Plugging into the power of sewage
19:00 10 March 2004
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Bruce Logan, Penn State University
Microbiology, University of Massachusetts
Sewage treatment, Wessex Water
The waste you flush down the toilet could one day power the lights in your home. So say researchers at Pennsylvania State University who last week revealed they have developed an electricity generator fuelled by sewage.
Even better, the device breaks down the harmful organic matter as it generates the electricity, so it does the job of a sewage-treatment plant at the same time. Penn State's microbial fuel cell (MFC) harnesses chemical techniques similar to those the body uses to break down food - but diverts the electrons liberated in the reactions to produce electrical energy.
"There are extraordinary benefits if this technology can be made to work," comments Bruce Rittmann, an environmental engineer at Northwestern University in Illinois.
Many developing countries urgently need sewage processing plants, for example, but they are prohibitively expensive, largely because they use so much power. Offsetting this cost by producing electricity at the same time could make all the difference, says Bruce Logan, who led the development team at Penn State.
Slurry of bacteria
Sewage contains a slurry of bacteria and undigested food, consisting of organic matter such as carbohydrates, proteins and lipids. The bacteria found in sewage treatment works use enzymes to oxidise organic matter, in a process that releases electrons.
Normally the electrons power respiratory reactions in the bacterial cells, and are eventually combined with oxygen molecules. However, by depriving the bacteria of oxygen on one side of the MFC, the electrons can be wrested from them and used to power a circuit.
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 (see graphic).
Bacteria cluster around the anodes and break down the organic waste as it is pumped in, releasing electrons and protons. With no oxygen to help mop up the electrons, the bacteria's enzymes transfer them to the anodes, while the protons migrate through the water to the central cathode.
Polarised molecules on the PEM encourage the protons to pass through to the cathode. There they combine with oxygen from the air and electrons from the cathode to produce water. It this transfer of electrons at the electrodes that sets up the voltage between them, enabling the cell to power an external circuit.
The Penn State team's device is the first MFC that is specifically designed to produce electricity by processing human waste. Previous designs have only run on glucose solutions.
As yet his design is only producing a tenth of what he calculates 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.
Microbiologist Derek Lovley of the University of Massachusetts at Amherst believes the most elegant aspect of Logan's MFC is its single-chamber design, which makes it very easy to scale up. Most glucose-powered MFCs comprise two anode and cathode chambers, separated by a PEM.
However, Lovley believes generating power from waste water on a large scale is a long way off: "One way to think of this technology is that it is currently at the state of development that solar power was 20 to 30 years ago - the principle has been shown, but there is a lot of work to do before this is widely used."