Thermal processes, such as multi-flash and multi-effect distillation, heat sea water into steam, which is diverted, cooled and condensed into fresh water. The processes occur through multiple stages, gradually reducing the original sea water to a heavily saline brine stream. Thermal desalinization is much more energy intensive than membrane technologies, and also more costly. Energy requirements and greenhouse gas emissions can be reduced through using waste heat from power plants, or alternative energy sources. However, as of 2011, nearly all energy used in desalinization is derived from fossil fuels.
Membrane desalinization is the most popular form of desalinization in the United States. With membrane technologies such as reverse osmosis, sea water is forced through a semi-permeable membrane, which keeps salts and minerals on one side, while allowing fresh water to pass through. Membrane desalinization generally results in equal parts fresh water and brine byproduct. As of 2011, membrane technologies were used in 30 to 40 percent of desalinization plants.
Even with membrane technologies, it can be many times more expensive to desalinate sea water than to transport fresh water over great distances. New technologies and a greater emphasis on membrane processes are bringing costs and energy requirements down. At the same time, thermal desalination remains a growing industry, especially in resource-rich countries in the Middle East. While energy intensive processes entail high cost, they also result in significant greenhouse gas emissions, making desalinization a contributor to global warming.
In addition to energy use and greenhouse gas emissions, desalinization has numerous impacts on local ecosystems. Desalinization plants must gather sea water from the local area. The intake process often damages local marine life, particularly larvae, fish eggs, plankton and other small creatures. After desalinization, the brine flow is normally sent back into the local marine environment, raising saline levels, and changing ocean chemistry and temperature. These changes can be highly detrimental to the local ecosystem.