• Energy Research

Abstract The interest in the use of electroactive microorganisms for different applications by means of bioelectrochemical systems (BES) has been constantly increasing since the last decade. The main application of BES among others, which has received a widespread attention and researched extensively, is the microbial fuel cell (MFC) technology that relies on the electrogenic nature of certain bacteria to simultaneously treat different wastewaters and produce electric power. Various types of wastewaters have been examined as substrates for feeding bacteria in MFCs. The number and complexity of wastewaters have increased rapidly over the last few years thus necessitating the need for documenting this data further. This review provides a comprehensive and the state-of-the-art information on various wastewater substrates that have been used in MFCs. The performance of different types (designs) of MFCs in terms of electric current and power outputs together with the wastewater treatment efficiency in terms of chemical oxygen demand (COD) removal and columbic efficiency (CE) is presented. Some of the challenges and future perspectives with regard to the energy recovery from wastewaters using MFCs are briefly discussed.

The pH-value played a crucial role for the development and current production of anodic microbial electroactive biofilms. It was demonstrated that only a narrow pH-window, ranging from pH 6 to 9, was suitable for growth and operation of biofilms derived from pH-neutral wastewater. Any stronger deviation from pH neutral conditions led to a substantial decrease in the biofilm performance. Thus, average current densities of 151, 821 and 730 μA cm(-2) were measured for anode biofilms grown and operated at pH 6, 7 and 9 respectively. The microbial diversity of the anode chamber community during the biofilm selection process was studied using the low cost method flow-cytometry. Thereby, it was demonstrated that the pH value as well as the microbial inocula had an impact on the resulting anode community structure. As shown by cyclic voltammetry the electron transfer thermodynamics of the biofilms was strongly depending on the solution's pH-value.

Electroactive microorganisms (EAMs) are a group of microbes that can access solid extracellular electron donors or acceptors via extracellular electron transfer processes. EAMs are useful in developing various microbial electrochemical technologies. This protocol describes the use of bioelectrochemical systems (BESs) to enrich EAMs at the cathode from an extreme haloalkaline habitat. It also provides information for a detailed characterization of enriched cathodic biofilms via various cross-disciplinary techniques, including electrochemical, analytical, microscopic, and gene sequencing techniques. For complete details on the use and execution of this protocol, please refer to Chaudhary et al. (2021).

Biogas is one of the promising futuristic renewable energy sources with enormous market potential. However, the presence of CO2 lowers down the calorific value of biogas. Hence, various biogas upgradation technologies are under intense investigation to increase the methane content to the desired level. This study reports on enhancing methane content in biogas through CO2 sequestration into acetic acid via microbial electrosynthesis (MES) process. The previously enriched mixed chemolithoautotrophic microbial culture dominated by Acetobacterium spp. used CO2 present in the biogas as the sole carbon source. After establishing a stable performing biocathode at a fixed cathodic potential of −1 V (vs. Ag/AgCl) through batch mode operation, biogas was fed continuously at different feed rates, viz., 0.5, 0.3, and 0.2 ml/min to the cathode chamber. The highest feed rate of 0.5 ml/min was least effective both for methane content increment (from 61 ± 3% to 86 ± 2%) and acetic acid titer (1.5 ± 0.5 g/L; 0.107 ± 0.02 g/L/d.). In comparison, the lowest flow rate of 0.2 ml/min was the most effective for the intended process (methane upgradation from 62 ± 7% to 93 ± 3% and acetic acid titer 3.4 ± 0.6 g/L produced at 0.24 ± 0.04 g/L/d rate). Both acetic acid bioproduction and biogas upgradation occurred best at an Ecell of 3.3 ± 0.35 V at the low feed rate. A maximum of 84 ± 7%, 57 ± 10% and 29 ± 2% coulombic, carbon and energetic efficiencies, respectively, were achieved in acetic acid. Cyclic voltammograms of biocathodes revealed the decrease in hydrogen evolution potential and increased bioelectrocatalysis, thereby suggesting the contribution of microbes in the process. Acetobacterium, which is known for CO2 fixation, was found to be the dominant microbial genus in biogas fed reactors. The demonstrated approach not only offers the advantage of obtaining two products, one in the bulk phase and the other in the off-gas, it also validates the applicability of the bioelectrochemical biogas upgradation technology.

