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Water engineering

Removing salt from water

Ceará plans to build Brazil’s first large-scale desalination facility as new and more cost-effective technologies emerge in Brazil and abroad

James Grellier / Wikimedia Commons A reverse osmosis desalination plant in Barcelona, SpainJames Grellier / Wikimedia Commons

Ceará, one of the driest and most water-stressed states in Brazil, is preparing to build what will become the largest desalination plant in the country. Once completed, the project, which began with basic design in 2016, will provide increased water security to the state capital, Fortaleza, and surrounding municipalities in a metropolitan area with a population of 4 million people. The project owner, CAGECE, the state water utility, expects to issue the call for bids for construction within the first half of the year. The technical, operational, environmental, and economic studies that are being used to inform the call for bids were prepared by a consortium led by South Korean GS Group.

Construction of the facility, which should start operation in 2022, will be through a public-private partnership (PPP) arrangement, the particulars of which will be outlined in the call for bids, explains CAGECE’s sustainability director, Ronner Gondim. Mucuripe beach, near the port of Fortaleza, is being considered to host the project. The plant will be designed to produce 1 cubic meter (m3) (or 1,000 liters) of water per second—the Metropolitan Area of Fortaleza consumes 8 m3 per second. “Water supply capacity will increase by 12%, enough for 720,000 people,” says Gondim. The cost of the project is estimated to be approximately R$480 million.

“The successful bidder will be responsible for construction and will acquire the right to operate the plant for 30 years,” he explains. The Government of Ceará will stipulate a maximum rate per liter of desalinated water and will award the contract to the bidder offering the lowest rate. CAGECE currently charges an average of R$3 per cubic meter of treated water. The global average cost of desalinated water starts at US$2 (approximately R$8) per cubic meter, depending on the process.

The new plant will likely use the reverse osmosis process for desalination, although this decision will be left to the successful bidder. Reverse osmosis is currently the cheapest known method of desalination. In this process, a high-pressure pump forces water across polymer membranes with minute pores, which retain the salts. The energy expenditure of this process is as high as 4 kilowatt-hours (kWh) per m3 of purified water. The technology is used in 84% ​​of desalination operations globally. Israel, a pioneer in desalination by reverse osmosis, processes 600 million m3 of sea water into drinking water each year, providing water supply to 6.5 million people, or about 75% of the Israeli population.

There are currently 15,900 desalination plants in operation worldwide, with a capacity to purify about 95 million m3 of water per day, according to a study published in December 2018 in Science of the Total Environment. The majority of plants are in the Middle East, North Africa, the US, China, and Australia. In Europe, Spain has the largest desalination capacity. The plant in Barcelona, on the Mediterranean coast, with a capacity to process 2.3 m3 of water per second, is one of the largest on the continent.

Marcelino Lourenço Ribeiro Neto / Embrapa Semiárido An irrigation system in northeastern Brazil using effluent from the desalination processMarcelino Lourenço Ribeiro Neto / Embrapa Semiárido

Cost is the primary constraint
Saudi Arabia and the United Arab Emirates have the largest desalinization capacity globally. The world’s biggest plant, in Ras Al-Khair, Saudi Arabia, produces 1 million m3 of water per day (11.5 m3 per second). The most commonly used method there is thermal desalination, in which saltwater is stored in heated tanks. When it evaporates, steam builds up in the upper part of the tank and then condenses into pure water without salts.

One of the drawbacks of thermal desalination is its high energy requirement, to the tune of 15 kWh/m3. Most of the thermal energy used in the process comes from fossil fuels, such as oil and gas, which are abundant in the region. Thermal desalination costs about three times as much as reverse osmosis, at about US$6 per m3 of treated water.

With high energy consumption remaining the biggest barrier to large-scale adoption of desalination, especially in developing nations, researchers in several countries are working to develop new, more efficient, and more cost-effective technologies for desalination. One of the new technologies that is currently being explored is capacitive deionization, an electrochemical process that retains the ions contained in the water as they pass between two electrically charged porous carbon electrodes. “Capacitive deionization is still a young technology, but has shown promise. For brackish water, it is even more cost-effective than reverse osmosis,” says chemical engineer Luís Augusto Martins Ruotolo of the Department of Chemical Engineering at the Federal University of São Carlos (UFSCar). Globally, the leading institutions advancing this method are Stanford University, the Lawrence Livermore National Laboratory, the Massachusetts Institute of Technology (MIT), and the European Center of Excellence for Sustainable Water Technology (WETSUS), in the Netherlands.

The biggest advantage of this system is that high pressures are not required to operate the equipment, unlike in reverse osmosis, says Ruotolo. “This reduces energy requirements and operating costs,” he explains. In Brazil, Ruotolo’s group developed a capacitive deionization process using activated carbon with nanometric pores.

Another emerging strategy that has gained interest is the use of graphene oxide membranes—atom-thick sheets of carbon—in reverse osmosis systems to filter salt out of seawater or brackish water, such as the water found in wells drilled in Brazil’s semiarid region. This technology is estimated to reduce energy consumption by up to 50%—pumping saltwater across graphene membranes creates less friction compared to traditional polymer membranes.

