Advancements in medium chain fatty acids production through chain elongation: Key mechanisms and innovative solutions for overcoming rate-limiting steps

The depletion of fossil fuels has prompted an urgent search for alternative chemicals from renewable sources. Current technology in medium chain fatty acids (MCFAs) production though chain elongation (CE) is becoming increasingly sustainable, hence the motivation for this review, which provides the detailed description, insights and analysis of the metabolic pathways, substrates type, inoculum and fermentation process. The main rate-limiting steps of microbial MCFAs production were comprehensively revealed and the corresponding innovative solutions were also critically evaluated. Innovative strategies such as substrate pretreatment, electrochemical regulation, product separation, fermentation parameter optimization, and electroactive additives have shown significant advantages in overcoming the rate-limiting steps. Furthermore, novel regulatory strategies such as quorum sensing and electronic bifurcation are expected to further increase the MCFAs yield. Finally, the techno-economic analysis was carried out, and the future research focuses were also put forward.

Variability in n-caprylate and n-caproate producing microbiomes in reactors with in-line product extraction

Medium-chain carboxylates (MCCs) are used in various industrial applications. These chemicals are typically extracted from palm oil, which is deemed not sustainable. Recent research has focused on microbial chain elongation using reactors to produce MCCs, such as n-caproate (C6) and n-caprylate (C8), from organic substrates such as wastes. Even though the production of n-caproate is relatively well-characterized, bacteria and metabolic pathways that are responsible for n-caprylate production are not. Here, three 5 L reactors with continuous membrane-based liquid-liquid extraction (i.e., pertraction) were fed ethanol and acetate and operated for an operating period of 234 days with different operating conditions. Metagenomic and metaproteomic analyses were employed. n-Caprylate production rates and reactor microbiomes differed between reactors even when operated similarly due to differences in H2 and O2 between the reactors. The complete reverse β-oxidation (RBOX) pathway was present and expressed by several bacterial species in the Clostridia class. Several Oscillibacter spp., including Oscillibacter valericigenes, were positively correlated with n-caprylate production rates, while Clostridium kluyveri was positively correlated with n-caproate production. Pseudoclavibacter caeni, which is a strictly aerobic bacterium, was abundant across all the operating periods, regardless of n-caprylate production rates. This study provides insight into microbiota that are associated with n-caprylate production in open-culture reactors and provides ideas for further work.IMPORTANCEMicrobial chain elongation pathways in open-culture biotechnology systems can be utilized to convert organic waste and industrial side streams into valuable industrial chemicals. Here, we investigated the microbiota and metabolic pathways that produce medium-chain carboxylates (MCCs), including n-caproate (C6) and n-caprylate (C8), in reactors with in-line product extraction. Although the reactors in this study were operated similarly, different microbial communities dominated and were responsible for chain elongation. We found that different microbiota were responsible for n-caproate or n-caprylate production, and this can inform engineers on how to operate the systems better. We also observed which changes in operating conditions steered the production toward and away from n-caprylate, but more work is necessary to ascertain a mechanistic understanding that could be predictive. This study provides pertinent research questions for future work.

Influence of pH and temperature on the performance and microbial community during the production of medium-chain carboxylic acids using winery effluents as substrate

Winery effluents containing high ethanol concentrations and diverse organic matter are ideal substrates for producing medium-chain carboxylic acids via fermentation and chain elongation. However, the process needs to be better understood. This study presents novel insights into the bioconversion mechanisms of medium-chain carboxylic acids by correlating fermentation and chain elongation kinetic profiles with the study of microbial communities at different pH (5 to 7) conditions and temperatures (30 to 40 °C). It was found that high productivities of MCCA were obtained using a native culture and winery effluents as a natural substrate. Minor pH variations significantly affected the metabolic pathway of the microorganisms for MCCA production. The maximal productivities of hexanoic (715 mg/L/d) and octanoic (350 mg/L/d) acids were found at pH 6 and 35 °C. Results evidence that the presence of Clostridium, Bacteroides, and Negativicutes promotes the high productions of MCCA. The formation of heptanoic acid was favor when Mogibacterium and Burkholderia were present.

