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Mud power: how bacteria can turn waste into electricity

Microbial fuel cells: a bit of mud or sewage, a few bacteria and, bingo: electricity. engineering for change

When you read the word “bacteria” you probably think about illness, advertisements for “probiotic” food supplements, and maybe about brushing your teeth.

Chances are, you probably don’t think about electricity.

As it turns out, bacteria can be used as a source of electricity, and recent research has produced some surprising findings.

It seems that bacteria have been plugged into each other all along and this finding could open entirely new opportunities in bioenergy production.

Breathing metal

Derek Lovley, a Distinguished Professor at the University of Massachusetts Amherst and the head of the Geobacter project, found that some bacteria naturally produce electricity through their ability to “breathe” solid lumps of iron in the soil.

We can harvest this electricity from the bacteria using devices known as microbial fuel cells (MFCs).

An MFC is made up of two electrodes – an anode and a cathode – linked by an electrical connection. Bacteria breathe out electrons, produced as part of their normal metabolism, onto the anode. These electrons can be collected as an electrical current.

The electrons are then passed to the cathode to complete the electrical circuit and create a kind of battery.

Bacteria capable of producing electricity occur naturally in almost any type of mud, sewage or waste. The bacteria usually use this process to breath without oxygen.

Turning this natural process into a functioning MFC is as simple as filling the MFC with mud, sewage or waste and waiting for the bacteria to grow.

Bacterial batteries

The bacteria that can produce electricity do so by directly transferring electrons to the anode. They do not use any intermediates or shuttles; in effect, they reach out and touch the anode.

How an MFC works. MFCGuy2010

To do this they have to produce some unique structures. Most notably, they produce filaments that can directly transfer electrons in the way which metal does. Previously, scientists thought it was impossible for biological systems to act like metal.

Recent research with the bacteria Geobacter sulfurreducens has demonstrated that these filaments, called nanowires, act as tiny wires.

When the billions of bacteria that are growing together combine, the nanowires join to form conductive biofilms. These can transfer electrons across considerable distances, opening up the possibility of growing organic, self-repairing electrical conductors for use in biocomputers.

A growing field

MFC applications are currently limited, due to the low power output. The most practical MFC currently in use by the United States Navy weighs 16 kilograms and produces the equivalent power of 16 D-Cell batteries, per year.

Problems with increasing the size of these devices have currently limited generating more power from them.

In many cases the electricity produced by the MFC may not be its most important function. Since MFCs can convert organic wastes into electricity, they are being investigated for use in bioremediation, where micro-organisms are used to remove pollutants.

Often the bacteria present in the ground can eat toxic compounds that might be present. But when there is nothing to accept the electrons produced by the bacterial metabolisms, it slows down the degradation processes.

Placing an MFC in the ground speeds up the process manyfold. While a small amount of electricity may be produced – enough, say, to power a flashing light – clean soil without toxic chemicals is a lot more valuable.

Play it backwards

MFCs can be run in reverse and used to produce organic compounds, instead of consuming them. This process, called microbial electrosynthesis, was discovered less than a year ago.

Humble dirt is the main ingredient in microbial fuel cells. Ashley Franks

A great advantage of microbial electrosynthesis is that the bacteria consume electricity and carbon dioxide and produce organic compounds as waste. These organic compounds can include butanol, a direct replacement for petrol, or sugars. In other words, bacteria can be used to produce biofuels.

When solar power is used as a power source, this process is identical to photosynthesis but does not require the land and water use of traditional agriculture.

Importantly, this process also doesn’t need to divert potential food crops from people’s tables – a lingering problem of producing biofuels from cellulose.

A further advantage of microbial electrosynthesis is that fuels such as butanol can be directly used in our current infrastructure without costly modification. This is not the case with ethanol.

Making methane and electricity?

One of the earliest sources of bioenergy was the production of methane from organic waste.

In this process, bacteria consume organic compounds and work together to produce methane. This methane can then be used for heating, electricity generation and for powering gas engines.

It has been shown these bacteria are likely transferring electricity to one another while making methane.

Knowing that electricity is involved in this process will allow design changes and improvements, all of which could make methane an important source for bioenergy in the future.

That would make sense, given how much energy we are currently flushing down the toilet.

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