Microbial fuel cell (MFC) technology, which produces electricity from bacteria found in wastewater and sewage, was first demonstrated in the early 20th century. However, research still remains largely at laboratory and pilot stages and the technology has yet to be scaled up and commercialised.
But that doesn't mean scientists are giving up. Among the research projects currently underway, one particularly interesting study was announced by scientists and engineers from Stanford University in the US, who last September unveiled a microbial battery that can produce electricity from bacteria found in wastewater using MFC technology.
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The battery is designed to use naturally occurring 'wired microbes' as mini power plants. The microbes produce electricity as they digest plant and animal waste. According to the scientists, the battery holds certain advantages over traditional microbial technologies.
After further development, they hope the technology will be used to create electricity in sewage treatment plants, or to break down organic pollutants in the 'dead zones' of lakes and coastal waters, where fertiliser runoff and other organic waste can deplete oxygen levels and suffocate marine life. Since releasing the original paper, the researchers say they have already made significant progress in overcoming initial design hurdles.
The science behind the microbial battery
Currently the battery looks like a school chemistry experiment. It is the size of a household D cell battery and has two electrodes - one positive and one negative - that sit in a bottle of wastewater.
It works by harnessing the power of exoelectrogenic microbes, organisms which evolved in airless environments and developed the ability to react with oxide minerals rather than breathe oxygen to convert organic nutrients into biological fuel.
Historically it has been extremely difficult for scientists and researchers to harness the power from these unruly microbes to use with bio-generators.
However, the Stanford team has managed to create a simple yet efficient design that puts the exoelectrogenic bacteria to work and allows them to harness the power produced. Using a scanning electron microscope the researchers recorded the microbes attaching what they describe as milky tendrils to carbon filaments at the battery's negative electrode. About 100 of the microbes could fit, side by side, in the width of a human hair.
"You can see that the microbes make nanowires to dump off their excess electrons," explains Craig Criddle, an environmental engineer working on the project.
As these microbes ingest organic matter and convert it into biological fuel, their excess electrons flow into the carbon filaments, and across to the positive electrode, which is made of silver oxide. The electrons on the positive side gradually reduce the silver oxide to silver, storing the spare electrons while doing so. Over the course of a day the positive electrode will absorb the full load of electrons and will largely have been converted into silver, which is removed from the battery and re-oxidized back to silver oxide, releasing the stored electrons.
This technique is different from other typical MFC research because it doesn't rely on oxygen being directly in the water and can therefore be more efficient, as researcher Xing Xie explains: "In the previous system the oxygen is directly in the system, so even though there is a membrane separating the two chambers, the oxygen can cause the membrane to go to the other chamber and then the efficiency can be significantly lower. When we separate these two things, theoretically efficiency can be close to 100 %, so that means most of the glucose can be used for electricity generation."
Hurdles to overcome
One of the main hurdles for the research is finding a replacement material for silver - which is too expensive to use at large scale - in order to make the battery more cost efficient, explains Yi Cui, an associate professor of materials science and engineering. The team has already had some success in finding a replacement; however, the researchers are not willing to go into more detail until a new paper has been finalised.
As well as scaling up the technology, Xie says, they need to find out how to better "switch the condition" enabling the two-step process to take place at a larger scale, which might be done automatically. "This kind of switch hasn't been tested so when we do the final tests we need to do this process," he adds.
In regards to efficiency, the researchers estimate that the microbial battery can extract about 30% of the energy locked in wastewater, which is roughly equivalent to the efficiency of the best commercially available solar cells. This rate, however, depend on how much organic matter is found in the waste, Xie points out. "Normally if you have more than 500mg chemical oxygen demand (COD) per litre you can produce about 0.6 kwh per cubic metre. That is the energy, not the power, and then it depends on how much wastewater you receive," he adds.
Other benefits of the microbial battery
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Generating electricity is the main focus of the battery, but the technology may also have other benefits. "Wastewater treatment technology costs a lot of energy to do, but to do it this way, even though you can not recover a lot [of energy], you save a lot by not using too much energy," Xie explains.
Treating wastewater currently accounts for about 3% of the total electrical load in developed nations, according to the university. In a conventional treatment plant, where ordinary bacteria use oxygen in the course of digestion, the majority of this energy goes toward pumping air into the wastewater.
"The second benefit is this technology may produce less sludge. In previous wastewater treatment technology they use a lot of sludge and it costs a lot of money to treat the sludge," Xie adds.
Xie believes that in five years' time the technology will be much more ready to scale up "to a real wastewater treatment plant scale."
Scientists and engineers aren't the only ones excited by microbial technology. Many believe it can play an important role in our future energy pool.
The US Department of Energy is backing the development of microbes and states on its website: "Identifying and harnessing their unique capabilities will offer us new solutions to longstanding challenges in environmental and waste cleanup, energy production and use, medicine, industrial processes, agriculture, and other areas."
Scientists at the University of Massachusetts are also working on unlocking the potential of MFC and have developed a "microbial fuel cell latrine" that purifies human waste, turns it into compost for farming and generates electricity.
A report released by the university in 2008 also supports further development of MFC potential. It states some of the uses of microbial fuel as harvesting electricity from sediments at the bottom of the sea, as a power source for soldiers in remote locations and as a way to make household electricity from garbage. Other universities in the US with ongoing microbial research projects include University of Colorado and Arizona State University.
It seems that in the future microbial fuel cell technology is likely to become one particularly niche way in which businesses, and maybe households, can generate their own electricity, and Xie and his team's microbial battery could be just one of perhaps many devices to help make this happen.