By Nils Zimmermann (VT / ELM)
This summer, the Spanish company Abengoa announced the signing of a contract with a Moroccan government agency to build the world's largest renewable energy-powered seawater desalination plant.
Abengoa will be in charge of the engineering, construction, operation and maintenance of the plant for 27 years. This will generate 275,000 cubic meters (m?) Of desalinated seawater per day in order to produce 150,000 m? of drinking water and 125,000 m? for the irrigation of some 13,600 hectares of agricultural land near Agadir, a city on the western coast of Morocco.
According to the Moroccan government, the electrical energy to supply the plant will come through high-voltage cables from the Noor Ouarzazate solar plant, almost 400 kilometers east of Agadir.
Desalination in Spain
Spain is the fifth country in number of desalination plants in the world with a total of 900 plants that have a capacity of 1.45 million cubic meters per day.
The first desalination plant in Spain (and Europe) was built in 1965 in Lanzarote with evaporation technology, through solar energy, which is hardly used today, and which has been replaced by reverse osmosis. In Las Palmas de Gran Canaria the first desalination plant in Spain was built with the reverse osmosis method in 1971.  It is currently managed by Emalsa. [Citation needed] Among the most recent construction, the one in El Prat de Llobregat stands out in the highly populated metropolitan region of Barcelona, affected by intermittent droughts and relatively polluted surface waters of the Llobregat basin.
Desalination in numbers
Currently, less than one percent of the world's population relies on seawater desalination for its daily supply of drinking water. There are around 21,000 operating desalination plants, most of them in the Middle East.
While the Agadir plant will draw water from the ocean to turn it into drinking water, "only about half of the desalination plants in the world do this. The rest treat water from impure sources, such as polluted rivers or brackish groundwater." says Klemens Schwarzer of the Jülich Solar Institute.
Can the supply of fresh water be increased?
In theory, the potential to increase freshwater supplies with the help of desalination technologies is enormous. About 97.5 percent of the 1,385 million cubic kilometers of water on Earth is salt water. The remaining 2.5 percent is fresh water. However, about 90 percent of that fresh water is found in the ice sheets of Antarctica, Greenland, and other glaciers. Humanity, on the other hand, only consumes a small fraction of the remaining fresh water.
Could seawater desalination plants turn deserts and semi-arid lands into abundant green plantations? Theoretically yes, but in practice it is not easy "since it takes a lot of energy and equipment to obtain fresh water from sea water", explains Schwarzer, adding that this process is very expensive.
Quality of produced water
The osmotized water or the permeate of the reverse osmosis modules must be conditioned to meet certain high quality characteristics, since the water produced has an acid pH and a low carbonate content, which makes it a highly corrosive product. . This requires its preparation before distribution and consumption. The pH is adjusted with calcium carbonate to a value of 7.7. Additionally, if required by municipal regulations for the use of drinking water, sodium fluoride and hypochlorite are also added.
There are two types of processes for converting seawater into drinking water: thermal desalination and "reverse osmosis" (RO) desalination. Both require a lot of energy.
Thermal desalination consists of the evaporation of water in order to separate salts and other impurities. Reverse osmosis requires high-pressure pumps that filter water through thin membranes that only allow water molecules to pass through. It is a complex filtration process. The Agadir plant will use OI technology.
According to Schwarzer, more pure water is usually obtained through thermal desalination than with RO.
On average, human beings directly or indirectly consume 3.8 cubic meters of water per day, this includes the water they drink, the water they use for personal hygiene and at home, as well as the water used in agriculture and industry. This means that the Agadir plant will produce enough water to supply the needs of some 72,500 average global citizens.
Since there are currently about 7.5 billion people in the world, it would take some 104,000 desalination plants the size of the one in Agadir to cover the global supply of fresh water.
It all depends on the location of the plant
In oil- or gas-rich countries, such as Saudi Arabia, most large desalination plants are powered with waste heat, a by-product of oil-fired thermoelectric plants.
Solar desalination plants are especially relevant in regions where both fresh water and fossil fuels are scarce. Schwarzer points out that in sub-Saharan Africa there is a huge need for clean water. At least near the coast, solar desalination might make sense. However, desalination equipment is too expensive for most Africans. Hence, Klemens Schwarzer advocates partially subsidizing this technology.