"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."
Microbial reverse dialysis test cell. (Credit: Penn State, Dept of Public Information)
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
the 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 organic matter in water will foul the membranes
without extensive precleaning and treatment of the water. Seawater use
restricts MRC operation to coastal areas, but food waste, domestic waste
and animal waste contain about 17 gigawatts of power throughout the
U.S. One nuclear reactor typically produces 1 gigawatt.
Rather than rely on seawater, the researchers used ammonium
bicarbonate, an unusual salt. An ammonium bicarbonate solution works
similarly to seawater in the MRC and will not foul the membranes. The
ammonium bicarbonate is also easily removed from the water above 110
degrees Fahrenheit. The ammonia and carbon dioxide that make up the salt
boil out, and are recaptured and recombined for reuse.
"Waste heat makes up 7 to 17 percent of energy consumed in industrial
processes," said Logan. "There is always a source of waste heat near
where this process could take place and it usually goes unused."
The researchers tested their ammonium bicarbonate MRC and found that
the initial production of electricity was greater than that from an MRC
using seawater.
"The bacteria in the cell quickly used up all the dissolved organic
material," said Logan. "This is the portion of wastewater that is
usually the most difficult to remove and requires trickling filters,
while the particulate portion which took longer for the bacteria to
consume, is more easily removed."
The researchers tested the MRC only in a fill and empty mode, but
eventually a stream of wastewater would be run through the cell.
According to Logan, MRCs can be configured to produce electricity or
hydrogen, making both without contributing to greenhouse gases such as
carbon dioxide. The MRC tested produced 5.6 watts per square meter.
Logan also said not having to process wastewater would save about 60 gigawatts.
The King Abdullah University of Science and Technology supported this work.
From sciencedaily
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