A cathodic electro-fermentation system for enhancing butyric acid production from rice straw with a mixed culture

Efficient methane fermentation from the waste of a novel straw alkali-heat pretreatment-butyric acid fermentation process

ABSTRACTThe butyric acid biorefinery technology for straw is highly significant for environmental protection and the restructuring of the energy system. However, this process produces waste from alkali-heat pretreatment (PW) and butyric acid fermentation (FW). In this study, the feasibility of methane fermentation from the wastes was confirmed, with the methane production from PW and FW of 351.1 ± 11.8 and 741.5 ± 14.2 mLCH4/gVS, respectively. The initial pH and VFW/VPW of methane fermentation using the mixed waste of PW and FW were optimized at 7.5 and 1.8, respectively. The methane fermentation using the mixed waste was also verified by operating two anaerobic digesters in sequencing batch mode. At the VFW/VPW of 0.25 (actual ratio), methane production was 301.20 mLCH4/gVS with the waste load of 0.64 kgVS/m³/d. When the VFW/VPW was 1.8 (optimal ratio), methane production reached 396.45 mLCH4/gVS at the waste load of 1.20 kgVS/m3/d. This study facilitates the comprehensive utilization of all components within rice straw.

13C-Labelled Glucose Reveals Shifts in Fermentation Pathway During Cathodic Electro-Fermentation with Mixed Microbial Culture

Cathodic Electro-Fermentation (CEF) is an innovative approach to manage the spectrum of products deriving from anaerobic fermentation. Herein, mixed microbial culture fermentation using a ternary mixture containing labelled 13C glucose and non-labelled acetate and ethanol was studied to identify the role of polarization on the metabolic pathways of glucose fermentation. CEF at an applied potential of -700 mV (vs. SHE, Standard Hydrogen Electrode) enhanced the production yield of acetate, propionate, and butyrate (0.90±0.10, 0.22±0.03, and 0.34±0.05 mol/mol; respectively) compared to control tests performed at open circuit potential (OCP) (0.54±0.09, 0.15±0.04, and 0.20±0.001 mol/mol, respectively). Results indicate that CEF affected the 13C labelled fermented product levels and their fractional 13C enrichments, allowing to establish metabolic pathway models. This work demonstrates that, under cathodic polarization, the abundance of both fully 13C labelled propionate and butyrate isotopomers increased compared to control tests. The effect of CEF is mainly due to intermediates initially produced from the glucose metabolic transformation in the presence of non-labelled acetate and ethanol as external substrates. These findings represent a significant advancement in current knowledge of CEF, which offers a promising tool to control mixed cultures bioprocesses.

Efficient caproate production from lignocellulose via single-step electro-fermentation platform without organic electron donor

Caproate production by microbial fermentation gained the advantages of sustainability and eco-friendliness, but challenged by sterile fermentation environment, necessity of organic electron donors. Here, a single-step electro-fermentation (EF) process of mixed culture was proposed for caprate production from rice straw. At the optimal potential of -0.8 V, caproate concentration, yield and selectivity in the neutral red (NR)-mediated EF system were 2.4 g/L, 0.2 g/g and 26.6%. Long-term operation accumulated 5.3 g/L caproate with the yield and selectivity of 0.2 g/g and 34.2% in the EF+NR system. Bioaugmentation by dosing chain-elongation microbial consortium further improved the caproate production, yield and selectivity to 9.1 g/L, 0.3 g/g and 41.5%, respectively. The improved caproate production in the bioaugmented EF+NR system was likely due to the enhanced interspecies electron transfer, reconstructed microbial community, multiple electron donors and suitable pH environment. Present study offers a feasible strategy for cost-effective caprate production directly from waste biomass.

The convergence of lactic acid microbiomes and metabolites in long-term electrofermentation

Regulating electron transfer in predominantly fermentative microbiomes has broad implications in environmental, chemical, food, and medical fields. Here we demonstrate electrochemical control in fermenting food waste, digestate, and wastewater to improve lactic acid production. We hypothesize that applying anodic potential will expedite and direct fermentation towards lactic acid. Continued operation that introduced epi/endophytic communities (Lactococcus, Lactobacillus, Weissella) to pure culture Lactiplantibacillus plantarum reactors with static electrodes was associated with the loss of anode-induced process intensification despite 80% L. plantarum retention. Employing fluidized electrodes discouraged biofilm formation and extended electrode influence to planktonic gram-positive fermenters using mediated extracellular electron transfer. While short-term experiments differentially enriched Lactococcus and Klebsiella spp., longer-term operations indicated convergent microbiomes and product spectra. These results highlight a functional resilience of environmental fermentative microbiomes to perturbations in redox potential, underscoring the need to better understand electrode induced polymicrobial interactions and physiological impacts to engineer tunable open-culture or synthetic consortia.

Acidogenic fermentation of potato peel waste for volatile fatty acids production: Effect of initial organic load

As a renewable carbon source produced from organic wastes by acidogenic fermentation, volatile fatty acids (VFAs) are important intermediates in chemical and biological fields and beneficial to resource recovery and carbon neutrality. Maximizing VFA production by some strategies without additional chemicals is critical to increasing economic and environmental benefits. In this study, the effects of initial organic load (OL) on the performance of VFA production, variations of intermediate metabolites, and the thermogravimetric properties of potato peel waste (PPW) during batch acidogenic fermentation were studied. The results showed that the concentration of VFAs increased with the increase of initial OL, while the VFA yield decreased with the increase of initial OL. When the initial OL was in the range of 28.4 g VS/L-91.3 g VS/L, the fermentation type of PPW was butyric acid fermentation. The highest butyric acid proportion of 61.3% was achieved with the initial OL of 71.5 g VS/L. With the increase of initial OL, the proportion of acetic acid and the utilization rate of protein in the PPW decreased. VFAs were produced from proteins and carbohydrates in the early stage and mainly produced from carbohydrates in the later stage. The production efficiency of VFA was relatively high with the initial OL of 71.5 g VS/L, because more easily-biodegradable compounds were solubilized. The results showed that suitably increased initial OL could accelerate acidogenesis, reduce hydrolysis time, and increase the proportion of butyric acid. The findings in this work suggest that PPW is a promising feedstock for butyric acid biosynthesis and appropriate initial OL is beneficial to VFA production.