Co-electron donors driven medium-chain fatty acids production: Roles of electron donors, reaction kinetics and metabolic pathways

Energy conversion of waste activated sludge alkaline fermentation liquor (WASAFL) to medium-chain fatty acids (MCFAs) is promising for sludge treatment and carbon recovery. However, the single electron donor (ED) fermentation for MCFAs production has irreparable defects. To resolve the respective shortcomings of single electron donor (ED) and improve the MCFAs production efficiency from WASAFL, a novel biotechnical process utilizing ethanol and lactate as co-EDs within different combination ratios were investigated. The results verified that MCFAs production was highest with ethanol to lactate ratio of 1:3 (6988.54 ± 208.18 mg COD/L), being 1.46 and 1.87 times of that with ethanol and lactate as single ED. The kinetic analysis results confirmed that ethanol to lactate ratio of 1:3 resulted in the highest MCFAs yield and formation rate. The microbial taxa results uncovered that the relative abundance of Sphaerochaeta and Haloimpatiens showed positive correlation with MCFAs production. The metabolic pathway analysis indicated that the ethanol oxidization, lactate oxidization, acrylate pathway, reverse β oxidization and fatty acid biosynthesis pathway might take place in the WASAFL fermentation system, contributing to the WASAFL-to-MCFAs conversion.

Conceptual system for sustainable and next-generation wastewater resource recovery facilities

Shifting the concept of municipal wastewater treatment to recover resources is one of the key factors contributing to a sustainable society. A novel concept based on research is proposed to recover four main bio-based products from municipal wastewater while reaching the necessary regulatory standards. The main resource recovery units of the proposed system include upflow anaerobic sludge blanket reactor for the recovery of biogas (as product 1) from mainstream municipal wastewater after primary sedimentation. Sewage sludge is co-fermented with external organic waste such as food waste for volatile fatty acids (VFAs) production as precursors for other bio-based production. A portion of the VFA mixture (product 2) is used as carbon sources in the denitrification step of the nitrification/denitrification process as an alternative for nitrogen removal. The other alternative for nitrogen removal is the partial nitrification/anammx process. The VFA mixture is separated with nanofiltration/reverse osmosis membrane technology into low-carbon VFAs and high-carbon VFAs. Polyhydroxyalkanoate (as product 3) is produced from the low-carbon VFAs. Using membrane contactor-based processes and ion-exchange techniques, high-carbon VFAs are recovered as one-type VFA (pure VFA) and in ester forms (product 4). The nutrient-rich fermented and dewatered biosolid is applied as a fertilizer. The proposed units are seen as individual resource recovery systems as well as a concept of an integrated system. A qualitative environmental assessment of the proposed resource recovery units confirms the positive environmental impacts of the proposed system.

Effect of granular activated carbon and chloroform on chain elongation with simple substrate ethanol and acetate

Chain elongation is a promising technology for production of medium-chain fatty acids (MCFAs). Granular activated carbon (GAC) is commonly used in anaerobic fermentation. Low level CHCl₃ can inhibit methanogenesis and homoacetogenesis at the same time. However, the effect of them on chain elongation performance with highly enriched consortia and simple substrate (i.e., ethanol and acetate) was still unclear. Hence, the effects of CHCl₃ and on MCFAs production and the microbial community was studied here. CHCl₃ displayed fatal effect on chain elongation system when its concentration was higher than 0.1% v/v. 0.05% v/v CHCl₃ was enough to inhibit homoacetogens and further decreased the caproate production efficiency without altering the core bacteria tremendously. GAC was found to be adverse for chain elongation with simple substrate (i.e., ethanol and acetate) and highly enriched microbial consortia dominated by Clostridium sensu stricto, less than 20% electrons were finally distributed in caproate. It might be attributed to other electron consuming activities induced by GAC.

