Water desalination plants can transform brackish seawater into drinking water.
Water desalination is the process of removing minerals from seawater or brackish water. It works by removing salt from the water, leaving clean water. There are several types of water desalination processes, including reverse osmosis, thermal distillation, and electrodialysis.
Reverse osmosis pushes water through a membrane while retaining unwanted salts and minerals. It is said to remove 95-99% of unwanted materials. Before processing, the water is pre-treated and sent through microfilters to remove larger particles and help protect the membrane. The water is then pushed through membranes that retain salt or minerals so that fresh water is produced.
There are three methods of desalination: reverse osmosis, electrodialysis, and thermal distillation.
Thermal distillation uses heat to remove minerals. Bringing salted water to a boil causes the water to evaporate, leaving the salt behind. The vapor is then condensed in a second container and collected. The process of boiling water can be very fuel intensive, so to keep costs down, other processes can be used. These processes are called multi-stage flash distillation, multi-effect distillation, and vapor compression distillation. It’s about lowering the vapor pressure of water to lower its boiling point, so it uses less energy.
One method of desalination uses solar energy to vaporize and collect water.
Electrodialysis uses the electrical charge of salt ions to separate them from water. In this process, the water is pretreated and sent to a stack of membranes. Two electrodes, a positive one placed on one side of the cell and a negative one on the other, conduct an electrical current through the water. Ions are attracted to oppositely charged electrodes and flow through membranes that are designed to trap anions, which are negatively charged ions, or cations, which are positively charged ions. Once the ions are captured, fresh water remains.
Other types of desalination processes include solar humidification, membrane distillation, and freezing. Freezing takes advantage of the fact that salt is not included in the formation of ice crystals when water freezes, so this mineral can be removed when the water is nearly frozen. Solar humidification uses the sun’s energy to vaporize water. The water can then condense on another surface, where it collects.
In membrane distillation, the water is vaporized first. It is then sent through membranes that allow the passage of vapor but not liquid. Salt and other substances get trapped in the membrane and get trapped there, and water vapor condenses on a surface on the other side. Once liquid, fresh water cannot flow back through the membrane and can pool freely.