MSU microbial electrolysis cell produces ethanol from glycerol, reduces wastewater in biodiesel production
|The MEC uses syntrophic cooperation within a bacterial consortium (red and green) in the anode chamber to ferment ethanol from glycerol and to remove inhibiting H2. Credit: ACS, Speers et al. Click to enlarge.
Researchers at Michigan State University have developed a microbial electrolysis cell (MEC) which will allow biodiesel plants to eliminate the creation of hazardous wastes while reducing their dependence on fossil fuel.
The platform, which uses microbes to produce ethanol from glycerol, has the added benefit of cleaning up the wastewater, will allow producers to reincorporate the ethanol and the water into the fuel-making process, said Gemma Reguera, MSU microbiologist and one of the co-authors. The ethanol replaces petrochemical methanol in the biodiesel production. A paper on their work is published in the ACS journal Environmental Science & Technology.
The MEC is driven by the synergistic metabolisms of the exoelectrogen (a microorganism that can transfer electrons extracellularly) Geobacter sulfurreducens and the bacterium Clostridium cellobioparum, which ferments glycerol into ethanol in high yields (90%) and produces fermentative byproducts that serve as electron donors for G. sulfurreducens.
Syntrophic [mutual dependency of two organisms for nutritional requirements] cooperation stimulated glycerol consumption, ethanol production, and the conversion of fermentation byproducts into cathodic H2 in the MEC. The platform was further improved by adaptively evolving glycerol-tolerant strains with robust growth at glycerol loadings typical of biodiesel wastewater and by increasing the buffering capacity of the anode medium.
This resulted in additional increases in glycerol consumption (up to 50 g/L) and ethanol production (up to 10 g/L) at rates that greatly exceeded the capacity of the anode biofilms to concomitantly remove the fermentation byproducts. As a result, 1,3-propanediol was generated as a metabolic sink for electrons not converted into electricity syntrophically. The results highlight the potential of consortia to process glycerol in MECs and provide insights into genetic engineering and system design approaches that can be implemented to further improve MEC performance to satisfy industrial needs.—Speers et al.
Geobacter are naturally occurring microbes that have proved promising in cleaning up nuclear waste as well in improving other biofuel processes. Much of MSU microbiologist Gemma Reguera’s research with these bacteria focuses on engineering their conductive pili or nanowires. These hair-like appendages are the managers of electrical activity during a cleanup and biofuel production.
First, Reguera, along with lead authors and MSU graduate students Allison Speers and Jenna Young, evolved Geobacter to withstand increasing amounts of toxic glycerol. The next step, the team searched for partner bacteria that could ferment it into ethanol while generating byproducts that fed the Geobacter.
C. cellobioparum (Ccel) has a naturally high glycerol-to-ethanol conversion yield (0.9), likely reflecting a adaptation of the bacterium to its natural habitat, the bovine rumen, where dietary glycerol is rapidly fermented upon ingestion, the authors noted. Efficient fermentation of dietary substrates in the rumen depends largely on syntrophic interactions among rumen bacteria—most significantly H2 transfer, to prevent acidosis and feedback inhibition. The researchers essentially reproduced natural cooperation by co-culturing Ccel with the H2-oxidizing Geobacter.
It took some tweaking, but we eventually developed a robust bacterium to pair with Geobacter. We matched them up like dance partners, modifying each of them to work seamlessly together and eliminate all of the waste. They feast like they’re at a Las Vegas buffet. One bacterium ferments the glycerol waste to produce bioethanol, which can be reused to make biodiesel from oil feedstocks. Geobacter removes any waste produced during glycerol fermentation to generate electricity. It is a win-win situation.—Gemma Reguera
The MECs do not harvest electricity as an output. Rather, they use a small electrical input platform to generate hydrogen and increase the MEC’s efficiency even more.
The promising process already has caught the eye of economic developers, who are helping scale up the effort. Through a Michigan Translational Research and Commercialization grant, Reguera and her team are developing prototypes that can handle larger volumes of waste.
Reguera also is in talks with MBI, the bio-based technology “de-risking” enterprise operated by the MSU Foundation, to develop industrial-sized units that could handle the capacities of a full-scale biodiesel plant. The next step will be field tests with a Michigan-based biodiesel manufacturer.
Allison M. Speers, Jenna M. Young, and Gemma Reguera (2014) “Fermentation of Glycerol into Ethanol in a Microbial Electrolysis Cell Driven by a Customized Consortium” Environmental Science & Technology doi: 10.1021/es500690a