Microbial Fuel Cell For Electricity Generation During Organic Wastewater Treatment

World-wide depletion of energy reservers and environmental contamination are inspiring the search for renewable and environment-friendly technologies to recover useful energy and materials from organic wastes. This has been particularly emphasised as a significant approach so as to make the wastewater treatment more sustainable and economical in the field of environmental engineering. Microbial fuel cell (MFC) has been addressed able to generate electrical currents via oxidizing orgianic compounds by using microorganisms as the bio-catalysts. Electricity generation during organic degradation represents a process of directly converting chemical energy within organic matters to electrical energy, which gives rise to a potential for MFC to produce electricity from organic wastewater along with wastewater treatment. Currently, MFC is still in its infant and majority of works have been emerged only focusing on its technical feasibility and fundamental characteristics. It has been taken into account that three bottlenecks in terms of low power density, high costs of materials and indetermination of available MFC configuration should be the main limiting elements for MFC technology to apply commercially.This study systematically investigated the feature and corresponding mechanisms of two-chambered MFC and single-chambered MFC, including the basic characteristics of power generation, the effects of anodic substrate, cathode enhancement as well as improvements of air-cathode MFC design mode. Some innovative findings have been made as follows.The results demonstrated that both the two-chambered and single-chambered MFC after being started up by using anaerobic activated sludge as inoculated source could stably generate power with simultaneous COD removal when acetate was offered as the single carbon source. The maximum power density produced followed the order of K3[Fe(CN)6]-cathode MFC>air-cathode MFC>aeration-cathode MFC. CE and COD in such three MFCs was 49 %, 28.2 %, 33.8 % and 60 %, 45.3 %, 80 %, respectively. Besides, the performance of the two-chambered MFC was limited by the anode while the performance of the single-chambered MFC was mainly limited by the cathode, which was determined by the inherent nature of the MFC configuration. The main reason for Coulombic loss occurring in the two-chambered MFC and single-chambered MFC should be anaerobic loss and aerobic loss, respectively.Additionally, MFC was demonstrated capable of using a wide variety of organic compounds for power generation, including single-sugar, multi-sugar as well as several types of small molecular organic fatty acid and ethanol. The dependence of power density on initial COD concentration exhibited"saturation effect", which could be described and explained with traditional Monod equation. The mixing was observed to have different impact on the MFC performance, depending upon the COD concentrtion, which was most likely due to the difference in diffusion of substrate into the biofilm. In the two-chambered MFC, buffering capacity of the anodic electrolyte played an important role in maintaining pH within a stable level. Increasing ion strength could result in a decrease of internal resistance as well as an increase of power density. High concentration of NH3-N was observed to have no obvious adverse influence on MFC performance. The oxidation of partial NH3-N in single-chambered MFC was achieved with oxygen as electron acceptor and independent on current output. In the two-chamber MFC, NH3-N could be transferred with the form of NH4+ cation into the cathode through PEM in order to balance the charge.Using acidic permanganate as the cathodic electron acceptor could increase the open circuit potential (OCP) of the two-chambered MFC up to 1.48 V with a higher corresponding maximum power density than that using ferricyanide and oxygen. Unlike permanganate concentration, the cathode potential was shown more sensitive to pH value. At permanganate concentration of 20 mg·L-1 and pH=3.5, the maximum power density produced in bushing MFC could reach a level as high as 3986.72 mW·m-2. Both SEM and XPS analysis confirmed the mechanisms that the main reduced product of permanganate was solid-state MnO2 deposited on the electrode surface. Its redox potential as great as +1.70 V could explain the most probable reason for high OCP in the permanganate-cathode MFC, which was well consistent with the experimental results shown above. For single-chambered MFC, power density could be apparently increased by means of either reducing the electrode spacing or increasing the cathode area because of the corresponding decrease of internal resistance of the cell. The Pt loading and Nafion content were revealed to have a co-effect on power output, i. e., power density was substantially increased with the increase of Pt loading in the absence of Nafion binder solution, while such effect was absent in the presence of Nafion solution.An up-flow air-cathode MFC (UAMFC) was designed for more available applications for wastewater treatment. As the organic loading rate (OLR) increased, COD removing efficiency, CE, EE and pH tended to decline and power density revealed a setpoint, which might be attributed to the limitation of treating capacity and acid accumulation. As indicated by electrochemical impedance spectroscopy (EIS) analysis, charge transfer resistance (Rc), ohmic resistance (Rohm) and diffusion resistance (Rd) accounted for 22.6 %,50.2 % and 26.3 % of total internal resistance Large fraction of ohmic resistance mainly originated from the inherent limitation of the system in relation to the reactor design structure and electrolyte characteristics. Recirculation was found to have a substantial impact on electrochemical performance (power, Rohm and Rd) at low OLR, while such effect was absent at high OLR. Removal of nitrogen compounds in the UAMFC was obtained along with power generation. It was also shown here that the long-term operation might lead to the performance degradation of the UAMFC, as indicated by a decrease of power density at a rate of 0.051 W·m-3·d-1 and an increase of Rohm and Rd. Such design is more advantageous by its virtue of low internal resistance and high power density, which provides a proof-in-concept and new approach to optimize the air-cathode MFC design and operation.