Wastewater into electricity gets an ionic twist

What would happen if you jumped into a lake and then, still wet, went swimming in the ocean?

According to Penn State researchers, the ionic different between fresh water and salt water automatically begins the process of reverse electrodialysis (RED), or spontaneous energy flow, as positive and negative ions flow back and forth upon contact. 

Although scientists previously thought they would have to build special devices near the ocean to capture this special kind of energy, the Penn State researchers have succesfully recreated this effect in the laboratory, using an ammonium bicarbonate salt solution.

But there is a further reason the researchers chose to build the system in their lab. Roland Cusick, a graduate student in environmental engineering, and Younggy Kim, a postdoctoral fellow, figured out how to combine the salt-fresh water system with microbial fuel cells (MFC), devices that use bacteria to oxidize organic matter to produce an electric current. 

The combined technology creates a microbial reverse-electrodialysis cell (MRC), the details of which were published in the March 1 edition of Science Express.

“We are taking two technologies, each having limitations, and putting them together,” said Bruce Logan, Penn State’s Kappe Professor of Environmental Engineering. “Combined, they overcome the limitations of the individual technologies.”

RED is difficult to use alone because it requires a very large number of cells to produce significant energy. But using exoelectrogenic bacteria that is found in wastewater allows RED to use fewer cells. At the same time, the presence of RED helps the bacteria to produce even more electricity.

Scientists have increasingly been combining technologies in recent years in order to maximize efficiency of individual technologies that often have specific flaws. At times, these combinations have been as simple as wind and solar—one of which operates in sunny hot day and wind powering away through the night. Both are intermittent but together can provide electricity nearly around the clock.

In this case, the combined technology was also limited by geographic issues. For years, researchers thought MFC operation was limited to areas with natural seawater available because organic matter in the system can contaminate the cell membranes. 

And, of course, much of the United States is not located near an ocean. But by using ammonium bicarbonate, an unusual salt solution, researchers have found a way to prevent fouling of the cell mebranes. And if they choose to remove the product, they can easily do so by boiling to water to 110 degrees Fahrenheit.

Shifra Mincer, EarthTechling