Electron flux and microbial community in microbial fuel cells (open-circuit and closed-circuit modes) and fermentation

An intermittent power supply scheme to minimize electrical energy input in a microbial electrolysis cell assisted anaerobic digester

From the perspective of energy saving in the operation of microbial electrolysis cell assisted anaerobic digester (MEC-AD), this study focused on developing an intermittent power supply scheme. The applied potential was switched off for 12 and 6 hours/day during the operation of a laboratory-scale MEC-AD system fed with glucose. The results from the operation under continuous applied potential served as the control. The overall biomethane generation and net energy income from the process were unaffected when the applied potential turned off for 6 hours/day. Both quantitative and qualitative analyses of microbial communities suggested that a balanced microbiome could be maintained under short-term switching-off the applied potential. However, performance substantially deteriorated when the applied potential turned off for 12 hours/day. Overall, the results of this study suggest that MEC-AD operation does not need a continuous power supply, and higher energy efficiency can be effectively achieved by intermittently powering the reactor.

Bio-electrochemical reactors using AMI-7001S and CMI-7000S membranes as separators for silver recovery and power generation

Bioelectrochemical reactors using AMI-7001S and CMI-7000S membranes were investigated for silver recovery and power generation. High silver removal (83.73-92.50%) and columbic efficiency (11.50-19.89%) were obtained in CMI-based reactor after 24h, although, some diffusion of Ag⁺ ions was observed. In contrast, substrate loss was found in AMI-based reactor, which caused low overall performance. At an initial Ag⁺ concentration 2000mg/L, a maximum power density of 5396mW/m³ and 3385mW/m³ were obtained in CMI and AMI-based reactor, respectively. Under SEM analysis, different morphologies of metallic silver deposits were detected on the cathode surfaces, and was confirmed by EDX and XRD technique. High diversity of anodic microbial communities were also found, in which Firmulates, Proteobacteria, and Bateroidetes were dominant phyla. Geobacter sp. and other exoelectrogens could be detected while no Shewanella-like sequences were retrieved. The study suggests that CMI-7000S showed better performance when compared to AMI-7001S at the same operational conditions.

Factors influencing silver recovery and power generation in bio-electrochemical reactors

The recovery of silver from Ag⁺ solution coupled with power generation was investigated in bio-electrochemical system (BES). In this system, chemical energy existing in the organic matter in the anode chamber can be converted biologically to electrical energy which can be used for the reduction of Ag⁺ ions in the cathode chamber. Results showed that type of substrate influenced the metabolic pathway and affected the cell voltage progression, and columbic efficiency. Silver recovery was not affected by increasing initial pH (2.0 to 7.0) and Ag⁺ concentration (100 to 1000 mg/L) in the catholyte, whereas power generation was improved. A maximum power density of 8258 mW/m³ and a columbic efficiency of 21.61% could be achieved with 1000 mg/L of Ag⁺. Ag⁺ ions were reduced to form metallic deposits as Ag⁰ crystals on the cathode surface, which were then confirmed by scanning electron microscope (SEM) image and energy dispersive X-ray (EDX) spectrum. The BES reactor had high silver removal (i.e., >96%) after 24 h of operation. When considering the crossover of Ag⁺ ions through the cation exchange membrane, the removal was in the range of 83.73-92.51%. This crossover was not considerable as compared to the Ag⁺ initial concentration. At higher initial Ag⁺ concentration (2000 mg/L), the silver removal decreased to 88.61% and the maximum power density decreased to 5396 mW/m³. This study clearly showed that BES can be employed for silver recovery, wastewater treatment, and also electricity generation.

Impact of Anodic Respiration on Biopolymer Production and Consequent Membrane Fouling

Microbial fuel cells (MFCs) have recently been integrated with membrane bioreactors (MBRs) for wastewater treatment and energy recovery. However, the impact of integration of the two reactors on membrane fouling of MBR has not been reported yet. In this study, MFCs equipped with different external resistances (1-10 000 ohm) were operated, and membrane-fouling potentials of the MFC anode effluents were directly measured to study the impact of anodic respiration by exoelectrogens on membrane fouling. It was found that although the COD removal efficiency was comparable, the fouling potential was significantly reduced due to less production of biopolymer (a major foulant) in MFCs equipped with lower external resistance (i.e., with higher current generation) as compared with aerobic respiration. Furthermore, it was confirmed that Geobacter sulfurreducens strain PCA, a dominant exoelectrogen in anode biofilms of MFCs in this study, produced less biopolymer under anodic respiration condition than fumarate (anaerobic) respiration condition, resulting in lower membrane-fouling potential. Taken together, anodic respiration can mitigate membrane fouling of MBR due to lower biopolymer production, suggesting that development of an electrode-assisted MBR (e-MBR) without aeration is feasible.