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Microbial electromethanogenesis (EM), as a sustainable bioderived carbon-neutrality catalyzing platform, can be accelerated and regulated by weak power input for carbon fixation into value-added bioenergy. Solar electricity as a day-night intermittent renewable resource has been verified to effectively directly drive microbes to capture CO2. However, understanding the influence mechanisms of higher CO2 loading on EM is of intrinsic significance yet lacking. Herein, natural solar-powered bioelectrocatalytic CO2 reduction to CH4 under increasing bicarbonate concentrations was investigated. CH4 recovery for the long-term measurement showed that CH4 production rate positively responded to improved bicarbonate concentrations from 2.5 to 10 g HCO3-·L-1, exhibiting a robust and potent competence in CH4 yield compared to reported EM. Whereas exceed bicarbonate mainly contributed to raised pH in the solution resulting in the proton limitation despite the intermittent driven-mode could mitigate pH shock. Electrochemistry results demonstrated that higher bicarbonate concentrations promoted the redox activity of electrode biofilm and lowered the system resistances, especially the charge transfer resistance. Adequately improving CO2 loading can dynamically optimize the structure of anodic electroactive microorganisms and facilitate electron transfer. Furthermore, more functional cathodic mcrA genes were upregulated with elevated bicarbonates and the species of basophilic Methanobacterium alcaliphilum occupied predominated at the cathode. These findings open up a perspective avenue to carbon reduction using natural solar intermittent-powered EM.

Microbe-electrode interacted microbial electrochemical systems (MESs) encompasing (electro)microbiology, electrochemistry, and material science, play an ever-increasing role in waste(water) treatment and resource recovery, which are perceived as eco-friendly and bioderived carbon-neutral catalysis technologies. However, external electricity input to drive the microbial metabolism in MESs can be expensive or not environmentally friendly, hampering the broader development of MESs. This perspective summarizes present renewable electricity sources from microbial full cells, salinity gradients, and solar light that have been demonstrated to drive MESs, followed by underexploited renewable power supplies from waste heat, self-powered triboelectric nanogenerators (mechanical energy harvester), etc. Future directions emphasizing electromicrobiology for MESs toward carbon-neutral are remarked.

Wastewater, as the used water, carries huge energy that is frequently ignored and unexploited. Microbes, as bioelectrocatalysts in microbial electrochemical systems (MESs), can convert chemical energy stored in biodegradable matrixes from wastewater to bioelectricity and chemicals. However, due to sluggish wastewater treatment rates and bioenergy production, wider applications have been hampered. Currently, photo-assisted MESs, combined electrochemical/photochemical driving force with microbial catalysis, have emerged as a sustainable platform to enhance pollutants degradation and bioenergy recovery from wastewater with the aid of solar light, accompanied by reasonable energy investment and minimal environmental disturbance. Nevertheless, the development of photo-assisted MESs is still in its infancy. This work broadly concludes present photo-assisted MESs wastewater treatment technologies and their overall limitations in terms of performance and the future advancements that will be necessary to make them more widely applicable. Herein, crucial factors influencing the performance of these photo-assisted MESs, such as the reactor types (bioanode-photocathode, photoanode-biocathode, photomicrobial electrode, and photosynthetic bacteria/algae MESs), the bandgap of semiconductor and microbe species are discussed. Furthermore, prominent research accomplishments of photo-assisted MESs with an emphasis on eliminating contaminants (initial concentration, removal efficiency, and removal rate) and recovering bioenergy (product types, production rate, and current density) from various wastewaters are systematically summarized. Finally, present challenges and prospects in the field of photo-assisted MESs technology are discussed, mainly including the optimization of electrode materials, screening and culture of microorganisms, scale-up of bioreactors, intermittency of solar energy, and other complications overarchingly shared with photo-assisted microbial electrochemical wastew

Lately, extracellular electron transfer (EET) is widely disclosed on the surface of the bioelectrodes, and conductive (bio)carriers involved in anaerobic biodegradation/biosynthesis. By electrostimulation, microbial consortia colonize the electrodes and accelerate substrate (waste/wastewater) metabolization on the bioanode or biosynthesize value-added products (methane, acetate, etc.) on the biocathode. However, the connections and contributions of planktonic microbial communities have not been effectually understood. Herein, electromethanogenesis were comprehensively investigated in response to different driving-force modes: intermittent electric field applied by manual on-off or natural solar power and continuous electrical field. Intermittent modes implied preferable electron transfer efficiency, higher methane yield and energy recovery efficiencies from wastewater by the microbes in the bulk solutions. Microbial community analysis revealed that less electroactive microorganisms and acetotrophic methanogens in the bulk solutions were accommodated under the intermittent modes than the continuous electrical field, whereas more fermentative bacteria and hydrogenotrophic methanogens evolved in the intermittent driving modes, implying that the interspecies electron transfer both on and away from the electrodes were favorably regulated. Redundancy and network analysis proved that more complicated ecological interactions were shown in the bulk solutions with the periodic on/off of electrical field. These results hinted that the electrostimulation effectively regulated EET bacteria, even in the bulk solutions, while more efficient electron flow to methane through interspecies electron transfer was developed during the intermittent driving regulation.