Efficient butyrate production from rice straw in an optimized cathodic electro-fermentation process

Butyrate production from renewable biomass shows great potential against climate change and over-consumption of fossil fuels. Herein, key operational parameters of a cathodic electro-fermentation (CEF) process were optimized for efficient butyrate production from rice straw by mixed culture. The cathode potential, controlled pH and initial substrate dosage were optimized at -1.0 V (vs Ag/AgCl), 7.0 and 30 g/L, respectively. Under the optimal conditions, 12.50 g/L butyrate with yield of 0.51 g/g-rice straw were obtained in batch-operated CEF system. In fed-batch mode, butyrate production significantly increased to 19.66 g/L with the yield of 0.33 g/g-rice straw, but 45.99% butyrate selectivity still needs to be improved in future. Enriched butyrate producing bacteria (Clostridium cluster XIVa and IV) with proportion of 58.75% on the 21st day of the fed-batch fermentation, contributed to the high-level butyrate production. The study provides a promising approach for efficient butyrate production from lignocellulosic biomass.

Highly selective butanol production by manipulating electron flow via cathodic electro-fermentation

Butanol production by solventogenic Clostridia shows great potential to combat the energy crisis, but is still challenged by low butanol selectivity and high downstream cost. In this study, a novel cathodic electro-fermentation (CEF) system mediated by methyl viologen (MV) was proposed and sequentially optimized to obtain highly selective butanol production. Under the optimal conditions (-0.60 V cathode potential, 0.50 mM MV, 30 g/L glucose), 7.17 ± 0.55 g/L butanol production were achieved with the yield of 0.32 ± 0.02 g/g. With the supplement of 4 g/L butyric acid as co-substrate, butanol production further improved to 13.14 ± 1.14 g/L with butanol yield and selectivity as high as 0.43 ± 0.01 g/g and 90.44 ± 1.66%, respectively. The polarized electrode enabled the unbalanced fermentation towards butanol formation and MV further inhibited hydrogen production, both of which contributed to the high-level butanol production and selectivity. The MV-mediated CEF system is a promising approach for cost-effective bio-butanol production.

Highly selective butyric acid production by coupled acidogenesis and ion substitution electrodialysis

Selective production of carboxylic acids (CAs) from mixed culture fermentation remains a difficult task in organic waste valorization. Herein, we developed a facile and sustainable carbon loop strategy to regulate the fermentation micro-environment and steer acidogenesis towards selective butyric acid production. This new ion substitution electrodialysis-anaerobic membrane bioreactor (ISED-AnMBR) integrated system demonstrated a high butyric acid production at 11.19 g/L with a mass fraction of 76.05%. In comparison, only 1.04 g/L with a mass fraction of 30.56% was observed in the uncoupled control reactor. The carbon recovery reached a maximum of 96.09% with the assistance of ISED. Inorganic carbon assimilation was believed to be an important contributor, which was verified by ¹³C isotopic tracing. Microbial community structure shows the dominance of Clostridia (80.16%) in the unique micro-environment (e.g., pH 4.80-5.50) controlled by ISED, which is believed beneficial to the growth of such fermentative bacteria with main products of butyric acid and acetic acid. In addition, the emergence of chain elongators such as Clostridium sensu stricto 12 was observed to have a great influence on butyric acid production. This work provides a new approach to generate tailored longer chain carboxylic acids from organic waste with high titer thus contributing to a circular economy.

Recent advances in electro-fermentation technology: A novel approach towards balanced fermentation

Biotransformation of organic substrates via acidogenic fermentation (AF) to high-value products such as C1-C6 carboxylic acids and alcohol serves as platform chemicals for various industrial applications. However, the AF technology suffers from low product titers due to thermodynamic constraints. Recent studies suggest that augmenting AF redox potential can regulate the metabolic pathway and provide seamless electron flow by lowering the activation energy barrier, thus positively influencing the substrate utilization rate, product yield, and speciation. Hence, the augmented AF system with an exogenous electricity supply is termed as electro-fermentation (EF), which has enormous potential to strengthen the fermentation technology domain. Therefore, this critical review systematically discusses the current understanding of EF with a special focus on the extracellular electron transfer mechanism of electroactive bacteria and provides perspectives and research gaps to further improve the technology for green chemical synthesis, sustainable waste management, and circular bio-economy.

Food Waste Biorefinery: Pathway towards Circular Bioeconomy

Food waste biorefineries for the production of biofuels, platform chemicals and other bio-based materials can significantly reduce a huge environmental burden and provide sustainable resources for the production of chemicals and materials. This will significantly contribute to the transition of the linear based economy to a more circular economy. A variety of chemicals, biofuels and materials can be produced from food waste by the integrated biorefinery approach. This enhances the bioeconomy and helps toward the design of more green, ecofriendly, and sustainable methods of material productions that contribute to sustainable development goals. The waste biorefinery is a tool to achieve a value-added product that can provide a better utilization of materials and resources while minimizing and/or eliminating environmental impacts. Recently, food waste biorefineries have gained momentum for the production of biofuels, chemicals, and bio-based materials due to the shifting of regulations and policies towards sustainable development. This review attempts to explore the state of the art of food waste biorefinery and the products associated with it.