Desalinating ocean water may be the solution for the water crisis; however, the technology needs to overcome cost and energy demand challenges. Learn how the process unfolds and what’s done in Brazil and worldwide
Global water crisis is a reality and forecasts are far from hopeful: the UN says that “by 2030, the planet will experience a 40% deficit in water, unless the management of such precious resource is drastically improved.” Increase in water demand is mostly due to population growth combined with industrial and agricultural progress, which develops in the same pace water resources deplete.
Water itself is far from being scarce – about 70% of the Earth’s surface is covered by H2O; moreover, we can find large amounts of liquid in aquifers and groundwater reservoirs. Potable water is scarce for human consumption – only 0.92% of the planet’s water is available for our use and to supply the industry and agriculture, who are responsible for 70% of consumption of the Earth’s water.
The solution looks simple: why don’t we turn this massive volume of salt and unfit water for consumption into water potable? The answer is just as simple: purifying seawater is costly.
But there’s reason for hope. In 2018, desalination costs had the lowest decline of all time. For the first time, says the International Desalination Association (IDA), desalinating one cubic meter of water – equivalent to 1000 liters – costs less than US$ 0.50.
“Desalination and water reuse: non-conventional, environmentally sound water supply solutions are alligned with the circular water economy and offer solutions to water scarcity,” said Shannon McCarthy, IDA Secretary General.
“The process isn’t complex. Besides removing salt from water, it also removes impurities such as bacteria, virus and fungi,” explains Kepler Borges França, professor and researcher of chemical engineering at the Federal University of Campina Grande (UFCG) and former coordinator of the program Água Doce (Freshwater Program), the most comprehensive water desalination project in Brazil.
How is water desalination process done?
There are plenty of techniques to purify saline water (whose salinity is equivalent to or higher than 30%) or brackish water (with salinity between 0.5% and 30%), and two of them stand out as the most popular: thermal distillation method and membrane osmosis method (also called reverse osmosis).
What is thermal distillation?
Thermal distillation is relatively simple, but requires large amounts of energy. At desalination plants with thermal processes, a source of energy – solar, at most sustainable plants – heats the water and salt at temperatures from 80 °C to 100 °C. Next, the water evaporates and separates from salt solid particles. The vapor is then decanted and filtered.
This method is the oldest and prevailed on the early days of desalination. From the 60s to late 80s, nearly 85% of all purified seawater underwent this process. Wired magazine currently informs that 69% of the world’s desalinated water is produced by the reverse osmosis system.
What is reverse osmosis?
Reverse osmosis has more steps and is a little more complex to be described, but also easy to understand. It starts by removing large particles of impurities from brackish or saline water, such as sand and mud. The process requires water pumping through a series of filters that perform this work, leaving the water clean, but still salty.
The liquid is then moved for desalination. This process occurs by saltwater passing through a semipermeable membrane, which allows the passage of water, but not salt. This process requires water pumping with pressure, with relative energy cost, but also ensures more efficiency in the production of potable water from seawater.
Reverse osmosis benefits
Compared to thermal desalination, osmosis has three great advantages. The most significant one is low energy consumption. Thermal processes require about 9 kw/h to desalinate 1,000 liters of water. Modern membrane osmosis processes can desalinate the same volume with 3 kw/h for seawater and 1.5 kw/h for brackish water. This represents at least three-fold energy efficiency – and may reach six-fold.
Thermal desalination is also less efficient in terms of production: on average, thermal processes results in 25% of potable water and 75% of brine – term used to denominate “water and leftovers.” Yielding for membrane osmosis process is 50% to 50%. “Distillation [thermal] costs are 10 to 15 times higher than membrane techniques. With reverse osmosis, costs are as low as BRL 1 to desalinate one thousand liters of brackish water and between BRL 1.50 and BRL 2 for seawater,” informs UFCG professor.
Worse: in thermal processes, all water salts are removed, which isn’t good. “Mineral salts in water are much needed for human consumption, and that is in accordance with the WHO (World Health Organization). Thermal processes remove all salts and, if directed at potable water, require the addition of a certain amount of salts,” adds Kepler.
Recycling water in the desalination process
By the end of 2018, there were more than 20 thousand desalination plants worldwide. Altogether, production at these units may reach 105 million cubic meters of water a day. This means 105 billion liters of water. Enough to serve the water needs of 950 million people a day (it’s worth noting that not all desalinated water is meant for human consumption; it’s also used in the agriculture, industry, etc.) And the outlook is that the production of potable water by this method grows 9% a year by 2022.
But the growth of this industry comes with a caveat: the generation of waste by the desalination process grows at the same rate. Adequate production of potable water is estimated in 95 million cubic meters a day; production of brine is 141.5 million cubic meters a day. And 55% of this is concentrated in only four countries: Saudi Arabia, United Arab Emirates, Kuwait, and Qatar.
“The antifoulants used in the process, particularly in the pretreatment process of the source water (filtering before osmosis), accumulate and discharge to the environment in concentrations that can potentially have damaging effects on the ecosystems,” warns Edward Jones, at Wageningen University (the Netherlands), to Wired.
On the other hand, this concentrate loaded with salts, if well handled, may significantly help in agriculture, especially in regions with a scarcity of resources. In 2003, the program Água Doce, related to the Ministry of the Environment – now Regional Development Ministry -, developed a system to reuse this waste in the Brazilian semi-arid region in conjunction with Embrapa (Brazilian Agricultural Research Corporation).
In this system, the concentrate is deposited in two large tanks (330 cubic meters), used as a sharing system, to raise tilapia – a type of fish that grows well under such conditions. A third tank receives the remaining water and brine, and organic matter is added. This mixture is used to water salinity-tolerant plants.
“We understand that saline water is an alternative to increase efficiency of production systems in the semi-arid region. It’s not a problem, but an alternative solution for families to increase production,” says Gherman Araújo, researcher at Embrapa Semiárido. However, ponders Araújo, “we need to use it with technical knowledge, rationally and respecting soil, water, climate, plant and animal features.”
Today, besides pisciculture, the system is used to raise halophyte plants, microalgae and hydroponic systems, which generate food and income to communities – in addition to domestic uses for cleaning purposes.
Water desalination in Brazil: what do we do?
Since 1997, Brazil has a program dedicated to the desalination method focused, especially, on the northern region. Dubbed Programa Água Boa (Good Water Program), it was redesigned and expanded in 2004 as PAD (Freshwater Program), involving social participation, local communal management and environmental protection principles.
PAD acts in the Brazilian semi-arid, covering an area of 969,589.40 km² (11% of Brazilian territory), involving all nine states in the Northeast, totalizing a population of 21 million people – with 9 million inhabitants in rural zones. All this territory has water shortage due to rain temporal and spatial variability, high evaporation rates and a predominance of crystalline rocks – all this together makes it impossible for humans to access untreated potable water.
“The program aims to bring potable water to communities without it, aiming the groundwater potential of the Northeast, where most water is brine,” explains Kepler França, who coordinated PAD nationwide. The program goal is to implement 1,357 desalination systems in 170 municipalities, reaching more than 500 thousand people.
Off the continent, PAD also works to turn seawater into potable water. Surrounded by ocean on all sides, the Fernando de Noronha Island is supplied by two sources: 60% from Xaréu reservoir (rainwater) and 40% from desalination. Desalination is expected to grow. In 2018, an investment of BRL 22 million was approved to expand the supply of desalinated water by 50%, from 48 thousand liters of water per hour to 72 thousand liters of water per hour.
“This program came to stay and accomplished its goal,” completes Kepler França.
Content published in April 4, 2019