Much like the Salton Sea, many inland bodies of water suffer from rising salinity, which can harm biota and prevent beneficial water use. This salinization occurs when soil, which contains salts and minerals, is mobilized from clearing natural vegetation or when fresh water is diverted for irrigation.  As irrigation water and drinking water sources become increasingly salty, different solutions become necessary to recover freshwater. Saudi Arabia is the world’s leader in desalination, which is the industrial process of removing salt from water, with 50% of the nation’s drinking water recovered from seawater.  At the Salton Sea, desalination is being explored as a part of habitat restoration efforts. The Salton Sea Restoration and Renewable Energy Initiative (SSRREI) proposes the use of existing desalination and renewable energy to create wetland pools for fish and bird species that suffer from the salinization of the Sea.   As water quality and water scarcity becomes an increasingly global issue, we will likely see increased innovation in desalination.
Industrial desalination employs one of two main techniques: membrane filtration or thermal distillation. For this post, I will focus on distillation. Distillation mimics the hydrologic cycle; salt water is boiled and the dissolved salts remain behind while the fresh water is evaporated off and collected. Multi-stage flash distillation (MSF) (Figure 1) and multiple effect distillation (MED) (Figure 2) are both widely used industrial desalination technologies.
Figure 1. The distillation process of multi-stage flash distillation.
To reduce the heat required for desalination, MSF distillation is accomplished in low-pressure environments; as the pressure decreases, the boiling point of water decreases. In MSF distillation, seawater is heated in a brine heater before traveling through consecutive stages; each of these stages has a lower pressure than the previous one (Figure 1).   At each stage, the introduction of heated water to a low-pressure environment causes the water to boil rapidly. About 15% of the water in each stage flashes into steam and the rest cools to below boiling before traveling to the next stage and repeating the process. The steam is condensed on tubes of cool feed water that run through each stage.
MED also uses low-pressure vessels to achieve multiple boiling events. In MED, seawater is preheated in tubes before entering the first effect where it is sprayed onto evaporator tubes and transformed into vapor. Spraying seawater directly on the tubes allows more efficient heat transfer than in MSF distillation.  The evaporator tubes are heated by steam from a boiler that is repeatedly vaporized and condensed. The seawater that does not vaporize is transferred to the next lower pressure effect.
Figure 2. The desalination process of multi-effect distillation. 
The two main challenges of distillation techniques are high energy loads and large waste production. Two types of energy are necessary in distillation plants, thermal and electrical. Thermal energy is introduced when the feed water is heated for evaporation and electrical energy is used for all other plant processes. MSF distillation uses 4-6 kilowatt-hour per cubic meter (kWh/m3) of freshwater produced in electrical energy and 50-110 kWh/m3 of thermal energy, and MED uses 1.5-2.5 kWh/m3 electrical and 60-110 kWh/m3 thermal.  In comparison, the membrane process of reverse osmosis only uses 3-5.5 kWh/m3 of electrical energy and no thermal energy. Freshwater from non-desalinated sources (i.e. river or lake) uses 0.2 kWh/m3 or less total energy.  In many cases, desalination plants are built next to power plants to supply this energy, such as the Diablo Canyon desalination and nuclear power plants in San Luis Obispo, CA.  Many distillation plants could be altered to utilize renewable energy sources, such as geothermal and solar, thus reducing the environmental impact of desalination. 
Countries in the Arabian Gulf rely heavily on desalination for daily water needs, producing over 5 million cubic meters per day. Through this process, brine that contains 2.5 times the concentration of seawater and contaminated with anti-corrosion reagents is produced at a rate of 3 million cubic meters per day and dumped into the ocean.   Eventually the salt will be diluted in the vast ocean, but dumping high concentrations of salt and chemical contaminants is harmful to marine life near the shore where the water is shallow and even a subtle change in salinity upsets the balance. Researchers are finding new ways to utilize and minimize brine by extracting the minerals and targeting Zero Liquid Discharge.  The brine contains valuable metals such as lithium, rubidium, cesium, and uranium that could be recovered using selective membranes in reverse osmosis. Brine mining is more favorable than hard rock mining from an environmental standpoint, but membrane technology is still in its infancy and the cost of high grade ores will have to surge before the energy return of brine mining is worth the energy investment. 
Currently, there is one desalination plant at the Salton Sea, which uses MED with a vertical tube evaporator (VTE). This plant was funded by the Bureau of Reclamation and is in the testing phase for geothermal distillation of Salton Sea water. An additional pilot project between CalEnergy, Sephton Technologies, and the Imperial Irrigation District plans to test Salinity Gradient Solar Ponds (SGSP) to power the VTE plant, which will supply freshwater to a marine habitat and discharge brine to the SGSP.   Due to the high density of saltwater, in a SGSP a vertical salinity gradient naturally forms with the top layer having the lowest salt content and bottom layer having the highest salt content. There is minimal convective movement between salt layers so the SGSP becomes a heat sink as it absorbs solar energy that can later be converted to electrical energy to power the VTE plant. This project could prove the efficacy of zero liquid discharge and renewable energy powered desalination.
Figure 3. Salton Seawater Marine Habitat schematic for the SSRREI. 
This SGSP pilot project is one of many projects within the SSRREI proposal for restoring the Salton Sea. The Salton Seawater Marine Habitat Demonstration has a dual purpose of establishing brackish habitats for wildlife and increased surface coverage for dust suppression. On a larger scale, solar ponds and habitats could be built along the shoreline where the water recedes to capture and store thermal energy that can later power desalination or add solar energy to the grid. Upon approval of the SSRREI proposal, we hope to see many innovative projects like this get underway at the Sea.
Written by Melissa Morgan
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