Water that contains saline levels less than 1,000 ppm is generally considered fresh water, and is safe to drink and use for household and agricultural purposes. For a reference point, typical ocean water contains about 35,000 ppm, the Great Salt Lake contains variations of 50,000 – 270,000 ppm, and the Caspian Sea contains an average of about 12,000 ppm. The more concentrated saline is in a body of water, the more energy and effort it takes to desalinize it.
Desalination ProcessesThere are numerous methods of desalination described below. Reverse osmosis is currently the most commonly found type of desalination, and multistage flash distillation is the method that currently produces the most amount of desalinated water. (There are several other less frequent types of desalination methods and energy sources not discussed here.)
Reverse OsmosisReverse osmosis is a process where pressure is used to push the water solution through a membrane, with the membrane preventing the larger solutes (the salt) to pass through. Reverse osmosis is generally considered to be the least energy consuming of all the large-scale processes.
There are several setbacks of reverse osmosis. The membranes are currently prone to gather too much bacteria and “clog up,” although they have improved since they were first used. The membranes deteriorate when chlorine is used to treat the bacteria. Other setbacks are the arguable water quality that reverse osmosis produces, along with the considerable pre-treatment that the salt water requires.
Forward OsmosisForward osmosis utilizes the natural osmotic process; a substance moving from an area of low concentration to an area of high concentration. It generally requires about half of the cost of reverse osmosis, due to less energy being used to complete the process. Instead of forcing the solution through a pressure gradient, this process allows it to naturally occur. When desalinating water, a solution of seawater moves across a semi-permeable membrane to a highly concentrated solution of ammonia salts, leaving the sea salts on the other side of the membrane. Afterwards, the solution is heated to evaporate the ammonia salt, and that salt is reusable.
The main setback to forward osmosis is that it has great potential, but is still fairly new to large-scale desalination and therefore needs funding and research to explore the possibilities that could improve it and reduce energy costs.
ElectrodialysisElectrodialysis reversal utilizes a membrane, like that in reverse osmosis, but sends an electric charge through the solution to draw metal ions to the positive plate on one side, and other ions (like salt) to the negative plate on the other. The charges are periodically reversed to prevent the membrane from becoming too contaminated, as typically found in regular electrodialysis. The ions located on both plates can be removed, leaving pure water behind. Recently developed membranes reportedly have been chlorine resistant, and generally remove more harmful ions (not just salt) than reverse osmosis. The primary setback to electrodialysis reversal is the upfront cost to create the facility, as well as the energy costs.
Thermal DesalinationThermal desalination is a method of cleaning water that can occur through many different processes, and includes removing salt as well as other contaminants. All thermal desalination is the process of heating the water solution and gathering pure water when the vapor cools and condensation occurs. Two types frequently used to desalinate water are:
Multistage Flash DistillationMultistage flash distillation occurs when the product of the heated water is reheated multiple times, each time functioning on lower pressure than the last. Multistage flash distillation plants are built alongside power plants in order to use the wasted heat. It requires much less energy than reverse osmosis plants. Several large facilities in Saudi Arabia use multistage flash distillation, accounting for around 85% of all desalinated water, though there are more reverse osmosis plants than there are multistage flash distillation plants. The main disadvantages of multistage flash distillation are that it requires more intake of salt water than reverse osmosis and the upfront and maintenance costs are considerably high.
Multiple-Effect DistillationMultiple-effect distillation is a simple process similar to multistage flash distillation. The salt water solution is heated and the pure water that is produced flows into the next chamber. The heat energy that it carries is used to boil it again, producing more vapor. The main setback is that it is best used for smaller-scale desalination. The costs are very high for large facilities.
Negatives of DesalinationA few general setbacks for the processes of desalination also exist. Dumping the wasted salt solution back into the ocean makes the process more difficult and has the potential to harms ocean life. The energy required to start up and power desalination plants is a huge expense and because most current power sources are derived from burning fossil fuels, it is generally looked upon as just a matter of choosing one environmental crisis over another. Within the energy issue, nuclear energy is potentially the most cost-effective energy source, but remains largely untapped due to public opinion on having a local nuclear power plant or waste facility. If regions situated away from the coast or in a high altitude try to use desalinated water, it is an even more expensive process. Higher altitudes and far distances require great resources to transport the water from the ocean or body of salt water.
Geography of DesalinationGeography of Desalination Desalination is currently used by countries that have an extreme need for fresh water, have enough money to fund it, and posses the amount of energy required to produce it. The Middle East holds the top spot for desalinated water, due to several countries’ large facilities, including Saudi Arabia, the United Arab Emirates, and Israel. Also large producers of desalinated water are: Spain, the United States, Algeria, China, India, Australia, and Aruba. The technology is expected to spread increasingly, particularly in the United States, Libya, China, and India.
Saudi Arabia is currently the world’s number one producer of desalinated water. They use multi-flash distillation in several large plants, providing water for many large cities, including the largest city, Riyadh, situated hundreds of miles from the coast.
In the United States, the largest desalination plant is located in Tampa Bay, Florida, though it has a very small output compared to most facilities in the Middle East. Other states that are developing plans for large desalination plants include California and Texas. The United States need for desalination plants is not as severe as many other countries, but as the population continues to explode in dry, coastal areas, the need increases.