T. Krieg, F. Mayer, D. Sell, D. Holtmann
Microbial fuel cells (MFCs) are often discussed as a part of a sustainable generation of electricity for the coming "energy revolution". In particular the applications of MFCs in wastewater treatment are often regarded as an attractive alternative method to reduce the costs of wastewater treatment and generate electricity. At the moment there is a lack of application of MFCs in real wastewater treatment plants (WWTPs). However, investigations in real plants are absolutely necessary to show the applicability of MFCs in WWTPs considering typical daily fluctuations and environmental effects like rain events affecting MFC performance remarkably. In this study a MFC system was tested in four municipal WWTPs using different modes of operation. A correlation between the current densities and the sludge loading was identified. At low sludge loadings the activated sludge needs a large amount of the energy derived from the substrate for the maintenance metabolism and therefore the resulting current densities of the MFC are quite low. At high sludge loadings much more of the energy can be transferred from the activated sludge to the electrode, resulting in higher currents. Furthermore, the effect of harsh environmental conditions (e.g. an extreme rainfall in winter) on the current densities was evaluated. In general, WWTPs have daily fluctuations depending on the wastewater composition, weather phenomena and population equivalents. However, our data show that these daily fluctuations can be only observed in the MFC performance at smaller WWTPs below 50,000 population equivalents.