One-stage anaerobic fermentation of excess sludge for caproate production by supplementing chain elongation enrichments with ethanol as electron donor

Medium chain fatty acids (MCFAs) production from excess sludge have recently received great research interest due to higher energy densities, easy-separation capability and high economic benefits. Here, the addition of chain elongation (CE) enrichments with ethanol as electron donor was used to enhance caproate production from one-stage sludge fermentation. Compared with 0.20 g/L of controls, caproate production reached 9.00 g/L by supplementing CE enrichments with ethanol/acetate ratio of 3:1 after 7 days of acidification of organic matter in pretreated sludge fermentation. Clostridium_sensu_stricto_12, that refers to CE, was enriched in the first and second transfer of the sludge microbial consortium. Maintaining the stability of the microbial consortium would be the key that enables stable and efficient caproate production from sludge fermentation by supplementing CE enrichments.

Produce individual medium chain carboxylic acids (MCCA) from swine manure: Performance evaluation and economic analysis

Environmental issues caused by untreated animal manure require the development of resource recovery from waste through a circular economy approach. Producing medium chain carboxylic acids (MCCA) with higher value than biogas from manure has become promising. The objective of this study was to develop an effective individual MCCA produce process utilizing manure. In this study, animal manure was firstly anaerobic fermentation into short chain fatty acids (SCFA), then acidified manure and ethanol were fed into the chain elongation reactor with gradually increasing the organic loading rate (OLR) from 7.0 to 18.5 gCOD/L/d, and the mixed MCCA was separated individually via a fractional distillation process. The SCFA fermentation occurred mainly at the first 10 days, and the optimum concentrations of SCFA for treatments at 2 %VS, 4 %VS and 6 %VS were 6.58, 10.40 and 14.10 g/L, respectively. For the chain elongation reactor, the maximum concentrations of n-caproate and n-caprylate were 10.25 and 0.63 g/L, respectively, which were comparable with that obtained from other complex wastes. Over 90% MCCA can be recovered from the fermentation broth via the optimized extractant of methyl tert-butyl ether (MTBE) and the fractional distillation system. Preliminary economic analysis shows that this MCCA production process presented a higher economic benefit (9.25 $/m³ manure) than traditional biogas production (2.65 $/m³ manure), making MCCA production from swine manure economically competitive. This work provides a new route for manure resource recovery besides the biogas process.

Recovery Techniques Enabling Circular Chemistry from Wastewater

In an era where it becomes less and less accepted to just send waste to landfills and release wastewater into the environment without treatment, numerous initiatives are pursued to facilitate chemical production from waste. This includes microbial conversions of waste in digesters, and with this type of approach, a variety of chemicals can be produced. Typical for digestion systems is that the products are present only in (very) dilute amounts. For such productions to be technically and economically interesting to pursue, it is of key importance that effective product recovery strategies are being developed. In this review, we focus on the recovery of biologically produced carboxylic acids, including volatile fatty acids (VFAs), medium-chain carboxylic acids (MCCAs), long-chain dicarboxylic acids (LCDAs) being directly produced by microorganisms, and indirectly produced unsaturated short-chain acids (USCA), as well as polymers. Key recovery techniques for carboxylic acids in solution include liquid-liquid extraction, adsorption, and membrane separations. The route toward USCA is discussed, including their production by thermal treatment of intracellular polyhydroxyalkanoates (PHA) polymers and the downstream separations. Polymers included in this review are extracellular polymeric substances (EPS). Strategies for fractionation of the different fractions of EPS are discussed, aiming at the valorization of both polysaccharides and proteins. It is concluded that several separation strategies have the potential to further develop the wastewater valorization chains.

