Desalination is a process that removes salt and other minerals from sea water, providing fresh water for human consumption and agriculture. While desalinization may offer a solution to water scarcity problems in arid regions of the world, the process is expensive and has significant environmental impacts. Desalinization is energy-intensive, making it costly and a potential emitter of greenhouse gases into the atmosphere. The process also can lead to changes in the local marine environment, often making it less suitable for supporting human life.
Because of its energy requirements, desalinization is a very expensive way to provide for fresh water needs. From a cost perspective, transporting fresh water over long distances is more economical. Thermal processes are particularly energy-intensive. While thermal desalinization is viable in many oil-rich countries in the Middle East, rising energy costs may make thermal processes less viable. Membrane desalinization, for example through reverse osmosis, requires a quarter of the energy, and costs half as much as the thermal alternatives.
Along with the cost, greenhouse gas emissions and their effects on the climate are also a concern. As of 2011, nearly all of the energy used for desalinization comes from fossil fuels. An environmental impact assessment of a proposed desalinization plant in Sydney, Australia, found that the plant would produce emissions equal to those of 53,000 cars, the Sydney Morning Herald reported in 2004. Emissions can be significantly lower if a desalinization plant uses alternative energy sources or waste heat from power plants.
Desalinization plants must gather salt water from the ocean in the local area. The intake process normally gathers a large volume of marine life along with the water, especially small creatures, plankton, fish eggs and larvae. Open intake systems are particularly destructive, and are often seen as the only option for large-volume plants. Many localities insist on maintaining low intake mortality rates, resulting in considerable efforts to improve intake technologies. Vertical beach wells provide one solution that greatly reduces mortality.
Desalinization results in a fresh water flow, and a brine by-product flow. Disposal of the brine flow represents one of desalinization's biggest problems. The brine flow contains an extremely high concentration of salt and minerals, leading to buildup near the plant and salt deposits on the ocean floor. The outflow can also alter the temperature and other aspects of the chemical composition of the surrounding ocean water. Such changes are invariably detrimental to marine life.