Barbados should give MFC’s a look!
by Robert D. Lucas, Ph.D.
Recently there has been much talk about the diverse means available of obtaining energy from renewable sources (solar, wave, wind as-well-as bio-diesel/gas). However, no mention has been made locally of the use of microbial fuel/factory cells (MFC’s). MFC’s are devices that convert chemical energy into electrical energy as do batteries, via the use of micro-organisms.
Unlike batteries, MFC’s can sustain their output of electricity as long as the chemical input is maintained. Most bacteria are electrochemically inactive and cannot be used in MFC’s. Those bacteria which are capable of producing an electric current are called exoelectrogens. Exoelectrogens, when placed into a suitable medium, transfer electrons (negatively charged particles) to an electrode which has been inserted into the medium. This flow of electrons is facilitated by an active electron transport system, which carries electrons directly from the microbe’s respiratory system to the anode ((negatively charged electrode).
“MFC’s do not depend on sunlight to be able to function. There is no need to have storage facilities for storing electricity as is the case with solar energy.”
Bacteria which form biofilms on the anode are more efficient in facilitating electron transfer. (Biofilms are thin layers of densely packed micro-organisms encapsulated within an aqueous matrix of proteins, nucleic acids and polysaccharides (complex sugars). The different organism which form the biofilm, communicate with each other through cell-to-cell signaling called quorum sensing. Biofilms are resistant to the action of biocides and disinfectants and are commonly found on food contact surfaces).
In order to turn this capability of exoelectrogens into a useable supply of electricity, the process has to be accommodated in a MFC. To generate a useful current, a complete circuit must be created just as is the case found in an ordinary torch light battery.
MFC’s consist of two chambers. The first chamber contains a suitable substrate such as waste water from food processing or sewage treatment plants (as a matter of fact, any organic material can be used as a substrate); and the anode on which the microbial bioflim has attached itself. This chamber is sealed thereby promoting anaerobic conditions. The second chamber consists of the cathode or positive electrode. The two sections are separated by a special membrane which allows only protons (positively charged particles) to pass through, while inhibiting the flow of electrons. Since the electrons cannot flow through the membrane, they flow into the anode which is connected to the external circuit cathode. The MFC can also be used to generate pure water from sewage waste water under aerobic conditions.
Unlike the production of bio-fuel (ethanol/diesel), bio-gas and hydrogen production from glucose (glucose is the starting substrate for all of the products mentioned using green technology), theoretically MFC’s are more efficient in producing electricity since the process in their case is one-step. In the case bio-ethanol and bio-gas, separation of carbon dioxide and other impurities have to be effected before use.
MFC’s can be used on a small-scale, do not have to be recharged, can operate well at temperatures of 20-40°C and pH values of about 7. MFC’s can also use crops directly to generate electricity. The current output from MFC’s is directly proportional to the energy content of the waste water. MFC’s do not depend on sunlight to be able to function. There is no need to have storage facilities for storing electricity as is the case with solar energy. Since microbial growth approximate a first order reaction, during peak demand one can maintain microbial growth in the logarithmic stage (rapid growth). When demand is low one can easily alter the energy content of the waste water.
MFC’s should be looked at seriously as a means of solving some of the energy demands of Barbados.
Robert D. Lucas, Ph.D.
Wikipedia: Microbial fuel cell
Earth Times: Microbial fuel cell – Eco-friendly sewage treatment (thanks for the above photo Earth Times!)