Transforming waste activated sludge into medium chain fatty acids in continuous two-stage anaerobic fermentation: Demonstration at different pH levels

Bioenergy recovery in the form of medium-chain fatty acids (MCFAs) from waste activated sludge (WAS) is increasingly attractive, which are valuable building blocks for fuel production. This study experimentally demonstrated the long-term MCFAs (C6-C8) production from WAS in two-stage anaerobic sludge fermentation at different pH conditions, using continuously operated bench-scale anaerobic reactors. The WAS was continuously converted to short chain fatty acids (SCFAs, 3500-3800 mg chemical oxygen demand (COD)/L) at the first stage via alkaline anaerobic fermentation, which was directly fed into the second stage as both substrates and inoculum for MCFAs production through chain elongation (CE). The productions of MCFAs at the second stage were continuously studied under three different pH conditions (i.e., 10, 7 and 5.5). The results demonstrated that there was no significant MCFAs production at pH 10 during the steady state, whereas the MCFAs productions were clearly observed at both pH 7 and pH 5.5, with much higher MCFAs production from WAS at pH 7 (i.e., 10.32 g COD/L MCFAs) than that at pH 5.5 (i.e., 8.73 g COD/L MCFAs) during the steady state. A higher MCFAs selectivity of 62.3% was also achieved at pH 7. The relatively lower MCFAs production and selectivity at pH 5.5 was likely due to the higher undissociated MCFAs generated at pH 5.5, which would pose toxicity impact on CE microbes and thus inhibit the CE process. Microbial community analysis confirmed that the relative abundances of CE related microbes (e.g., Clostridium sensu stricto 12 sp. and Clostridium sensu stricto 1) increased at pH 7 compared to those at pH 5.5, which enabled more efficient MCFAs production from WAS.

Reactor microbiome enriches vegetable oil with n-caproate and n-caprylate for potential functionalized feed additive production via extractive lactate-based chain elongation

BACKGROUND

Biotechnological processes for efficient resource recovery from residual materials rely on complex conversions carried out by reactor microbiomes. Chain elongation microbiomes produce valuable medium-chain carboxylates (MCC) that can be used as biobased starting materials in the chemical, agriculture and food industry. In this study, sunflower oil is used as an application-compatible solvent to accumulate microbially produced MCC during extractive lactate-based chain elongation. The MCC-enriched solvent is harvested as a potential novel product for direct application without further MCC purification, e.g., direct use for animal nutrition. Sunflower oil biocompatibility, in situ extraction performance and effects on chain elongation were evaluated in batch and continuous experiments. Microbial community composition and dynamics of continuous experiments were analyzed based on 16S rRNA gene sequencing data. Potential applications of MCC-enriched solvents along with future research directions are discussed.

RESULTS

Sunflower oil showed high MCC extraction specificity and similar biocompatibility to oleyl alcohol in batch extractive fermentation of lactate and food waste. Continuous chain elongation microbiomes produced the MCC n-caproate (nC6) and n-caprylate (nC8) from L-lactate and acetate at pH 5.0 standing high undissociated n-caproic acid concentrations (3 g L^-1). Extractive chain elongation with sunflower oil relieved apparent toxicity of MCC and production rates and selectivities reached maximum values of 5.16 ± 0.41 g nC6 L^-1 d^-1 (MCC: 11.5 g COD L^-1 d^-1) and 84 ± 5% (e^- eq MCC per e^- eq products), respectively. MCC were selectively enriched in sunflower oil to concentrations up to 72 g nC6 L^-1 and 3 g nC8 L^-1, equivalent to 8.3 wt% in MCC-enriched sunflower oil. Fermentation at pH 7.0 produced propionate and n-butyrate instead of MCC. Sunflower oil showed stable linoleic and oleic acids composition during extractive chain elongation regardless of pH conditions. Reactor microbiomes showed reduced diversity at pH 5.0 with MCC production linked to Caproiciproducens co-occurring with Clostridium tyrobutyricum, Clostridium luticellarii and Lactobacillus species. Abundant taxa at pH 7.0 were Anaerotignum, Lachnospiraceae and Sporoanaerobacter.

CONCLUSIONS

Sunflower oil is a suitable biobased solvent to selectively concentrate MCC. Extractive reactor microbiomes produced MCC with improved selectivity and production rate, while downstream processing complexity was reduced. Potential applications of MCC-enriched solvents may include feed, food and biofuels purposes.

