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Generating electricity from human waste

The waste we flush down the toilet could one day power lights at home.

AN ELECTRICITY generator fuelled by sewage has been developed. The waste you flush down the toilet could one day power the lights at home.

The generator does the job of a sewage-treatment plant at the same time as it breaks down the harmful organic matter as it generates the electricity. The device has been developed by researchers at Pennsylvania State University and a report has appeared in New Scientist.

Harnessing chemical techniques similar to those the body uses to break down food, Penn State's microbial fuel cell (MFC) diverts the electrons liberated in the reactions to produce electrical energy.

Sewage processing plants are needed in developing countries but they are expensive, as they use so much power.

Producing electricity at the same time will offset this cost, says Bruce Logan, who led the team. A slurry of bacteria and undigested food, consisting of organic matter such as carbohydrates, proteins and lipids are contained in sewage.

In a process that releases electrons, the bacteria found in sewage treatment works use enzymes to oxidise organic matter. Normally the electrons power respiratory reactions in the bacterial cells, and are combined with oxygen molecules.

However, the electrons can be wrested from them and used to power a circuit by depriving the bacteria of oxygen on one side of the MFC.

The MFC comprises a sealed 15-centimetre-long can with a central cathode rod surrounded by a proton exchange membrane (PEM), which is permeable only to protons.

Eight anodes are arranged around the cathode. The organic waste is broken down by bacteria that cluster around the anodes as organic waste is pumped in, releasing electrons and protons.

With no oxygen to help mop up the electrons, bacteria's enzymes transfer them to the anodes, while the protons migrate through the water to the central cathode.

The protons are encouraged to pass through to the cathode by polarised molecules on the PEM. There they combine with oxygen from the air and electrons from the cathode to produce water.

It is this transfer of electrons at the electrodes that sets up the voltage between them, enabling the cell to power an external circuit.

As yet this design is only producing a tenth of what its potential power output could be. Even so, if scaled up, this system would produce 51 kilowatts on the waste from 100,000 people, Logan says.

He hopes to be able to boost its efficiency by increasing the surface area of the anodes or by finding more efficient anode material. — Our Bureau

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