Removing salt and other impurities from sea-, ground- and wastewater could solve the world’s looming freshwater crisis.
And yet, while industrial-scale seawater desalination plants do exist in coastal areas where the freshwater challenge is most acute, the process of making undrinkable water drinkable is largely out of reach for inland water sources due to the high cost of concentrate disposal.
“When we desalinate water, we are left with a pure water stream and a concentrated waste stream. Inland brackish water and wastewater desalination plants are costly to build and to operate because we don’t have easy disposal options for the concentrate stream,” said Meagan Mauter, associate professor of civil and environmental engineering at Stanford.
Compounding this problem is that some inland wastewaters from industrial sources can have up to 10 times higher concentration of dissolved solids than seawater. “Concentrating and disposing of concentrated brine could unlock vast new water resources, but it’s just too expensive at this time,” she said.
It is not for lack of trying, however, added Mauter, who in her newest paper in Proceedings of the National Academy of Sciences introduces a suite of new analytic methods that could help desalination engineers weigh the many technical and financial factors that go into building a desalination plant.
Still waters run deep
Mauter’s team applies this “innovation assessment model” to analyze membrane-based desalination in which impure water is separated from freshwater by a permeable material with pores just large enough for water molecules to flow through, but too small for salt and other solid impurities. Under osmotic or hydraulic pressure, the molecules of freshwater migrate through the membrane barrier and leave the impurities behind.
While it sounds easy, membrane separation is technically quite difficult. High-salinity membrane separation processes can involve hundreds of interdependent components or design variables – each with bearing on the ultimate efficiency and cost of the underlying process. Using Mauter’s approach, engineers aiming to lower the cost of desalination can now test their innovative ideas before they build their prototypes.
“Innovation is not always intuitive. Often, the cost increases of these new technologies negate any performance improvements,” Mauter says. “A better process or component is not much good if the end result is a further increase in overall separation costs.”
Her approach helps desalination designers look at all components in a process when trying to understand the relationship between cost and performance. Many times, she said, the best way to reduce the costs of a treatment technology is not to improve performance, but to reduce the manufacturing costs of a particular component.
“That is a very different set of scientific questions to consider,” Mauter added. “Our method helps prioritize the research and development pipeline and helps to earmark scarce research dollars for innovations with the greatest potential benefit.”
Innovation into action
The method is actually three distinct approaches. The first is a relatively simple cost-benefit analysis of materials and manufacturing methods that helps winnow a long list to a few contenders with the most promise. The second increases the rigor a bit, balancing performance gained with the cost to make a new component. The most advanced method in the suite is a simulation of expected impact of a component innovation on reducing costs that also accounts for the impact of improvements in other, coupled components.
Mauter and co-authors then used their newly developed approaches to suggest one potential innovation that has high probability of substantial reductions in the “levelized cost of water” – the industry standard criterion – for treating high-salinity brine.
High-pressure reverse osmosis processes, she says, could hit the sweet spot for cost-effective high-salinity water desalination. For these technologies to displace existing thermal processes, however, will require new high-pressure membranes able to withstand pressures of up to 4,000 pounds per square inch without compromising water permeability or salt rejection.
“Anyone in the desalination research and development spectrum – a researcher, an investor or a corporate executive – should be very interested in these techniques for bringing down the cost of desalination,” Mauter said.