Biological production of medium-chain carboxylates through chain elongation: An overview

Medium chain carboxylates (MCCs) have wide applications in various industries, but the traditional MCCs production methods are costly and unsustainable. Anaerobic fermentation offers a more scalable, economical and eco-friendly platform for producing MCCs through chain elongation which converts short chain carboxylates and electron donor into more valuable MCCs. However, the underlying microbial pathways are not well understood. In this review, biological production of MCCs through chain elongation is introduced elaborately, including the metabolic pathways, electron donor and substrates, microorganisms and influencing factors. Then, the strategies for enhancing MCCs production are extensively analyzed and summarized, along with the technologies for MCCs separation from the fermentation broth. Finally, challenges and perspectives concerning the large-scale MCCs production are proposed, providing suggestions for the future research. Extensive review demonstrated that anaerobic fermentation has great potential in achieving economical and sustainable MCCs production from complex organic substrates, including organic waste streams, which would significantly broaden the application of MCCs, especially in the renewable energy field. An interdisciplinary approach with knowledge from microbiology and biochemistry to chemical separations and environmental engineering is required to use this promising technology as a valorization method for converting organic biomass or organic wastes into valuable MCCs.

Membrane bioreactors for the production of value-added products: Recent developments, challenges and perspectives

The potential of membrane bioreactors to produce value-added products such as biofuels, biopolymers, proteins, organic acids and lipids at high productivities is emerging. Despite the promising results at laboratory scale, industrial deployment of this technology is hindered due to challenges associated with scale-up. This review aims to address these challenges and create a framework to encourage further research directed towards industrial application of membrane bioreactors to produce value-added products. This review describes the current state-of-the art in such bioreactor systems by exploiting membranes to increase the mass transfer rate of the limiting substrates, reach high cell concentrations and separate the inhibitory substances that may inhibit the bioconversion reaction. It also covers the current trends in commercialization, challenges linked with membrane usage, such as high costs and membrane fouling, and proposes possible future directions for the wider application of membrane bioreactors.

Opportunities and challenges in microbial medium chain fatty acids production from waste biomass

Medium chain fatty acids (MCFAs) that produced from affordable waste biomass via chain elongation (CE) technology are recognized as the potential alternatives to part fossil-derived chemicals, contributing to the sustainable development of economy and environment. The purpose of this review is to provide comprehensive analyses on the opportunities and challenges of MCFAs production and application. First, both two microbial MCFAs synthesis pathways of reverse β-oxidation and fatty acid biosynthesis were introduced/compared in detail to give readers a thorough understanding of the CE process, with the expectation of further boosting MCFAs production by well distinguishing them. Furthermore, the six key MCFAs production bottlenecks, corresponding research progresses, and possible solutions were analyzed. Five major MCFAs production strategies with their production mechanism, performances, and characteristics were also critically assessed. Additionally, the commercial production status was introduced, and future alternative production mode and research priorities were also recommended.

Long-Term Continuous Extraction of Medium-Chain Carboxylates by Pertraction With Submerged Hollow-Fiber Membranes

Medium-chain carboxylic acids (MCCAs), which can be generated from organic waste and agro-industrial side streams through microbial chain elongation, are valuable chemicals with numerous industrial applications. Membrane-based liquid-liquid extraction (pertraction) as a downstream separation process to extract MCCAs has been applied successfully. Here, a novel pertraction system with submerged hollow-fiber membranes in the fermentation bioreactor was applied to increase the MCCA extraction rate and reduce the footprint. The highest average surface-corrected MCCA extraction rate of 655.2 ± 86.4 mmol C m⁻² d⁻¹ was obtained, which was higher than any other previous reports, albeit the relatively small surface area removed only 11.6% of the introduced carbon via pertraction. This submerged extraction system was able to continuously extract MCCAs with a high extraction rate for more than 8 months. The average extraction rate of MCCA by internal membrane was 3.0- to 4.7-fold higher than the external pertraction (traditional pertraction) in the same bioreactor. A broth upflow velocity of 7.6 m h⁻¹ was more efficient to extract MCCAs when compared to periodic biogas recirculation operation as a means to prevent membrane fouling. An even higher broth upflow velocity of 40.5 m h⁻¹ resulted in a significant increase in methane production, losing more than 30% of carbon conversion to methane due to a loss of H₂, and a subsequent drop in the H₂ partial pressure. This resulted in the shift from a microbial community with chain elongators as the key functional group to methanogens, because the drop in H₂ partial pressure led to thermodynamic conditions that oxidizes ethanol and carboxylic acids to acetate and H₂ with methanogens as the syntrophic partner. Thus, operators of chain elongating systems should monitor the H₂ partial pressure when changes in operating conditions are made.

