Growing a Better Membrane
While nearly 70 percent of the world is covered by water, just a slim amount of it—2.5 percent—is freshwater, meaning that as the world population continues to boom, the demand for fresh, consumable water will intensify.
When freshwater supplies are diminished, we can turn to water desalination to produce potable water from seawater. Unfortunately water desalination, or the process of removing salt and other minerals from water, is energy intensive. One approach for reducing the energy intensity of water desalination processes is to use waste heat as the energy input via processes like membrane distillation. Unfortunately, there are no commercially available membranes specifically designed for membrane distillation processes.
Recent PhD graduate Megan Leitch and her advisors, Professors Gregory Lowry and Meagan Mauter, created new membrane materials that are specifically tailored for membrane distillation and could make the process more efficient.
Their research, outlined in the research paper Bacterial Nanocellulose Aerogel Membranes: Novel High-Porosity Materials for Membrane Distillation, was published as the cover story for the March 2016 issue of Environmental Science & Technology Letters.
Membrane distillation (MD) is a heat-based process that causes the water molecules to evaporate, pass through a membrane, and then condense on the other side of the membrane, with the salt molecules left behind.
The process has been researched for more than 60 years, "but there are no large-scale MD manufacturers," Leitch explains, as the commercial implementation of MD has been limited by the low permeability and high conductive heat loss of membranes that are currently used in the process.
Leitch theorized that aerogels would make for a better MD membrane than the current commercial membrane material. An aerogel is a very light material in which the liquid component of the gel has been replaced with gas. It is more porous and retains heat more efficiently than the currently used membrane materials.
"Initially I tried to use traditional silica aerogels but couldn't produce a thin aerogel big enough or flexible enough to test in MD," she says.
So she applied for, and received, a Graduate Research Opportunities Worldwide (GROW) grant from the National Science Foundation, and went to work with Professor Olli Ikkala's research group at Aalto University in Finland to learn how to make nanocellulose aerogels.
"The method I chose to make the nanocellulose was to grow it using a special type of bacteria that, under the right conditions, produces a biofilm by extruding a web of nanocellulose fibers," Leitch explains. "If you've ever had Kombucha tea, the mother culture is composed of yeast and a nanocellulose-producing bacteria. The glob swirling around at the bottom of the drink is nanocellulose."
With the help of Ikkala's research group, she was able to produce nanocellulose aerogels that successfully worked as prototype membranes for MD.
Her continued tests of the membranes when she returned to CMU, with the help of chemical engineering master’s student Chenkai Li, showed that the aerogel membrane was more permeable to water and retained heat better than the currently used commercial membranes.
“Megan’s work provides critical insight into structure-function relationships of membrane distillation membranes. This research paves the way for the design of high porosity membrane materials using commercially relevant fabrication processes” says Mauter.
"Because pressure-driven separation methods like reverse osmosis are more energy-efficient for sea water desalination, MD will likely never be used for big municipal drinking water applications," Leitch says. "More likely it will be used for industrial processes, for concentrating brines, or possibly for desalination in remote locations where electrical energy is very expensive or not readily available."
But her research, and further research on membrane improvements, could make MD more efficient and make it a more competitive option for desalinating water.