The external resistance ( R _ext ) of microbial fuel cells (MFCs) regulates both the anode availability as an electron acceptor and the electron flux through the circuit. We evaluated the effects of R _ext on MFCs using acetate or glucose. The average current densities ( I ) ranged from 40.5 mA/m ² (9,800 Ω) to 284.5 mA/m ² (150 Ω) for acetate-fed MFCs (acetate-fed reactors [ARs]), with a corresponding anode potential ( E _an ) range of −188 to −4 mV (versus a standard hydrogen electrode [SHE]). For glucose-fed MFCs (glucose-fed reactors [GRs]), I ranged from 40.0 mA/m ² (9,800 Ω) to 273.0 mA/m ² (150 Ω), with a corresponding E _an range of −189 to −7 mV. ARs produced higher Coulombic efficiencies and energy efficiencies than GRs over all tested R _ext levels because of electron and potential losses from glucose fermentation. Biogas production accounted for 14 to 18% of electron flux in GRs but only 0 to 6% of that in ARs. GRs produced similar levels of methane, regardless of the R _ext . However, total methane production in ARs increased as R _ext increased, suggesting that E _an might influence the competition for substrates between exoelectrogens and methanogens in ARs. An increase of R _ext to 9,800 Ω significantly changed the anode bacterial communities for both ARs and GRs, while operating at 970 Ω and 150 Ω had little effect. Deltaproteobacteria and Bacteroidetes were the major groups found in anode communities in ARs and GRs. Betaproteobacteria and Gammaproteobacteria were found only in ARs. Bacilli were abundant only in GRs. The anode-methanogenic communities were dominated by Methanosaetaceae , with significantly lower numbers of Methanomicrobiales . These results show that R _ext affects not only the E _an and current generation but also the anode biofilm community and methanogenesis.

In recent years, there has been a significant accumulation of waste in the environment, and it is expected that this accumulation may increase in the years to come. Waste disposal has massive effects on the environment and can cause serious environmental problems. Thus, the development of a waste treatment system is of major importance. Agro-industrial wastewater and waste residues are mainly rich in organic substances, lignocellulose, hemicellulose, lignin, and they have a relatively high amount of energy. As a result, an effective agro-waste treatment system has several benefits, including energy recovery and waste stabilization. To reduce the impact of the consumption of fossil energy sources on our planet, the exploitation of renewable sources has been relaunched. All over the world, efforts have been made to recover energy from agricultural waste, considering global energy security as the final goal. To attain this objective, several technologies and recovery methods have been developed in recent years. The microbial fuel cell (MFC) is one of them. This review describes the power generation using various types of agro-industrial wastewaters and agricultural residues utilizing MFC. It also highlights the techno-economics and lifecycle assessment of MFC, its commercialization, along with challenges.