Microbial fuel cell (MFC) is an electrochemical cell that converts chemical energy into electrical energy through catalytic reactions of microorganisms under anaerobic conditions. MFC consists of an invariant electrode electrolyte system. In the anode, electrons are produced as a result of catabolism in microorganisms consuming organic substrates. These electrons flow through an external circuit to cathode, where as the H+ ions produced in the anode flow to cathode through a cation exchange membrane and react with electrons producing H2O or other reduced products which depends on the type of electron acceptor.
MFC is an emerging fuel cell type due to its unique operating characteristics compared to other fuel cell types. No need of extensive feed stock conversion steps, ability to operate on range of biodegradable materials, high theoretical yields, no need of expensive catalysts, ability to operate at moderate temperatures and ease of fabrication are some of these unique features of MFC.
During the present study, a set of experiments were performed to identify the relationship between Chemical Oxygen Demand (COD) and cell voltage output. The experiments were carried out using a batch type MFC. The types of electrodes were Carbon Felt electrodes and the type of membrane was a Cation exchange membrane. Oxygen was used as the electron acceptor at the cathode for all the MFC configurations. Cow dung was used as the waste biomass substrate and the microorganisms already present in the cow dung used as the bio catalyst. Cell potential and the amount of COD reduction were measured using different concentrations of cow dung. This system gave the results for a mixed culture and undefined substrate. A set of experiments were performed for defined substrate (synthetic waste water/ glucose) with a known anaerobic culture (E.coli) and the electron acceptor used for this system was Potassium ferricyanide. Using the results, comparison of the two systems and the potential of MFC for waste water treatment were discussed.
This technology can be applied as a new waste water treatment method, since this removes organic matter from waste streams while generating electricity. According to the published literature some systems have removed COD levels up to 80% even though the obtained COD reductions for above experiments are below than this value. There are some pilot scale water treatment facilities available. But presently this technology is in its early developing stages. MFC gives only a power density of few thousands mW/m2. Therefore MFCs have been using for equipments which require little power. Some of these equipments are sensors and monitoring equipments.
Key words: Microbial fuel cell, waste water treatment, Bio energy
M. P. Gunathilake, S.H.P. Gunawardane, A.A.P. de Alwis
Department of Chemical and Process Engineering, University of Moratuwa, Sri Lanka