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Part II: Microbial Fuel Cells (MFC)

I can’t believe how fast time passes. I have been meaning on writing this post up for a long time now. In the last post, I just was introducing Shewanella onedeinensis and its ability to transfer electrons from a variety of organic substrates to metal oxides. Today, we shall look at one possible application for such a microbe: using bacteria to make electricity.

This is done through contraptions called Microbial Fuel Cells (MFCs) but to understand MFCs we need to first understand how fuel cells work. There is going to be some detail. But the take-home message here though is: We can use certain bacteria to make electricity out of waste.

A fuel cell is a device that produces electricity as long as one provides an external source of fuel and a source that accepts electrons known as oxidants. This is different from normal batteries because batteries simply release the electrical energy stored within them. They get tossed once they are used up. In the case of rechargeable batteries, energy must be put back into them first before they can be used again. A fuel cell will continue to work as long as the fuel is provided externally. In other words, a battery is an electrochemical storage device. A fuel cell is an electrochemical conversion device.

The diagram to the left is from a journal called "Emerging Environmental Issues" (Volume 10, 2005). The archetype of a fuel cell is a hydrogen fuel cell. Let’s look at how this fuel cell works going from left to right. In a hydrogen fuel cell, hydrogen flows into one side of the fuel cell (the anode side). Hydrogen consists of one positively charged particle (called proton) and one negatively charged particle (called electron). There is a special membrane that is able to separate electrons from protons and a solution that allows the protons to travel to the other side of the fuel cell (called electrons). This is an unstable state and for the reaction to become stable again, positive and negative charges must balance. The only way for the electron to catch up with the traveling proton is to travel through a wire to the other side. Hence, we have a flow of electrons – electricity that can be used to do work. At the other side, the oxidant is ready to accept the positively charged proton. 2 protons and 2 electrons combine with one oxygen atom to form water (H2O). In a nutshell, this is how a hydrogen fuel cell works.

Microbial fuel cells work in a very similar way. However, the fuel in this case is not hydrogen, it is a carbon source instead. Sugar is an example of a carbon source. Bacteria like our Shewenella that live on the anode are able to consume the sugars. Under anaerobic conditions, in the simplest case, these bacteria release a proton while depositing electrons externally onto the anode. Like with the hydrogen fuel cell, electrons and protons must combine with an oxidant at the cathode. In the process, electricity is created.

So why is this useful?

In many cases, urban waste waters contain many organic pollutants. These pollutants if released untreated into the environment can cause a lot of environmental damage. This is the reason why ideally waste water treatment is important, but can be very energy intensive because bacteria need to be constantly mixed and aerated in large tanks. What if instead of consuming a lot of electricity, we can use MFCs to clean up some of the waste in the water and at the same time generate part or all of electricity?

In reality, there are many challenges to make MFCs practical as discussed in review article titled “Towards practical implementation of bioelectrochemical wastewater treatment” (Trends in Biotechnology, Vol 26 No 8). Electricity generation in MFCs has been very small (< 1 amp/m2). In laboratory conditions, MFC performance has exponentially increased though. Over the last few years, it has reached 10 Amps/m2 and is likely to increase even more with more technological innovations. These, however, represent laboratory conditions only where optimal yields are obtained through use of special synthetic media in monocultures.

In waste water treatment, however, open souces of water means that organic food sources vary and are not optimal. Furthermore, monocultures cannot be maintained. There is much research done on this end. Research groups are experimenting with better anodes, MFC designs and ways to engineer “bacterial communities” to increase MFC performance in these scenario. And so I think it is only a matter of time until we will start seeing them used for waste water. Next time, we shall look at how these bacteria could be used in just cleaning up uranium waste.

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