The most common technology currently used to remove salt from seawater or brackish water from underground reservoirs is reverse osmosis. This process is considered costly because of the materials and electricity used: a high-pressure pump forces the water through a polymer membrane, which retains the salt. One alternative for desalination that uses less energy is capacitive deionization, which uses activated carbon with nanometric pores (1 nanometer is a millionth of a millimeter) to remove salt from water. Researchers from the Department of Chemical Engineering at the Federal University of São Carlos (UFSCAR) developed new types of carbon specifically suited for this application. “It is similar to the carbon used in water filters, but the number and size of pores provide more area to store ions and molecules,” explains chemical engineer Luís Augusto Martins Ruotolo, professor at UFSCAR.
Activated carbon can be made from different materials such as wood, sugarcane bagasse, coconut husks, and polymers. At UFSCAR, the carbon was made by heating polyaniline, an electricity-conducting polymer, at 800 degrees Celsius (°C) under conditions that remove volatile organic matter, resulting in a carbon-rich electrode. The researchers generated more efficient activated carbon with the capacity to retain more molecules or ions on its surface. Ruotolo and doctoral student Rafael Linzmeyer Zornitta, who work at the Environmental Technologies Laboratory (LATEA), inserted two of these electrodes into an electrochemical cell composed of acrylic plates and rubber gaskets. They were positioned on opposite sides of the cell (see infographic) and separated by a channel draining the water containing salt (sodium chloride) that will be treated.
To desalinate this water, a 1.2 volt (V) electrical current was applied to the electrochemical cell; this voltage is lower than the charge contained in a AA battery (1.5 V). One electrode was polarized with a negative charge, and the other with a positive charge. When the saltwater enters the cell, it passes between the electrodes. The sodium ions (Na+), which have a positive charge, are attracted to and held in the negative electrode, and chloride (Cl-) moves to the positive pole. When the electrodes become saturated with these elements the polarity is reversed, expelling the retained material, which is removed from the cell in a backwash process. In the future, the researchers intend to build a prototype powered by a solar panel.
Activated carbon that adsorbs salt is already available for sale, but is not suitable for capacitive deionization processes because it has small areas that retain salt ions. The carbon developed at LATEA has areas which hold six times more chemicals than the carbon currently available on the market. A patent application for this invention has been filed with the Brazilian National Institute of Industrial Property (INPI) by UFSCAR’s Innovation Agency. Innovation also involves the other possibilities for using this material, such as treatment of industrial effluents and extraction of other salts from water. “In a boiler that generates steam, for example, the water must be clean enough so that elements such as calcium, magnesium, and iron do not cause buildup in the pipes,” says Ruotolo. In this study, he received support from partners at the Madrid Institute for Advanced Studies (IMDEA-Energy) and the University of Málaga, both in Spain, and at the University of Wisconsin at Madison.
Advantages and disadvantages
Once the first stage of the project was complete, Zornitta moved to the Leibniz New Materials Institute in Germany, where he is part of a team led by professor Volker Presser that develops technology for capacitive deionization. He took with him in his luggage activated carbon made of lignin from sugarcane, supplied by the Brazilian Bioethanol Science and Technology Laboratory (CTBE) in Campinas, São Paulo. Lignin is part of the residue left over after second-generation ethanol production. It is being used in Germany to produce activated carbon for studies on capacitive deionization. “Second-generation [production] is expected to advance in the coming years in Brazil and large quantities of lignin will be left over in the refineries,” says Ruotolo.
The main advantage of capacitive deionization over reverse osmosis, the dominant technique in the desalination market, is low operating cost, since it uses less water pressure and less electricity. But deionization is not yet being used to desalinate seawater; this technology cannot handle volumes of salt exceeding 10 grams (g) per liter (l), and seawater contains 35 g/l. “Today there is a great mobilization of the scientific community in the search for new materials or operating strategies that make desalination of seawater via capacitive deionization feasible,” says Ruotolo. It will be a long, hard road to change this scenario, since reverse osmosis extracts salt so completely that the resulting water is in its distilled form. To make it potable, small quantities of mineral salts must be added back to the water.
