What is a Desalination Plant and How Does it Work?
Simply put, desalination is any process that removes salt and minerals from water. Whichever process is used, it will generate two water streams. The first is treated fresh water that’s safe for drinking and has very low concentrations of salt and minerals. The second is an increasingly concentrated mixture of salt, other minerals and water known as brine.
Two distillation technologies dominate around the world. The particular technology used in any case usually depends on several factors, including energy costs. It’s important to recognize, though, that even the most effective technology may extract a very high volume of freshwater from the source water, salts and other compounds remain in a highly concentrated liquid. This reject brine is directly harmful to the environment.
Desalination Using Phase Change Processes
Phase change technologies remove salts and minerals from source liquids at the point where the water changes state: between liquid and frozen (freezing), or between liquid and gas (evaporation). These processes are energy-intensive, so they tend to be most popular in locations where energy costs are relatively low, like the Middle East.
Multi-Stage Flash (MSF)
In multi-stage flash distillation, source water is heated to very high temperatures and compressed to very high pressures. As it passes through a series of chambers, the pressure is gradually reduced, which induces a rapid boil and releases water vapor. Each subsequent chamber reduces the pressure further and produces additional vapor from an increasingly concentrated brine. The vapor is cooled and condensed to its pure liquid form and collected.
Multi-effect Distillation (MED)
The concepts in MED are like that for MSF, but instead of a series of chambers in a single vessel, MED employs a series of vessels, which makes the overall process more efficient.
Vapor Compression Distillation (VCD)
Vapor Compression Distillation can be used alone or in combination with other thermal distillation processes. In VCD, seawater is combined with hot, recirculated brine and is sprayed onto pipes heated by compressed vapor. The steam that is produced is collected and compressed and passed back through the chamber where it heats the incoming seawater on its way to be cooled and collected as fresh water. Brine that is not converted to steam is collected; some of it is recirculated through the spray nozzles again and the rest is discharged as waste. VCD is commonly used to produce fresh water on a small to medium scale, such as for petroleum drilling sites or resorts.
Solar Distillation
Solar distillation is usually used for relatively small-scale operations. In these units, sometimes called solar stills, the sun provides thermal energy to evaporate source water from a basin, which then condenses on a clear glass or plastic cover. The condensed water is collected in a trough surrounding the basin, while the cover also serves to transmit and retain thermal energy. Since the water remains in the basin and both the water vapor and condensate moves with gravity, this is a passive system that only requires periodic removal of concentrated brine and salts.
This type of desalination is most often used in very arid regions where safe fresh water is not available. The amount of freshwater produced depends on the quality of the source water, the specific design of the still, and local climate.
Freezing
Desalination using freezing is based on the principle that ice crystals are essentially pure water. In one configuration, sea water is passed through a heat exchanger to reduce its temperature, then carried to a chamber where it is cooled to the point of forming ice crystals. The slurry of ice and brine is separated, and the ice is melted. The freshly melted freshwater passes through the heat exchanger to help cool incoming sea water and is then discharged. This kind of indirect freezing process requires complex equipment which is expensive and difficult to maintain.
Compared to other desalination processes, freezing potentially offers several advantages. It requires substantially less energy transfer, requires virtually no pre-treatment, and has minimal problems with scale formation and corrosion. In addition, its effectiveness isn’t hindered by the presence of contaminants or the salt concentration in the source water. This makes this a potentially valuable solution even for highly concentrated brine reject from RO plants.
Although current technology hasn’t yet developed to the point where freezing is a viable large-scale desalination process, it does offer some significant inherent advantages, and research and development continue.
