The cost of treating municipal and industrial wastewater using activated sludge processes has been increasing every year, mainly because of enormous energy requirement. Experts predict that nanofiber membrane based microbial fuel cells can replace activated sludge processes for waste water treatment due its potential to reduce the energy usage.
A typical microbial fuel cell (MFC) consists of an anode and a cathode, which are separated by a proton exchange membrane. Microbial fuel cell (MFC) uses microbes (cultured) that convert the organic wastes present in the water through chemical reaction (oxidation-reduction), and transport electrons in the electrical circuit for the generation of electric power. The oxidation process occurs at anode compartment, the generated electron transfers to cathode department where it involves in reduction process. Bacteria or yeasts, the microbes which act as biocatalysts and are capable of extracellular electron transfer are called ‘electricigens’. Thus, microbial fuel cells are capable of generating electricity while clearing organic waste from water that help reduce the amount of external electricity to power the plant’s operating equipment, thus greatly reduce the costs.
The power output in the microbial fuel cells has been presently limited by proton and electron transfer from the anode (biocatalyst) to the cathode. The factors that affect the performance of the MFC include the microorganisms, membrane, electrode, cell resistance, ionic strength of the solution, electrode spacing and the distance between the electrodes. Amongst various factors, the membrane is considered to be the most important, as it contributes nearly 35% of the overall capital cost of MFCs.
Nafion polymer membranes and dispersions have been choice for the fuel cell industries and are used as proton exchange membrane (PEM). The importance of the PEM is the conductivity of the protons, as well as blocking the oxygen migration from the cathode to the anode and the substrate migration from the anode to the cathode. Scientists explored ways to modify the Nafion membrane to improve the proton conductivity and block the crossover of oxygen by impregnating with porous substrate or cross linking or blending with hydrophobic polymer such as polyvinylidene fluoride (PVDF). Fuel cells R&D team from Malaysia and Iran led by Prof. Mostafa Ghasemi investigated the possibility of Nafion/PVDF nanofiber composite membrane to solve the limitation of Nafion membrane. Nanofiber membrane characteristics include one dimensional network structure, high surface area, high porosity, sufficient solvent uptake and high mechanical strength convinced them to choose nanofibers as a material for membrane.
|Dr. Mostafa Ghasemi|
The team produced PVDF nanofibres with diameter range of 100–400 nm using industry-capable electrospinning process from 15 wt.% PVDF sol-gel (DMF: Acetone solvent ratio of 5:5) mixed with Nafion/ isopropanol dispersion. The team found that the structure of the PVDF nanofibres and mechanical strength were unaffected by the addition of the Nafion solution, however noted that thermal stability of PVDF/Nafion membranes was decreased by increasing the amount of Nafion.
MFC built with yeast microorganisms as a biocatalyst, and Nafion/PVDF nanofiber composite as membrane showed the highest power generation, with 4.9 mW/m2 at 57.6 mA/m2,which was more than (16%) that of Nafion 117. Furthermore, high amount of organics (more than 70% COD) removal was noticed which was due to the conversion of the organic substrate by yeast microbes in the microbial fuel cells through the Nafion/PVDF nanofiber composite membrane.
This study demonstrated that PVDF nanofibres played a major role in Nafion composite membranes, and the MFC with nanofiber composite membrane offers multifold benefits; organic waste removal, electricity (energy) production, and lower operating costs.
This study has opened a new path for the commercialization of MFCs in wastewater treatment, as well as power generation. Industries that produce wastewaters high in easily degradable organic include the food industries, dairies, breweries, biotech industries can be benefitted from such MFCs application.