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Salt is removed from saline water by desalination. The two core methods are distillation, which evaporates water and then condenses the vapor, and reverse osmosis, which uses pressure to push water through a semipermeable membrane. A simple solar still can demonstrate distillation and collect a small amount of water; dependable large-scale supply requires engineered systems. Neither method, by salt removal alone, guarantees that the water is safe to drink.
What does “fresh water” mean here?
Salinity describes the amount of dissolved salt in water; it is not a complete measure of whether water is safe to drink. The U.S. Geological Survey (USGS) classifies fresh water as containing less than 1,000 parts per million (ppm) of dissolved salts, while ocean water contains about 35,000 ppm. Those figures are useful reference points, not a drinking-water safety standard. USGS explains desalination and salinity categories.
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How distillation removes salt
Distillation separates water from dissolved salts by changing water into vapor and then back into liquid. Heating saline water causes the water to evaporate while the salts remain in the source water. The vapor is captured and cooled so it condenses as liquid water.
What a solar still can do
A solar still uses sunlight as its heat source. In a basic arrangement, an enclosure lets the sun warm saline water; vapor forms and condenses on a cooler surface, where it can be collected. The USGS describes this as a low-tech way to make small amounts of fresh water, not a practical substitute for a regular household supply. It describes the method this way: “The low-tech approach to accomplish this is to construct a ‘solar still’ which uses heat from the sun to run a distillation process to cause dew to form on something like plastic sheeting.”
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A small still’s limited output matters: the process demonstrates how separation works, but the available evidence does not establish a validated homemade design or a predictable yield. A portable solar still is likewise best understood as a small-scale demonstration or contingency device, not as a guaranteed source of routine drinking water.
How reverse osmosis removes salt
Reverse osmosis (RO) uses pressure to drive water through a semipermeable membrane. The membrane retains much of the dissolved salt, producing a less salty water stream and a concentrated salty stream, often called brine. It is a widely used membrane process, but the feedwater and the system’s design determine whether it can operate effectively.
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As feedwater salinity rises, more pressure is needed. Very salty water can exceed a membrane system’s operating limits, so a household RO unit intended for ordinary tap water should not be assumed to treat untreated seawater. The U.S. Department of Energy (DOE) describes the pressure-driven membrane process and its salinity-related constraints in Desalination Basics.
Choosing between a simple still and engineered desalination
| Consideration | Solar still (distillation) | Reverse osmosis |
|---|---|---|
| Water output and scale | A simple still produces small amounts; it is not a dependable ordinary household supply. | Used in engineered systems at larger scales; output depends on system design and feedwater. |
| Energy and operation | Uses sunlight for heat in the basic demonstration; collection depends on the still’s setup and conditions. | Requires pressure to move water through a membrane and is an engineered process. |
| Salinity limits | Separates water by evaporation and condensation; no comparative operating limit is stated by USGS. | Higher salinity requires more pressure and may exceed membrane limits, according to DOE. |
| Residual salt or brine | Salt remains in the source water as water evaporates; the source water becomes more concentrated. | Produces a concentrated salty stream that must be managed. |
| Energy comparison | Thermal desalination processes typically use more energy than alternatives such as RO, even when efficient, according to DOE. | DOE identifies RO as an alternative that typically uses less energy than even efficient thermal processes. |
The comparison is general, not a device-performance rating: output and operating needs vary with the water and equipment. For large or dependable supplies, desalination relies on engineered systems rather than a simple household-scale still. USGS reports that the International Desalination Association estimated more than 300 million people receive water from desalination plants; the USGS page does not date that estimate. The same page attributes global production of 109.22 million cubic meters per day in 2022 to the association.
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Does removing salt make the water safe to drink?
Not necessarily. Desalination addresses salinity, but water can contain other contaminants that the process or a particular device may not remove. For home treatment, choose a system based on the contaminants actually present and check its label for the contaminants it is designed to reduce. The Centers for Disease Control and Prevention (CDC) explains how to select a home water treatment system at About Home Water Treatment Systems. Do not treat low salinity by itself as proof of potability.
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