The difference between saltwater and freshwater can produce electricity. Microbial fuel cells use bacteria to degrade organic matter to also produce energy. Great as they are, these two technologies have had limitations in the past, but now a team of scientists unite them in a system that eliminates most of their shortcomings.
The team, led by Bruce Logan from Penn State University (PSU), used microbial fuel cells and reverse electrodialysis (which uses the salty-fresh water difference) to create what they called a microbial reverse-electrodialysis cell (MRC).
We already covered the topic of REDs in past articles. To summarize, a reverse electrodialysis system is composed of several stacks of alternating ion exchange membranes (+ and -). Each membrane is like a battery added to a series of other batteries to increase the voltage. In real applications, however, a large number of stacks is needed, making the system become bulky.
The number of RED stacks can be reduced by using exoelectrogenic bacteria, or, in plain English, bacteria that eats organic material to produce electricity.
It has been established that RED systems work best in lab conditions. When taken out to the sea, the various organic impurities foul its membranes if water is not pre-treated.
What Logan and his colleagues did was using an ammonium bicarbonate solution instead of seawater. The main advantage besides working with MRCs just as well as regular saltwater, is that the ammonium bicarbonate is easily removable from the solution and easily recyclable from water above 100 °F.
That temperature can be obtained from various power stations or almost any industrial facility’s waste heat. “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 surprise when using ammonia bicarbonate instead of seawater was that the production of electricity was greater, and the bacteria dissolved organic materials better than a huge water filter.
“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.”
Logan so far tested the MRC device in lab conditions, but he’s now planning to run a stream of wastewater through it. He expects his device to produce electricity or hydrogen, thus recycling water by cleaning it through a process that also generates 5.6 watts of electricity per square meter cleanly.
Moreover, some 60 gigawatts of electricity would be saved by not having to clean the wastewater through traditional, energy-intensive processes, so MRCs are a win-win situation for both municipal facilities and the grid.