Boosting ethanol production rates from carbon dioxide in MES cells under optimal solventogenic conditions

Selective butyrate production from CO2 and methanol in microbial electrosynthesis - influence of pH

Methanol assisted microbial electrosynthesis (MES) enables butyrate production from carbon dioxide and methanol using external electricity. However, the effects of operational parameters on butyrate formation remain unclear. By running three flat plate MES reactors with fed-batch mode at three controlled pH values (5.5, 6 and 7), the present study investigated the influence of pH on methanol assisted MES by comparing the process performance, microbial community structure, and genetic potential. The highest butyrate selectivity (87 % on carbon basis) and the highest butyrate production rate of 0.3 g L-1 d-1 were obtained at pH 6. At pH 7, a comparable butyrate production rate was achieved, yet with a lower selectivity (70 %) accompanied with acetate production. Butyrate production rate was considerably hindered at pH 5.5, reaching 0.1 g L-1 d-1, while the selectivity reached was up to 81 %. Methanol and CO2 consumption increased with pH, along with more negative cathodic potential and more negative redox potential. Furthermore, pH affected the thermodynamical feasibility of involved reactions. The results of metagenomic analyses suggest that Eubacterium callanderi dominated the microbial communities at all pH values, which was responsible for methanol and CO2 assimilation via the Wood-Ljungdahl pathway and was likely the main butyrate producer via the reverse β-oxidation pathway.

Biochemical production with microbial bioelectrochemical systems

Microbial bioelectrochemical systems (BES) represent a promising platform for sustainable biochemical production by leveraging microbial electrocatalysis. These systems utilize electrical energy to drive microbial metabolic processes, enabling the recovery of CO₂ into valuable organic molecules such as methane, acetate, ethanol, and other biochemicals. This approach aligns with global efforts to mitigate greenhouse gas emissions and create circular carbon economies. The advancement of BES technology requires both scale-down and scale-up strategies to ensure feasibility and scalability. Scale-down approaches focus on optimizing operational parameters, enhancing electron transfer efficiencies, and understanding microbial community dynamics under controlled conditions. Scale-up efforts address the challenges of maintaining system stability, energy efficiency, and economic viability in larger, industrial-scale operations. Together, these strategies bridge the gap between fundamental laboratory research and real-world applications, positioning microbial BES as a key technology for sustainable biochemical production and captured carbon utilization.

Microbial Electrochemical Technologies: Sustainable Solutions for Addressing Environmental Challenges

Addressing global challenges of waste management demands innovative approaches to turn biowaste into valuable resources. This chapter explores the potential of microbial electrochemical technologies (METs) as an alternative opportunity for biowaste valorisation and resource recovery due to their potential to address limitations associated with traditional methods. METs leverage microbial-driven oxidation and reduction reactions, enabling the conversion of different feedstocks into energy or value-added products. Their versatility spans across gas, food, water and soil streams, offering multiple solutions at different technological readiness levels to advance several sustainable development goals (SDGs) set out in the 2030 Agenda. By critically examining recent studies, this chapter uncovers challenges, optimisation strategies, and future research directions for real-world MET implementations. The integration of economic perspectives with technological developments provides a comprehensive understanding of the opportunities and demands associated with METs in advancing the circular economy agenda, emphasising their pivotal role in waste minimisation, resource efficiency promotion, and closed-loop system renovation.

Enhanced bioethanol production by evolved Escherichia coli LGE2-H in a microbial electrolysis cell system

Lignocellulose pretreated using pyrolysis can yield clean energy (such as bioethanol) via microbial fermentation, which can significantly contribute to waste recycling, environmental protection, and energy security. However, the acids, aldehydes, and phenols present in bio-oil with inhibitory effects on microorganisms compromise the downstream utilization and conversion of lignocellulosic pyrolysates. In this study, we constructed a microbial electrolysis cell system for bio-oil detoxification and efficient ethanol production using evolved Escherichia coli to overcome the bioethanol production and utilization challenges highlighted in previous studies. In electrically treated bio-oil media, the E. coli-H strain exhibited significantly higher levoglucosan consumption and ethanol production capacities compared with the control. In undetoxified bio-oil media containing 1.0% (w/v) levoglucosan, E. coli-H produced 0.54 g ethanol/g levoglucosan, reaching 94% of the theoretical yield. Our findings will contribute to developing a practical method for bioethanol production from lignocellulosic substrates, and provide a scientific basis and technical demonstration for its industrialized application.

Selective butyric acid production from CO2 and its upgrade to butanol in microbial electrosynthesis cells

Microbial electrosynthesis (MES) is a promising carbon utilization technology, but the low-value products (i.e., acetate or methane) and the high electric power demand hinder its industrial adoption. In this study, electrically efficient MES cells with a low ohmic resistance of 15.7 mΩ m2 were operated galvanostatically in fed-batch mode, alternating periods of high CO2 and H2 availability. This promoted acetic acid and ethanol production, ultimately triggering selective (78% on a carbon basis) butyric acid production via chain elongation. An average production rate of 14.5 g m-2 d-1 was obtained at an applied current of 1.0 or 1.5 mA cm-2, being Megasphaera sp. the key chain elongating player. Inoculating a second cell with the catholyte containing the enriched community resulted in butyric acid production at the same rate as the previous cell, but the lag phase was reduced by 82%. Furthermore, interrupting the CO2 feeding and setting a constant pH2 of 1.7-1.8 atm in the cathode compartment triggered solventogenic butanol production at a pH below 4.8. The efficient cell design resulted in average cell voltages of 2.6-2.8 V and a remarkably low electric energy requirement of 34.6 kWhel kg-1 of butyric acid produced, despite coulombic efficiencies being restricted to 45% due to the cross-over of O2 and H2 through the membrane. In conclusion, this study revealed the optimal operating conditions to achieve energy-efficient butyric acid production from CO2 and suggested a strategy to further upgrade it to valuable butanol.