As climate change fans a major drought in the American Southwest, the country is breaking some worrying records. Lake Mead’s water level, which provides water for millions of people, is at an all-time low. And in some places, the shrinking Colorado River, which irrigates some 5 million acres of farmland and quenches the thirst of more than 40 million people, is just desert and dust.
Meanwhile, as of 2018, about 80% of the country’s wastewater – including water used in agriculture, power plants and mines – is dumped back into the world, untreated and unusable, a wasted opportunity. And although today’s purification technologies, which use a process called reverse osmosis, are still the most cost-effective and energy-efficient way to treat seawater and brackish groundwater, traditional reverse osmosis can’t handle super-salty water. Could – which contain twice the water salt content of the sea. As the US water supply shrinks (and gets saltier), the country can no longer afford to dump back the saltiest sources in the world.
Now, in a new study published in desalinationMembers of the National Alliance for Water Innovation (NAWI) research consortium analyzed an emerging form of reverse osmosis called low-salt-rejection reverse osmosis. These innovative systems can also treat highly saline water. But the design is so new that it’s still theoretical.
So, to find out how these technologies might compete with other water treatment options, the NAWI research team developed a mathematical model that, with the help of a supercomputer, calculated the cost, clean water production, and efficiency of more than 130,000 possible systems. Can quickly evaluate energy consumption. Design. Their results suggest that, in many cases, low-salt-rejection reverse osmosis may be the most cost-effective option, potentially reducing the overall cost of producing clean water by up to 63%.
“The ultimate goal of this research is to conduct a thorough techno-economic evaluation of a new technology that has not yet been tested in the real world, but has the potential to enable high water-recovery desalination,” said Adam Attia, A senior engineer at the National Energy Technology Laboratory and lead author of the paper.
Although few studies have evaluated the potential cost and efficiency of low-salt-rejection reverse osmosis systems, this study provides a more comprehensive analysis of their design, operation, and performance. To better understand the potential promise of these theoretical systems, the team used a supercomputer to sift through the most optimal, cost-effective designs. Then they figure out how those designs can function in hundreds of thousands of scenarios (as opposed to just a handful).
Because low-salt-rejection reverse osmosis systems allow more salt to pass through each membrane, they require less force—and therefore less energy—to push the water through. But, if more salt can be squeezed through, the resulting water is, not surprisingly, still too salty to drink. To produce potable water, this still very salty water is recycled back past the membrane stages. Once the salt content is low enough, standard reverse osmosis can take care of the rest, producing high quality drinking water.
All of that adds to the complexity of the recycling system. So, the team needed to figure out: how many membrane phases is optimal? How many recycling loops are needed? And how much cost and energy do those loops add? To answer these questions, researchers can individually calculate how much clean water each design can produce from water with different concentrations of salt.
“It’s likely going to take a really, really long time to sort them out,” said Ethan Young, a researcher at the National Renewable Energy Laboratory (NREL) and an author of the study. “We were able to do it in a matter of minutes with high-performance computing.”
And, in those few minutes, he examined not one but hundreds of thousands of possible scenarios.
“The novelty of our study is the computational brute force that we brought to bear on this analysis,” said Bernard (Ben) Nueven, NREL researcher and author.
Young said that without the supercomputer, all those calculations would take about 88 days instead of an hour or a few minutes. Of course, the supercomputer also needed the mathematical magic of Nuven and Young to solve these complex design problems quickly and accurately.
With that fast math, the team found that low-salt-rejection reverse osmosis can outperform its competitors in both cost and energy use—at least for water containing less than 125 grams of salt per liter. But the team’s model could also help other research teams identify, build and test the most promising system designs.
“Hopefully, by doing these computational analyses, we can give experimentalists information to say, ‘Oh, here’s an interesting thing to study,’ or, ‘No, it’s probably completely ruled out. It is,” Nuven said.
The model can also be expanded to help experimenters on the best designs for reverse osmosis systems in general. Their study is the first to use and link to NAWI’s Water Treatment Technoeconomic Assessment Platform (WaterTap). A publicly available software tool, WaterTAP empowers users to model and simulate various water treatment technologies and evaluate their cost, energy and environmental trade-offs.
“I think it’s great. We’re building a tool that can help us and other researchers assess the potential of new and exciting technologies,” Nuven said of WaterTap, which is developed by NREL. , was built through a collaboration between Lawrence Berkeley National Laboratory, National Energy Technology Laboratory, Oak Ridge. National Laboratory, and the Regents of the University of California.
Next, the researchers hope to partner with experimental teams to build and evaluate how low-salt-rejection reverse osmosis systems work in the real world. For example, mineral build-up can slow down the system and should be accounted for in future evaluations.
Still, Atiya said, this emerging form of reverse osmosis could be a valuable tool for maximizing water recovery from high-salinity sources. “And our model can play an important role in supporting the deployment of the technology,” he said.
“To me,” Nuwen said, “this is a demonstration of what we can do with a little bit of calculation and a little bit of optimization.” – Credit: NAWI