Reverse osmosis and distillation are the two main processes used worldwide to desalinate seawater. They are subject to criticism due to their high energy consumption. Several projects are being developed to improve and/or decarbonize these two processes.
In a context of climate change where fresh water resources are becoming scarce, the water desalination of sea appears more and more as a leading solution. The majority of Gulf countries today depend largely on desalinated water for the consumption of their inhabitants and the construction of new desalination plants is increasing in all four corners of the planet. The seawater resource seems inexhaustible since it covers three-quarters of the surface of our planet. Despite everything, desalination is the subject of criticism due to its high energy consumption. Several technologies are being developed to improve and/or decarbonize the two main processes used around the world.
The one most used is called thereverse osmosis. It consists of applying sufficient pressure to salt water to make it pass through a semi-permeable membrane so that only the water molecules pass through it and thus produce fresh water. Research work is attempting to improve this device by, for example, adding reagents to these membranes, composed of polymer matrices, with the aim of improving their permeability. Except that this generally leads to a degradation of their selectivity. Clearly, they let some of the sodium chloride escape.
In Montpellier, the European Membrane Institute (IEM) may have found a solution by developing a biomimetic membrane. She is directly inspired by a protein present in the kingdom of life, called aquaporin, and which has a pore permeable only to water molecules. “By imitating this protein, we created artificial water channels that we graft onto polymer membranes used by manufacturers, explains Mihail Barboiu, research director at CNRS. We thus create a hybrid membrane which makes it possible to increase the permeability of reverse osmosis by a factor of 3 to 6, while maintaining its selectivity. Our process can be used to reduce the pressure needed and therefore the energy needed to pass water through the membrane. It is also possible to use the same pressure to produce more fresh water and thus meet greater demand. »
This process has been validated in the laboratory on industrial pilots. At the same time, a pilot machine was developed to manufacture large areas of hybrid membranes, up to around fifty m² per day. Scientists are currently looking for industrial groups to test their innovation on a large scale.
Desalinating sea water using renewable energies
In terms of reverse osmosis, the Canadian company Oneka has developed technology to exploit this process using only wave energy. The device is based on buoys attached to anchors placed on the ocean floor. THE oscillating wave motion is used to operate the system. When the buoy descends into the trough of a wave, the seawater is sucked up through a strainer and then filtered. And when it rises, the seawater is pressurized in a pump, then passes through a pressure and flow optimization system before being pumped to reverse osmosis membranes.
Several types of buoys are marketed by this company and those with the largest capacity can produce 50 m3 per day. Even if it is possible to install several buoys to supply the same site, this technology is mainly intended to meet local needs. For comparison, Saudi Arabia’s desalination capacities are estimated at more than 5 million m3 per day. Still in terms of reverse osmosis, and this time in France, the company Osmosun has developed a solution to implement this technology by exploiting solar energy. THE Project Kori Odyssey aims to make this process accessible in order to improve the conditions of access to drinking water for isolated populations in the Pacific Islands.
Distillation, also called heat treatment, is the second most used process in the world. It involves evaporating seawater so that only the water molecules escape, while the dissolved salts and all other substances contained in the seawater remain as a deposit. To obtain consumable fresh water, it is then sufficient to condense the water vapor. According to one study by Ifri (French Institute of International Relations)this process is more energy intensive, because it takes more than 7 kWh (kilowatt-hour) to desalinate a m3 of water, compared to an average of between 2.5 and 3 kWh for reverse osmosis.
Based in the United Arab Emirates, the Manhat company has developed technology to completely decarbonize this process. To do this, she made a still powered by solar energy and which works thanks to a greenhouse that floats on the surface of the ocean. The principle is simple: the sun has the effect of heating and evaporate water under the greenhouse, which separates the salt crystals, which are then released into the sea. When temperatures cool, the water condenses into fresh water and is then collected. The company announces that it has reached a technological maturity of 6 on the TRL (Technology readiness level) scale, which has 9. This technology is intended to supply water to floating plant farms installed in the coastal areas of the United Arab Emirates.