Extracting fresh water from the ocean relies on desalination, which separates salt and minerals from saline water. Thermal desalination methods employ heat energy to drive this separation, fundamentally replicating the Earth’s natural water cycle of evaporation and condensation. This technique involves heating seawater until it vaporizes, leaving behind the dissolved solids. The resulting pure steam is then captured and cooled back into liquid freshwater, allowing industrial plants to reliably produce high-purity water for various uses.
Preparing the Seawater
Before seawater enters the main evaporation unit, it undergoes preparation steps to protect the equipment and maintain efficiency. The initial stage involves physical screening to remove large debris, such as marine life, sand, and other suspended solids that could damage pumps and clog passages. This mechanical filtration ensures the raw feed water is clean enough for the subsequent heating stages.
Following the removal of large particles, the seawater is chemically treated, primarily to combat scale formation on hot surfaces. As water heats up, dissolved minerals like calcium carbonate and magnesium hydroxide precipitate out, forming a hard scale on the heat exchanger tubes. To prevent this mineral buildup, which severely reduces heat transfer efficiency, chemical additives known as anti-scalants are injected into the water flow. The pH of the seawater may also be adjusted, or other chemicals like coagulants might be added to promote the settling of fine suspended particles.
Multi-Stage Flash Distillation
Multi-Stage Flash (MSF) distillation uses a pressure-reduction technique to cause instantaneous vaporization. In this system, seawater is first heated to a high temperature (typically between 90°C and 120°C) while being kept under pressure to prevent boiling. This heated, pressurized water then flows into a series of interconnected chambers, called stages, each maintained at a progressively lower pressure than the last.
As the hot seawater enters the first stage, the sudden drop in pressure causes a portion of the water to instantaneously “flash” into steam. This rapid vaporization utilizes the latent heat already contained within the liquid without the addition of more heat energy. The resulting pure water vapor rises and condenses on cooler tubes that carry the incoming feed water, preheating it for the next cycle. The condensed vapor is collected as freshwater, and the remaining concentrated brine moves to the next stage, where the flashing process repeats.
Multi-Effect Distillation
Multi-Effect Distillation (MED) is a widely used thermal process that achieves high efficiency by reusing the heat of vaporization multiple times. The system consists of a sequence of separate chambers, known as “effects,” which are arranged to operate at successively lower temperatures and pressures. Heating steam is introduced into the first effect, causing a portion of the seawater that flows over a tube bundle to evaporate.
The vapor produced in the first effect is routed to become the heat source for the tube bundle in the second effect. Since the second effect is maintained at a lower pressure, the boiling point of the seawater is also lower. This allows the steam from the first effect to condense and release its latent heat to cause evaporation in the second. This process continues across multiple effects, multiplying the amount of freshwater produced from a single initial heat input. The design allows MED to operate at lower temperatures, often below 70°C, which minimizes the risk of scale formation and corrosion.
Handling the Byproducts
The thermal distillation processes produce two primary streams: the highly purified water, known as the distillate, and a concentrated saline waste stream, called brine. The distillate is exceptionally pure, having been separated from almost all salts and minerals, but this purity means it must undergo post-treatment to become potable. This typically involves aeration to adjust dissolved gas content, a final pH correction, and the addition of specific minerals like calcium and magnesium to improve its taste and prevent it from corroding distribution pipes.
The concentrated brine contains all the rejected salts and pre-treatment chemicals, requiring careful environmental management. Disposal often involves discharging it back into the sea, usually after mixing it with a larger volume of cooling water or through diffusers to rapidly dilute the stream. This dilution minimizes the risk of the dense, warm, and highly saline plume settling on the seabed and harming local marine ecosystems. In some cases, zero-liquid discharge systems may be employed to fully evaporate the remaining water and collect the solid salts for disposal.