Anaerobic digestion of organic waste: Recovery of value-added and inhibitory compounds from liquid fraction of digestate

Anaerobic digestion, as an eco-friendly waste treatment technology, is facing the problem of low stability and low product value. Harvesting value-added products beyond methane and removing the inhibitory compounds will unleash new vitality of anaerobic digestion, which need to be achieved by selective separation of certain compounds. Various methods are reviewed in this study for separating valuable products (volatile fatty acids, medium-chain carboxylic acids, lactic acid) and inhibitory substance (ammonia) from the liquid fraction of digestate, including their performance, applicability, corresponding limitations and roadmaps for improvement. In-situ extraction that allows simultaneous production and extraction is seen as promising approach which carries good potential to overcome the barriers for continuous production. The prospects and challenges of the future development are further analyzed based on in-situ extraction and economics.

Carboxylates and alcohols production in an autotrophic hydrogen-based membrane biofilm reactor

Microbiological conversion of CO₂ into biofuels and/or organic industrial feedstock is an excellent carbon-cycling strategy. Here, autotrophic anaerobic bacteria in the membrane biofilm reactor (MBfR) transferred electrons from hydrogen gas (H₂ ) to inorganic carbon (IC) and produced organic acids and alcohols. We systematically varied the H₂ -delivery, the IC concentration, and the hydraulic retention time in the MBfR. The relative availability of H₂ versus IC was the determining factor for enabling microbial chain elongation (MCE). When the H₂ :IC mole ratio was high (>2.0 mol H₂ /mol C), MCE was an important process, generating medium-chain carboxylates up to octanoate (C8, 9.1 ± 1.3 mM C and 28.1 ± 4.1 mmol C m^-2  d^-1 ). Conversely, products with two carbons were the only ones present when the H₂ :IC ratio was low (<2.0 mol H₂ /mol C), so that H₂ was the limiting factor. The biofilm microbial community was enriched in phylotypes most similar to the well-known acetogen Acetobacterium for all conditions tested, but phylotypes closely related with families capable of MCE (e.g., Bacteroidales, Rhodocyclaceae, Alcaligenaceae, Thermoanaerobacteriales, and Erysipelotrichaceae) became important when the H₂ :IC ratio was high. Thus, proper management of IC availability and H₂ supply allowed control over community structure and function, reflected by the chain length of the carboxylates and alcohols produced in the MBfR.

Selective Extraction of Medium-Chain Carboxylic Acids by Electrodialysis and Phase Separation

Carboxylic acids obtained via the microbial electrochemical conversion of waste gases containing carbon dioxide (i.e., microbial electrosynthesis) can be used in lieu of nonrenewable building-block chemicals in the manufacture of a variety of products. When targeting valuable medium-chain carboxylic acids such as caproic acid, electricity-driven fermentations can be limited by the accumulation of fermentation products in the culturing media, often resulting in low volumetric productivities and titers due to direct toxicity or inhibition of the biocatalyst. In this study, we tested the effectiveness of a simple electrodialysis system in upconcentrating carboxylic acids from a model solution mimicking the effluent of a microbial electrochemical system producing short- and medium-chain carboxylic acids. Under batch extraction conditions, the electrodialysis scheme enabled the recovery of 60% (mol mol-1) of the total carboxylic acids present in the model fermentation broth. The particular arrangement of conventional monopolar ion exchange membranes and hydraulic recirculation loops allowed the progressive acidification of the extraction solution, enabling phase separation of caproic acid as an immiscible oil with 76% purity.