Electroactive microorganisms (EAMs) can act as pseudocapacitor to store energy and discharge electrons on need, while electromethanogens acting as receptor are able to utilize electrons, protons and carbon dioxide for methanization. However, external energy is required to overcome thermodynamical barriers for electromethanogenesis. Herein, electro-driving power by solar light was established to accelerate conversion of waste organics to bioenergy. The intermittent power supply modes were elucidated for favourable performances (e.g., current density, methane production rate, energy recovery efficiencies and economic evaluation), compared with the control driven by continuous applied voltage. It was found that natural intermittent solar-powered mode was more beneficial for microorganisms involved in electron transfer and energy recovery than manual sharp on-off mode. Electrochemistry analysis unrevealed that a higher redox current and lower resistance were exhibited under the solar-powered mode. A high charge storage capacity and electron mobility were found through cytochrome c content and live cells ratio in the solar-power assisted bioreactor. The intermittent power driving modes can regulate electron transfer proteins with capacitive storage behavior in biomass, which helps to understand the responses of functional communities on the stress of intermittent electric field. These findings indicate a promising perspective of microbial biotechnology driven by solar power to boost bioenergy recovery from waste/wastewater.

Anaerobic digestion (AD) represents a potential way to achieve energy recovery from waste organics. In this study, a novel bioelectrochemically-assisted anaerobic reactor is assembled by two AD systems separated by anion exchange membrane, with the cathode placing in the inside cylinder (cathodic AD) and the anode on the outside cylinder (anodic AD). In cathodic AD, average methane production rate goes up to 0.070 mL CH4/mL reactor/day, which is 2.59 times higher than AD control reactor (0.027 m3 CH4/m3/d). And COD removal is increased ~15% over AD control. When changing to sludge fermentation liquid, methane production rate has been further increased to 0.247 mL CH4/mL reactor/day (increased by 51.53% comparing with AD control). Energy recovery efficiency presents profitable gains, and economic revenue from increased methane totally self-cover the cost of input electricity. The study indicates that cathodic AD could cost-effectively enhance methane production rate and degradation of glucose and fermentative liquid.

Methane production rate (MPR) in waste activated sludge (WAS) digestion processes is typically limitedby the initial steps of complex organic matter degradation, leading to a limited MPR due to sludgefermentation speed of solid particles. In this study, a novel microbial electrolysis AD reactor (ME-AD) wasused to accelerate methane production for energy recovery from WAS. Carbon bioconversion wasaccelerated by ME producing H2 at the cathode. MPR was enhanced to 91.8 gCH4/m3 reactor/d in themicrobial electrolysis ME-AD reactor, thus improving the rate by 3 times compared to control conditions (30.6 gCH4/m3 reactor/d in AD). The methane production yield reached 116.2 mg/g VSS in the ME-ADreactor. According to balance calculation on electron transfer and methane yield, the increasedmethane production was mostly dependent on electron contribution through the ME system. Thus, theuse of the novel ME-AD reactor allowed to significantly enhance carbon degradation and methaneproduction from WAS.

Background: Bioelectrochemical systems have been considered a promising novel technology that shows an enhanced energy recovery, as well as generation of value-added products. A number of recent studies suggested that an enhancement of carbon conversion and biogas production can be achieved in an integrated system of microbial electrolysis cell (MEC) and anaerobic digestion (AD) for waste activated sludge (WAS). Microbial communities in integrated system would build a thorough energetic and metabolic interaction network regarding fermentation communities and electrode respiring communities. The characterization of integrated community structure and community shifts is not well understood, however, it starts to attract interest of scientists and engineers. Results: In the present work, energy recovery and WAS conversion are comprehensively affected by typical pretreated biosolid characteristics. We investigated the interaction of fermentation communities and electrode respiring communities in an integrated system of WAS fermentation and MEC for hydrogen recovery. A high energy recovery was achieved in the MECs feeding WAS fermentation liquid through alkaline pretreatment. Some anaerobes belonging to Firmicutes (Acetoanaerobium, Acetobacterium, and Fusibacter) showed synergistic relationship with exoelectrogensin the degradation of complex organic matter or recycling of MEC products (H2). High protein and polysaccharide but low fatty acid content led to the dominance of Proteiniclasticum and Parabacteroides, which showed a delayed contribution to the extracellular electron transport leading to a slow cascade utilization of WAS. Conclusions: Efficient pretreatment could supply more short-chain fatty acids and higher conductivities in the fermentative liquid, which facilitated mass transfer in anodic biofilm. The overall performance of WAS cascade utilization was substantially related to the microbial community structures, which in turn depended on the initial pretreatm

Bioaugmented fermentation of cellulosic substrates to produce biohydrogen via co-culture of isolated strains was investigated. Two mesophilic anaerobic bacterial strains, known for their ability to hydrolyze cellulosic substrates, were taken in consideration: Shigella flexneri str. G3, which shows high cellulolytic activity but cannot ferment oligosaccharides to bioenergy, and Clostridium acetobutylicum X9, able to convert microcrystalline cellulose into hydrogen. The ability of the selected strains to effectively convert different cellulosic substrates to hydrogen was tested on carboxymethyl cellulose (AVICEL), as well as pretreated lignocellulosic material such as Bermuda grass, corn stover, rice straw, and corn cob. Results showed that co-culture of Shigella flexneri str G3 and Clostridium acetobutylicum X9 efficiently improved cellulose hydrolysis and subsequent hydrogen production from carboxymethyl cellulose. Hydrogen production yield was enhanced from 0.65 mol H2 (mol glucose)−1 of the X9 single culture to approximately 1.5 mol H2 (mol glucose)−1 of the co-culture, while the cellulose degradation efficiency increased from 50% to 95%. Co-culture also efficiently improved hydrogen production from natural lignocellulosic materials (which was up to 4-5 times higher than mono-culture with X9), with the highest performance of 24.8 mmol L-1 obtained on Bermuda grass. The results demonstrate that co-culture of S. flexneri G3 and C. acetobutylicum X9 was capable of efficiently enhance cellulose conversion to hydrogen, thus fostering potential biofuel applications under mesophilic conditions.