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Engineer designs system to put wastewater to work

Sparing the drain to power the neighborhood

In the midst of the worldwide energy crisis, researchers at Washington University in St. Louis have been continuing their work on a microbial fuel cell that generates electricity from wastewater. Advances in the design of this fuel cell in the last year have increased the power output by a factor of 10 and future designs, already in the minds of the researchers, hope to multiply that power output by 10 times again. If that goal can be achieved, the fuel cell could be scaled up for use in food and agricultural industries to generate electrical power - all with the wastewater that today goes right down the drain.

Lars Angenent, Ph.D., assistant professor of chemical engineering, and a member of the University's Environmental Engineering Science Program, has devised a continually fed upflow microbial fuel cell (UMFC). In a paper published online in the Environmental Science Technology, Angenent describes how wastewater enters from the bottom of a system and is continuously pumped up through a cylinder filled with granules of activated carbon. Many previous microbial experiments used closed systems with a single batch of nutrient solution, but because this system is continuously fed from a fresh supply of wastewater, Angenent's UMFC has more applications for industry since wastewater is continually outputted during industrial production.

The organic matter in the wastewater provides food for a diverse community of bacteria that have developed a biofilm (a thick-layered colony of bacteria) on a simple electrode in the anode chamber. An inexpensive U-shaped proton exchange membrane inside the anode chamber separates the anode from the cathode.

As the bacteria feed on the organic material in the wastewater they release electrons to the anodic electrode. These electrons then move to the cathodic electrode via a copper wire. The formed protons are transferred through the membrane towards the cathode where they react with electrons and oxygen to form water.

This is the second design of the UMFC. Last year, Angenent's design used a cathode on top of the anode. This time, with the U-shaped design, the surface area was increased and he reduced the distance between the anode and cathode, which helped reduce power loss due to resistance. These two changes are largely responsible for the boost in power by a magnitude of 10 times from a maximum of 3 watts per cubic meter of solution last year to a maximum of 29 w/m3 today. Sustained power in the system can average 20 watts per cubic meter - enough to run a small light bulb.

Angenent and his doctoral student Jason He are exploring other anode-cathode shapes, surface areas, and distances to both increase power and reduce the resistance in the system so that less power is lost as it runs. Angenent says that for the UMFC to be economical he needs "two more breakthroughs, but [he doesn't] know what they are yet."

The economic viability level for this microbial fuel cell is around 160 watts per cubic meter of solution and the goal of increasing the power output by 10 times would double that level to around 300. If that can happen, this microbial fuel cell system would be a proof of concept with far-reaching applications in the food and agricultural industries. Since this experiment uses common and inexpensive materials and wastewater is plentiful in industry, a scaleable version of this system at one food industry could one day generate enough power for 900 American single-family households. A clean and renewable energy source, all with what's already just going down the drain.

Tony Fitzpatrick | EurekAlert!
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