Although the technology needs to be improved, capacitive deionization is already being used commercially by a Dutch company, Voltea, with investment backing from Unilever Ventures and the Environmental Technologies Fund, a British venture capital group. The company has been selling desalination systems since 2009, although these are not suitable for seawater. The technology used by Voltea is based on applying voltage between two porous carbon electrodes which are parallel within a cell. The carbon electrodes are built layer by layer, in micrometer-thick films; these devices are sized and positioned depending on the volume of water that needs to be desalinated. Voltea’s technology is used in desalinating tap water for industrial use and agricultural irrigation. Its major advantage is low energy consumption. Even with these limitations, Voltea was recognized as one of the 21 Pioneering Technologies of 2013 at the World Economic Forum, and won an award at the 2010 Global Water Summit.
The expanding potential applications of capacitive deionization still require technological development, but it may be just a matter of time. “Reverse osmosis started to be commercially applied in 1965, but only in the 1980s did it begin to be widely used in desalination. It was a technological ripening, with new solutions that were appearing,” says chemical engineer Emilio Gabbrielli, an Italian living in Brazil who chaired the International Desalination Association (IDA) until October of this year. Today he is the director of Overseas Business Development in the Water Division of Toray, a Japanese company that manufactures reverse osmosis filters and membranes, among other products. He says that, like deionization, there are other experimental technologies which in the future may replace reverse osmosis at lower costs.
Gabbrielli estimates that the amount of water desalinated each day around the world is 100 million cubic meters (1 m3, the equivalent of one thousand liters), 20 times the average flow of London’s River Thames. This process takes place in approximately 19,000 desalination plants. Today between 300 and 400 million people use desalinated water, especially in countries such as Israel, Saudi Arabia, Singapore, Australia, and Spain. “Renewable energy such as solar and wind power are increasingly sought after to power desalination plants, and countries like Australia and Saudi Arabia are leading the way for this energy option.” As for the price of this process, Gabbrielli states that a cubic meter of desalinated water costs between US$0.60 and US$1.50 in larger-capacity plants, depending on the region where it is used. In comparison, water treated or transported by water supply companies in Brazil costs between R$0.10/m3 and R$0.20/m3, but in many cases where water must be transported by road, the costs may be similar to desalination.
For Gabbrielli, desalination will gain strength in Brazil in the near future, because populations require greater volumes of water and this option will become important during periods of drought. He predicts that more widespread knowledge of the technology and falling prices for desalination equipment could lead large Brazilian cities located on the coast to adopt this process. “But the reverse osmosis technology also applies to water reuse. I imagine that within 10 to 20 years, reused water will even be used in traditional water supplies,” says the chemical engineer.
Brazil’s broadest experience with desalination has taken place in the northeastern sertão badlands, through the government programs Água Boa (Good Water; 1998–2003) and Água Doce [Fresh Water; 2003–2010), both of which were Ministry of Environment initiatives that brought desalination equipment to isolated communities of the semiarid region. In this region, the groundwater is brackish because it comes into contact with crystalline rocks. Today, 3,000 desalination systems serve approximately 200,000 people.
The two government programs were initially coordinated by chemical engineer Kepler França, a professor at the Desalination Reference Laboratory (LABDES) of the Federal University of Campina Grande (UFCG) in Paraíba. He also helped install desalination equipment on the archipelago of Fernando de Noronha in 1998. The system installed there accounts for 40% of consumption on the islands, while the rest comes from rainwater stored in cisterns and subsequently treated.
The next large Brazilian center to rely on desalinated seawater is expected to be Fortaleza, Ceará. In August 2017, The Ceará Water and Sewage Corporation (CAGECE) opened a bid process to select two companies which will conduct the studies for a future plant. The Spanish companies GS Inima and Acciona were recently chosen, and will deliver the plans in 150 days. The project, which will cost R$500 million, is scheduled to come online in 2020 and is expected to supply 12% of drinking water for the Fortaleza metropolitan area.
1. Desalination using capacitive deionization: Development of new electrodes and optimization of the process (No. 15/16107-4); Grant Mechanism Regular Research Grant; Principal Investigator Luís Augusto Martins Ruotolo (UFSCAR); Investment R$228,804.27.
2. Desalination using capacitive deionization: Development of electrodes and optimization of the process (No 15/26593-3); Grant Mechanism PhD Grant; Principal Investigator Luís Augusto Martins Ruotolo (UFSCAR); Scholarship Beneficiary Rafael Linzmeyer Zornitta; Investment R$41,033.51; R$102,303.70 (Internship Grant for Research Abroad).
ZORNITTA, R. L.; RUOTOLO, A. M. Simultaneous analysis of electrosorption capacity and kinetics for CDI desalination using different electrode configurations. Chemical Engineering Journal. Online. Sept. 11, 2017.