Desalination is the process of removing dissolved salts and minerals from water to make it suitable for consumption or other uses. While thermal distillation involves boiling water and collecting the resulting steam, this technique requires a significant amount of energy. Traditional boiling is impractical for home use due to high energy costs and the volume of water needed. Several low-energy, non-boiling alternatives exist that harness natural processes to achieve effective separation. These methods offer simple, DIY approaches to produce fresh water from a saline source.
Passive Solar Desalination
Passive solar desalination, often accomplished with a solar still, uses the sun’s energy to drive a natural water cycle on a smaller scale. This technique utilizes solar radiation to warm the saltwater, causing it to evaporate at temperatures well below the 212°F (100°C) boiling point. The process relies on the principle that only pure water molecules become vapor, leaving the dissolved salts and contaminants behind.
A simple solar still requires a hole in the ground, a collection cup, and a sheet of plastic. Dig a basin in the soil and place a clean container in the center. Pour the saltwater into the surrounding trench, ensuring it avoids the collection cup. The hole is then covered with clear plastic sheeting, sealed around the edges with soil or rocks.
A small weight, such as a rock, is placed on the center of the plastic directly over the collection cup, creating a cone shape. Sunlight heats the water and the moist air inside the still, causing pure water vapor to rise. This vapor condenses into droplets on the cooler underside of the plastic sheet. The droplets trickle down the cone and drip directly into the collection cup, yielding highly pure distillate.
Freezing and Fractional Crystallization
Freezing desalination, also known as fractional crystallization, is based on the scientific principle that when salt water freezes, the resulting ice lattice naturally excludes salt molecules. Pure water molecules align to form the crystalline structure of ice, forcing the dissolved ions into the remaining liquid portion. This separation significantly lowers the energy requirement compared to evaporation, as the latent heat of fusion for freezing is substantially less than the latent heat of vaporization for boiling.
For home application, the process involves partially freezing the saltwater in a clean container. As the water begins to solidify, the salt concentration in the unfrozen liquid, or brine, increases dramatically. Once a thick layer of relatively pure ice has formed, the remaining concentrated brine should be poured off and discarded.
The resulting ice is then melted to yield fresh water with a much lower salt content. To achieve potable quality, this process often requires multiple cycles. The melted water from the first attempt is refrozen and separated again. Each subsequent freezing cycle further reduces the salt concentration, bringing the water closer to a drinkable standard.
Why Basic Filtration Fails
Common household filters, such as those found in pitcher filters or refrigerator systems, are designed to remove suspended solids, sediment, and chemical contaminants like chlorine. These filters typically use materials like activated carbon or a micro-mesh screen, which work by physically straining or chemically adsorbing impurities. However, they are incapable of removing dissolved salts.
Salt exists as dissolved ions—specifically sodium (Na+) and chloride (Cl-)—which are incredibly small. The pore size of a standard microfilter is typically around 0.1 microns. Since the effective size of a water-solvated salt ion is far smaller, it passes through unobstructed. Removing these dissolved solids requires a much more sophisticated barrier.
The commercial solution for removing dissolved ions without boiling is Reverse Osmosis (RO). RO systems use a semi-permeable membrane with an extremely fine pore size, typically around 0.0001 microns. A high-pressure pump is required to force water through this membrane, overcoming the natural osmotic pressure of the saline solution. The tiny membrane pores allow water molecules to pass through while physically blocking the larger, charged salt ions, which are flushed away in a concentrated stream of wastewater.