Scientists around the world are engaged in further researching this technology. In Brazil, researchers from the State University of Paraíba (UEPB), the Federal University of Rio Grande do Norte (UFRN), and the Federal University of Paraíba (UFPB) are developing nanostructured graphene membranes for use in desalinating brackish water. In addition to cost reduction, another benefit of this method is that graphene filters need cleaning less often than reverse osmosis membrane filters do. Graphene is also immune to the effects of chlorine, a chemical used in the cleaning process that reduces the structural integrity of polymer membranes, requiring them to be replaced more frequently.

Infographic Alexandre Affonso (concept and illustration)

Desalination in Brazil
The desalination plant in Ceará will be the largest, but not the first, in Brazil. Two decades ago, a small-scale desalination system was built in the Fernando de Noronha archipelago with a capacity to produce about 720 m3 of water per day. The plant meets 40% of the archipelago’s water requirement. The remainder is sourced from harvested rainwater. Some communities in the Brazilian semiarid region also rely on desalinators to make brackish well water potable. “All of them use the reverse osmosis method,” says chemical engineer Weruska Brasileiro Ferreira of the Department of Sanitation and Environmental Engineering at UEPB.

A number of institutions have distributed and installed these systems in the region since the 1990s. In 2011, the Federal Government developed a new methodology for its Água Doce program that included technical, social, and environmental recommendations to make desalination systems more sustainable. As part of the program, 605 desalinators were installed in 174 municipalities, benefiting approximately 240,000 people.

Civil engineer Alexandre Saia, desalination coordinator at the Ministry of Regional Development, says the average cost of building desalinators in the Northeast’s sertão (badlands) is R$278,000 per unit. Each unit produces 4 m3 per day, enough for 400 people. Many of the systems in the semiarid region, however, are out of service due to a lack of municipal funding for maintenance.

Industrial engineer Antonio Santos Sánchez, of the Polytechnic School at the Federal University of Bahia (UFBA), explains that desalinator maintenance basically consists of replacing and cleaning the salt-retaining membranes, which normally have a useful life of five to nine years. “But without proper cleaning, the useful life of the system is reduced,” he says. Annual maintenance costs per unit are approximately R$18,000, according to data from the Água Doce program.

The membranes used in Brazil are manufactured by US-based Dow Chemical. Desalination plants in Israel use the same material, differing only in the amount of energy consumption and the type of membrane used in the process. Salt concentration in sea water is much higher than in semiarid wells, and requires more energy and more membranes for desalination. Each liter of sea water has more than 30,000 milligrams (mg) of salt. Well water in the semiarid region has a concentration ranging from 5,000 to 18,000 mg of salt per liter. The concentration recommended by the World Health Organization (WHO) for human consumption ranges from 250 to 500 mg per liter.

“Every 2,000 liters of desalinated brackish water generates 1,000 liters of drinking water and another 1,000 liters of brine, which is the effluent from the process,” says food scientist Ângelo Paggi Matos of the Department of Food Science and Technology at the Federal University of Santa Catarina (UFSC). Brine disposal is among the environmental concerns associated with desalination.

Plants processing seawater discharge the brine effluent back into the ocean. But for facilities operating in the semiarid region, inappropriate disposal can affect the quality of the soil, making it infertile. To prevent this from happening, the Água Doce program uses holding tanks to store the brine. Through a partnership with the Brazilian Agricultural Research Corporation (EMBRAPA), a strategy has been developed to reutilize desalination brine in tilapia fish farms and to irrigate seedlings of Caatinga (semiarid scrubland) plant species such as oldman saltbush (Atriplex nummularia), which is used as feed for goats and sheep.

Brazilian government creates center for desalination research
The institution will assess the performance of desalination systems used in Brazil

To advance the development and deployment of desalination in Brazil, in January this year the Federal Government announced the launch of a Desalination Technology Test Center (CTTD) at the National Institute for the Semiarid Region (INSA) in Campina Grande, Paraíba. The center operates under the auspices of the Brazilian Ministry of Science, Technology, Innovation, and Communications (MCTIC). The CTTD was among the 35 priority goals set for the first 100 days of President Jair Bolsonaro’s administration. Researchers at the new facility also have access to infrastructure at the Desalination Laboratory of the Federal University of Campina Grande (UFCG).

“The purpose of the center is to assess the level of technological maturity of desalination systems in operation or being developed in Brazil,” says agricultural engineer Salomão Medeiros, a director at INSA. “This will include testing equipment for efficiency and assessing the quality of desalinated water, energy expenditure, maintenance costs, and other parameters.” The government has launched a public invitation to companies interested in entering their systems for performance assessment. Seven companies—all based in Brazil—have been selected so far.

Medeiros notes that the government has yet to announce the amount of funding to be allocated to the center, which began operating in April. The initiative is also designed to provide a platform for knowledge exchange among Brazilian and Israeli researchers. “Building desalinization capabilities is strategic in any country, even in those with abundant water resources, like Brazil,” says Medeiros.

Desalination by capacitive deionization: development of new electrodes and process optimization (nº 15/16107-4); Grant Mechanism Regular Research Grant; Principal Investigator Luís Augusto Martins Ruotolo (UFSCar); Investment R$228,804.27.