You can purify salt water by boiling it and collecting the steam, pushing it through a specialized membrane, or using the sun’s heat to evaporate and recondense it. Each method removes salt through a different mechanism, and the best choice depends on your situation: a survival scenario, an off-grid living setup, or simple curiosity about how desalination works.
Distillation: Boiling and Collecting Steam
Distillation is the oldest and most intuitive method. You heat salt water until it turns to steam, then cool that steam so it condenses back into liquid. The salt stays behind because it doesn’t evaporate at water’s boiling point. The result is high-purity water with more than 99.9% of dissolved minerals removed.
To do this at home, you need a pot with a lid, a heat source, and a way to collect condensation. Fill the pot with salt water, place a smaller collection bowl inside (floating or propped above the water line), and invert the lid so condensation drips down into the bowl. Adding ice on top of the inverted lid speeds up condensation significantly. On a stove, this setup can produce a usable amount of drinking water in about an hour, though the process is slow and energy-hungry compared to other methods.
The main drawback is energy cost. Heating large volumes of water to a full boil requires substantial fuel or electricity, which is why industrial distillation plants consume far more energy per liter than membrane-based systems. For small batches in an emergency, though, it works reliably with basic kitchen equipment.
What Distillation Doesn’t Remove
Simple boiling handles salt and most dissolved minerals, but it struggles with volatile organic compounds, which are chemicals with boiling points close to or below water’s. These compounds evaporate alongside the water and end up in your condensed “clean” water. Research on distillation membranes shows rejection rates below 54% for volatile compounds and below 80% for semi-volatile ones. If your salt water source might contain fuel, solvents, or industrial runoff, distillation alone won’t make it safe. Running the distilled water through an activated carbon filter afterward catches most of these contaminants.
Solar Stills: Low-Tech, No Fuel Required
A solar still uses sunlight instead of a stove to evaporate water. The simplest version is a shallow basin of salt water covered by a tilted sheet of glass or clear plastic. Sunlight heats the water, vapor rises and condenses on the cooler cover, and gravity channels the droplets into a collection trough at the low edge.
The tradeoff is output. Conventional solar stills produce between 2 and 5 liters per square meter per day, with efficiency ranging from about 20% to 46% depending on sunlight intensity, ambient temperature, and design. One study measuring an optimized flat-plate solar still recorded a peak output of roughly 6 liters per square meter over 8 to 9 hours under strong solar radiation (6.2 kWh/m²) at 23°C. That’s a best-case scenario. In cloudier or cooler conditions, expect closer to 2 liters.
For a survival situation, you can improvise a ground-level solar still by digging a hole, placing a container at the bottom, filling the area around it with salt water or wet material, and stretching clear plastic over the top. A small rock in the center of the plastic creates a low point where condensation drips into your container. This won’t produce much, but it requires zero equipment beyond plastic sheeting. If you’re in a location above 20° latitude, a single-slope design (angled cover facing the equator) captures more sunlight than a symmetrical one. Glass covers between 3 and 5 mm thick provide the best balance of light transmission and condensation performance.
Reverse Osmosis: The Most Efficient Option
Reverse osmosis (RO) works by forcing salt water through a membrane with pores so small that only water molecules pass through. Salt, bacteria, viruses, and most dissolved contaminants get left behind. Unlike distillation, there’s no heating and no phase change. The energy goes entirely into pressurizing the water, which makes RO significantly more energy-efficient per liter.
Industrial RO plants desalinate seawater at scale for coastal cities around the world. But the technology has also been miniaturized into portable, hand-powered units designed for boats, emergency kits, and off-grid communities. These devices use a manual piston mechanism that converts pumping effort into the hydraulic pressure needed to push water through the membrane. Field testing of hand-powered RO units shows salt rejection rates between 88% and 95%, depending on the membrane quality. A commercial-grade polyamide membrane achieved 95.25% rejection in testing, though it required roughly twice the pumping effort of lower-grade membranes.
Portable desalinators typically cost several hundred dollars and can produce enough drinking water for one or two people per day with sustained hand-pumping. They’re compact, require no electricity or fuel, and remove bacteria and viruses along with salt. For anyone planning extended time at sea or in coastal survival situations, they’re the most practical option available.
The DIY Bottle Still
If you have plastic bottles, some tubing, and a heat source, you can build a rudimentary distillation device. The basic concept: drill holes in a bottle cap and base, thread tubing through them, connect one end to a steam source (a pot or metal container over heat) and angle the bottle downward so condensation flows out the other end into a collection glass. Sealing the connections with hot glue or similar adhesive prevents steam from escaping. A second loop of tubing carrying cold water around the outside of the bottle speeds up condensation.
This setup is more effective than a pot-and-lid arrangement because the tubing gives steam a longer path to cool and condense. It’s still slow and fuel-dependent, but in a situation where you have scrap materials and a fire, it produces cleaner water than boiling alone since you’re physically separating the collection point from the salt water.
Why Desalinated Water Needs Minerals Added Back
Water that’s been fully distilled or passed through a high-rejection RO membrane tastes flat and slightly off. That’s because the process strips out calcium, magnesium, sodium, and potassium, the minerals that give water its familiar taste. More importantly, those minerals matter for your body. Studies have linked long-term consumption of water low in calcium and magnesium to muscle cramps, fatigue, weakness, and even cardiovascular problems.
If you’re drinking desalinated water regularly, rather than just in a short-term emergency, you should add minerals back. The simplest approach is adding a small pinch of clean sea salt (not enough to taste salty, just enough to reintroduce trace minerals) or using mineral drops sold for this purpose. Alternatively, running distilled water through a filter containing calcium carbonate or coral sand will remineralize it naturally. For occasional or emergency use, the mineral deficit is not an immediate concern since your food provides most of what you need.
What Happens to the Leftover Salt Water
Every desalination method produces a byproduct: brine, which is the concentrated salt water left after you’ve extracted the fresh water. This brine is saltier than the original source and, depending on what was in the water, can also contain heavy metals, organic compounds, and any pretreatment chemicals used in the process.
At home scale, the leftover brine is a small volume and can simply go down the drain if you’re connected to municipal sewage. If you’re doing this outdoors or near a natural water source, avoid dumping concentrated brine directly into streams, ponds, or shallow coastal areas. The elevated salinity can harm aquatic life, and heavy metals in the concentrate can accumulate in sediment. Spreading it thinly over dry, non-vegetated ground is a better option for small quantities. Industrial desalination plants face this challenge at massive scale, and improper brine disposal has been linked to eutrophication and pH swings in marine environments near discharge points.
Choosing the Right Method
- Emergency with minimal gear: A pot-and-lid distillation setup or improvised ground solar still. Low output, but requires almost nothing.
- Extended survival or boating: A portable hand-pump RO desalinator. Higher upfront cost, but produces cleaner water with less effort per liter.
- Off-grid living in a sunny climate: A properly built solar still with glass cover, sized to your daily water needs. At 2 to 5 liters per square meter per day, you’d need several square meters of still area to supply one person.
- Home experimentation or small batches: Stovetop distillation with a proper condenser tube. Effective and repeatable, though not energy-efficient enough for daily use.
Whichever method you choose, always consider what else might be in the water beyond salt. If the source could contain chemicals, fuel, or industrial pollutants, pair your desalination method with activated carbon filtration to catch what evaporation or membranes miss.