Production of energy from the difference between salt water and fresh water is most convenient near the oceans, but now, using an ammonium bicarbonate salt solution, Penn State researchers can combine bacterial degradation of waste water with energy extracted from the salt-water fresh-water gradient to produce power anywhere.
"We are taking two technologies, each having limitations, and putting them together," said Bruce E. Logan, Kappe Professor of Environmental Engineering. "Combined, they overcome the limitations of the individual technologies."
The technologies Logan refers to are microbial fuel cells (MFC)-- which use wastewater and naturally occurring bacteria to produce electricity -- and reverse electrodialysis (RED) -- which produces electricity directly from the salinity gradient between salty and fresh water. The combined technology creates a microbial reverse-electrodialysis cell (MRC). The researchers describe MRCs in today's (March 1) edition of Science Express.
RED stacks extract energy from the ionic difference between fresh water and salt water. A stack consists of alternating ion exchange membranes -- positive and negative -- with each RED membrane pair contributing additively to the electrical output. Unfortunately, using only RED stacks to produce electricity is difficult because a large number of membranes is required when using water at the electrodes, due to the need for water electrolysis.
Using exoelectrogenic bacteria -- bacteria found in wastewater that consume organic material and produce an electric current -- reduces the number of stacks needed and increases electric production by the bacteria.
Logan, working with Roland Cusick, graduate student in environmental engineering, and postdoctoral fellow Younggy Kim, placed a RED stack between the electrodes of an MFC to form the MRC.
While the researchers previously showed that an MRC can work with natural seawater, the org
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