Recovery of electrical energy is a key parameter for evaluating the performance of microbial fuel cells (MFCs). In this brief review, we analyze energy data in the sampled publications on continuously operated MFCs from the past 12 years and present a rough picture of energy recovery in MFCs. We observe that most MFCs produce a normalized energy recovery (NER) lower than 1.5 kWh/m3 or 1.0 kWh/kg of chemical oxygen demand (COD). The small MFCs (<100 mL) that produce high power densities do not exhibit any obvious advantage in NER compared with the larger MFCs. Pure substrates lead to better performance in both power and energy recovery. MFCs seem to be able to extract more energy (kilowatt hour per kilogram of COD) from low-strength substrates. The separator/membrane does not significantly affect NER. To establish an energy balance in MFCs and gain a better understanding of the MFC application niche, NER should be properly presented in future studies.

Two 4 L tubular microbial fuel cells (MFCs) were installed in a municipal wastewater treatment facility and operated for more than 400 days on primary effluents. Both MFCs removed 65-70% chemical oxygen demand (COD) at a hydraulic retention time (HRT) of 11 h and reduced about 50% suspended solids. The COD removal rates were about 0.4 (total) or 0.2 (soluble) kg m(-3) day(-1). They could handle fluctuation, such as emptying the anode for 1-3 days or different HRTs. The preliminary analysis of energy production and consumption indicated that the two MFCs could theoretically achieve a positive energy balance and energy consumption could be reduced using larger tubing connectors. Through linkage to a denitrifying MFC, the MFC system improved the removal of total nitrogen from 27.1 to 76.2%; however, the energy production substantially decreased because of organic consumption in the denitrifying MFC. Establishing a carbon (electron) balance revealed that sulfate reduction was a major electron scavenger (37-64%) and methane production played a very minor role (1.3-3.3%) in electron distribution. These results demonstrate the technical viability of MFC technology outside the laboratory and its potential advantages in low energy consumption, low sludge production, and energy recovery from wastes.

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A modularized microbial fuel cell system can effectively treat primary effluent and use the produced energy to offset part of its energy consumption.