22 May 1998
The Power Of Putrefaction
Top up your batteries with ditchwater? No chance! But the peculiar physiological characteristics of metal-reducing bacteria developed to decrease metal contamination in waste water can produce electricity. In a report from the American Society for Microbiology, a team led by Byung Hong Kim from the Korea Institute of Science and Technology, claim that their experimental 20ml cell system is capable of producing around 0.6 volts.
Most organisms metabolize to gain energy, tapping the energy liberated from food. Respiration using oxygen is part of this process in aerobic organisms like humans but anaerobic micro-organisms use other substances such as nitrates, sulfates or metals as electron acceptors. To oxidize metals, metal-reducing bacteria must deliver an electron to the large metal ions around them. Where the water-insoluble metal ions are so large they cannot be accepted into the bacterium cell, the electron must travel to the metal. The researchers found that some bacteria have a special electron transport pathway in which the electron transport protein is exposed to the bacterial outer membrane. The electron can thus move from the cell to the metal ion, making a (reverse) current flow.
Bacteria with these characteristics, though widely observed in nature, have only recently gained the attention of researchers. Whilst the metal-reducing bacteria are effective in water purification, the addition of organic compounds makes each micro-organism capable of acting like an electrochemical cell, transferring current from inside to its outer membrane. The microbe plays the role of a small dynamo, using waste water to generate electrical power.
Previously, a microbial fuel cell using water-soluble electron mediators, or electron shuttles, had been developed using micro-organisms as catalysts. These mediators carried electrons from within a cell to the pole of the fuel cell where they were oxidized. They then travelled back into the bacterial cells to gather further electrons, repeating the cycle.
A clear improvement over the previous microbial fuel cell, this new development eliminates the high costs of mediators, such as the commonly-used phenol which can cause environmental contamination. It is also more efficient, losing fewer electrons during the transport process, and is more stable than a mediated cell, with less electrode and membrane fouling which might prevent efficient operation.
Clearly, the sewage powered Scalextric set is closer than any of us imagined.