Saltwater, such as that found in oceans, contains a high concentration of dissolved salts, primarily sodium chloride. On average, ocean water has a salinity of about 3.5%, meaning every liter contains approximately 35 grams of dissolved salts. This high salt content makes it unsuitable for direct human consumption because the human body’s kidneys cannot process such a high concentration of salt. Drinking saltwater would lead to dehydration as the kidneys would require more fresh water to flush out the excess salt than the amount consumed. Therefore, removing these dissolved salts is necessary to make saltwater potable.
Distillation
Distillation is a method for purifying water by separating it from dissolved salts through a process of heating and condensation. The saltwater is heated to its boiling point, converting the pure water into steam while leaving the non-volatile salts and other impurities behind. This steam is then collected and cooled, causing it to condense back into liquid, salt-free water. This basic principle can be applied in simple home setups, such as boiling water in a pot and collecting the condensation on a slanted lid or a separate surface.
The effectiveness of distillation in removing dissolved solids, including salts, is high because only water molecules evaporate and then condense. However, distillation is energy-intensive, particularly when dealing with large volumes of water, as significant heat is required to boil the water. While effective for small-scale applications or emergencies, its energy demands make it less practical for continuous, high-volume freshwater production compared to other methods.
Reverse Osmosis
Reverse osmosis (RO) is a more advanced method for filtering saltwater, relying on pressure to force water through a specialized semi-permeable membrane. This membrane has extremely small pores, which are large enough for water molecules to pass through but too small for dissolved salt ions. When high pressure is applied to the saltwater, the water molecules are pushed across the membrane, leaving the concentrated salt solution behind.
RO systems are highly efficient in removing a wide range of contaminants, including salts, bacteria, viruses, and other dissolved solids, making them widely used in both home purification units and large-scale industrial desalination plants. For household applications, RO systems often include multiple stages of filtration, with pre-filtration steps necessary to remove larger particles, sediment, and chlorine before the water reaches the delicate RO membrane. This pre-treatment helps protect the membrane from fouling and damage, ensuring its longevity and maintaining system efficiency.
Solar Desalination
Solar desalination harnesses the sun’s energy to purify saltwater. This method mimics the natural hydrological cycle. A typical solar still consists of a shallow basin containing saltwater, covered by a transparent, sloped lid. Sunlight passes through the lid, heating the saltwater in the basin.
As the water heats, it evaporates, leaving the salts behind. The water vapor then rises and condenses on the cooler underside of the transparent lid. Due to the lid’s slope, the condensed, purified water trickles down into a collection trough. This passive process is simple, requires no external power source beyond sunlight, and is well-suited for remote locations or emergency situations. While effective, solar stills typically have a slower output, producing a limited volume of fresh water over a day.
What Standard Water Filters Don’t Remove
Water filters are not designed to remove dissolved salts from water. These filters, such as activated carbon filters, sediment filters, or gravity-fed systems, primarily target larger impurities. They are effective at removing contaminants like bacteria, protozoa, sediment, chlorine, and some organic compounds that affect taste and odor.
However, the pore sizes in these standard filters are larger than the size of dissolved salt ions. Therefore, while they can make brackish or contaminated freshwater safer to drink by removing biological and particulate matter, they do not alter the water’s salinity. Attempting to filter saltwater with these devices will not render it potable, as the dissolved salts will pass through unimpeded.