Across much of the planet in places like the Middle East, freshwater, being as scarce as it is, is something of a luxury for many people. Desalinating seawater helps people and communities cope.
The trouble is that desalination routinely comes with considerable environmental impacts. There are some 16,000 desalination plants around the planet and together they pump out 142 million cubic meters of salty brine every day to produce 95 million cubic meters of fresh water, according to a recent UN-sponsored study.
For every liter of freshwater produced, desalination plants create 1.5 liters of extra-salty brine on average. Many of these plants produce exceeding amounts of salty wastewater, which is also rich in toxic byproducts. In a year the overall volume amounts to 51.8 billion cubic meters of brine, which would be enough to cover the entire state of Florida in the U.S. under 30.5 cm of extra-salty water.
The colossal amounts of brine thus produced can inflict grave harm on marine ecosystems in coastal areas.
“Brine underflows deplete dissolved oxygen in the receiving waters,” explained Edward Jones, an expert at Wageningen University in the Netherlands who worked on the study.
“High salinity and reduced dissolved oxygen levels can have profound impacts on benthic organisms, which can translate into ecological effects observable throughout the food chain,” he added.
Then there is this: desalination is an energy-intensive process and its carbon footprint is considerable.
Yet that doesn’t have to be the case. Researchers at MIT in the United States, working in collaboration with colleagues in China, have devised a new solar-powered desalination system could provide plenty of drinking water with no extra carbon.
“The device is essentially a multilayer solar still with a set of evaporating and condensing components like those used to distill liquor. It uses flat panels to absorb heat and then transfer that heat to a layer of water so that it begins to evaporate,” they explain. “The vapor then condenses on the next panel. That water gets collected, while the heat from the vapor condensation gets passed to the next layer.”
In addition, the process works without the accumulation of salt or concentrated brine. “In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater,” the researchers say.
During a trial the device managed to generate 5.78 liters of pure water per square meter. This is more than twice the amount produced by other passive solar-powered desalination systems. The new solar-powered desalination device could be deployed in arid off-grid coastal areas to provide an efficient, low-cost water source.
The pilot unit was constructed from cheap, readily available materials, including black solar absorbers and paper towels. In similar passive solar desalination systems previously, the solar absorber material and the wicking material are a single component, which makes them expansive to produce. “We’ve been able to decouple these two,” a researcher on the new project says.
With more desalination stages and further optimization, the new passive solar system could reach efficiency levels of up to 800%, according to the researchers. The new system could especially come in handy in sun-drenched countries in the Middle Ease like Saudi Arabia, Kuwait and the United Arab Emirates, which produce well over half of brine on the planet.