Here, we present an unusual approach of modifying the growth pattern of electroactive bacteria for improving their communication with electrodes. Cells of a known exoelectrogen, Shewanella oneidensis MR-1, were treated with cisplatin, a drug that inhibits cell division but forces the cells to grow longer resulting in a characteristic elongated growth. Such elongated cells showed up to a 5-fold improvement in current densities compared to normal, untreated cells.

Using saline electrolytes in combination with halophilic CO2-fixing lithotrophic microbial catalysts has been envisioned as a promising strategy to develop an energy-efficient microbial electrosynthesis (MES) process for CO2 utilization. Here, an enriched marine CO2-fixing lithotrophic microbial community dominated by Vibrio and Clostridium spp. was tested for MES of organic acids from CO2. At an applied Ecathode of -1V (vs Ag/AgCl) with 3.5 % salinity (78 mScm-1), it produced 379 ± 53 mg/L (6.31 ± 0.89 mM) acetic acid and 187 ± 43 mg/L (4.05 ± 0.94 mM) formic acid at 2.1 ± 0.30 and 1.35 ± 0.31 mM day-1, respectively production rates. Most electrons were recovered in acetate (68.3 ± 3 %), formate (9.6 ± 1.2 %) besides hydrogen (11 ± 1.4 %) and biomass (8.9 ± 1.65 %). Notably, the bioproduction of organic acids occurred at a high energetic efficiency (EE) of ∼ 46 % and low Ecell of 2.3 V in saline conditions compared to the commonly used non-saline electrolytes (0.5-1 mScm-1) in the reported MES studies with CO2 (Ecell: >2.5 V and EE: <34 %).

The advent of renewable energy conversion systems exacerbates the existing issue of intermittent excess power. Microbial electrosynthesis can use this power to capture CO2 and produce multicarbon compounds as a form of energy storage. As catalysts, microbial populations can be used, provided side reactions such as methanogenesis are avoided. Here a simple but effective approach is presented based on enrichment of a robust microbial community via several culture transfers with H2:CO2 conditions. This culture produced acetate at a concentration of 1.29 ± 0.15 g L–1 (maximum up to 1.5 g L–1; 25 mM) from CO2 at a fixed current of −5 Am–2 in fed-batch bioelectrochemical reactors at high N2:CO2 flow rates. Continuous supply of reducing equivalents enabled acetate production at a rate of 19 ± 2 gm–2d–1 (projected cathode area) in several independent experiments. This is a considerably high rate compared with other unmodified carbon-based cathodes. 58 ± 5% of the electrons was recovered in acetate, whereas 30 ± 1...

This study aimed to understand the wastewater treatment and electricity generation performance besides the microbial communities of the integrated Hydroponics-Microbial Electrochemical Technology (iHydroMET) systems operated with water-saturated and water-unsaturated reactors. The organics removal was slightly higher in the water-unsaturated system (93 ± 4 %) than in the water-saturated system (87 ± 2 %). The total nitrogen removal and electric voltage were considerably higher in the water-saturated system (42 ± 5 %; 111 ± 8 V per reactor) than in the water-unsaturated system (18 ± 3 %; 95 ± 9 V per reactor). The enhanced organics and nitrogen removal and high voltage output in respective conditions were due to the dominance of polysaccharide-degrading aerobes (e.g., Pirellula), anammox bacteria (e.g., Anammoximicrobium), denitrifiers (e.g., Thauera and Rheinheimera), and electroactive microorganisms (e.g., Geobacter). The differential performance governed by distinct microbial communities under the tested conditions indicates that an appropriate balancing of water saturation and unsaturation in reactors is crucial to achieving optimum iHydroMET performance.