Continuous medium chain carboxylic acids production from excess sludge by granular chain-elongation process

Short chain carboxylic acids (SCCAs) production is one of the primary ways to recycle excess sludge (ES). However, the high cost for the SCCAs separation/extraction due to its complete miscibility in water hinders the practical application of SCCAs and the popularization of this recycling way. To overcome this barrier, this study performed an emerging chain elongation (CE) technology to upgrade the SCCAs-rich sludge fermentation broth into the highly hydrophobic medium chain carboxylic acids (MCCAs). In a continuous expanded granule sludge bed (EGSB) reactor, a maximal MCCAs yield of 67.39 % and the corresponding concentration of 9.80 g COD/L (224.97 mM C/L) were achieved. By supplying CO₂ at a loading rate of 2 [Formula: see text] to lower the hydrogen partial pressure, the ethanol utilization rate and the resulting MCCAs yield were further improved. In addition, three branched-MCCAs including iso-caproate, iso-heptylate, and iso-caprylate were obtained the first time from waste biomass with the average proportions of 6.17 %, 3.65 %, and 0.8 %, respectively. The branched-MCCAs came from the CE of branched-SCCAs. The granule sludges performing CE were mainly consisted of rod-shaped cells, and dominated by Clostridium sensu stricto and Clostridium IV. This study is expected to lay a foundation for recycling ES to MCCAs.

The Isolate Caproiciproducens sp. 7D4C2 Produces n-Caproate at Mildly Acidic Conditions From Hexoses: Genome and rBOX Comparison With Related Strains and Chain-Elongating Bacteria

Bulk production of medium-chain carboxylates (MCCs) with 6-12 carbon atoms is of great interest to biotechnology. Open cultures (e.g., reactor microbiomes) have been utilized to generate MCCs in bioreactors. When in-line MCC extraction and prevention of product inhibition is required, the bioreactors have been operated at mildly acidic pH (5.0-5.5). However, model chain-elongating bacteria grow optimally at neutral pH values. Here, we isolated a chain-elongating bacterium (strain 7D4C2) that grows at mildly acidic pH. We studied its metabolism and compared its whole genome and the reverse β-oxidation (rBOX) genes to other bacteria. Strain 7D4C2 produces lactate, acetate, n-butyrate, n-caproate, biomass, and H2/CO2 from hexoses. With only fructose as substrate (pH 5.5), the maximum n-caproate specificity (i.e., products per other carboxylates produced) was 60.9 ± 1.5%. However, this was considerably higher at 83.1 ± 0.44% when both fructose and n-butyrate (electron acceptor) were combined as a substrate. A comparison of 7D4C2 cultures with fructose and n-butyrate with an increasing pH value from 4.5 to 9.0 showed a decreasing n-caproate specificity from ∼92% at mildly acidic pH (pH 4.5-5.0) to ∼24% at alkaline pH (pH 9.0). Moreover, when carboxylates were extracted from the broth (undissociated n-caproic acid was ∼0.3 mM), the n-caproate selectivity (i.e., product per substrate fed) was 42.6 ± 19.0% higher compared to 7D4C2 cultures without extraction. Based on the 16S rRNA gene sequence, strain 7D4C2 is most closely related to the isolates Caproicibacter fermentans (99.5%) and Caproiciproducens galactitolivorans (94.7%), which are chain-elongating bacteria that are also capable of lactate production. Whole-genome analyses indicate that strain 7D4C2, C. fermentans, and C. galactitolivorans belong to the same genus of Caproiciproducens. Their rBOX genes are conserved and located next to each other, forming a gene cluster, which is different than for other chain-elongating bacteria such as Megasphaera spp. In conclusion, Caproiciproducens spp., comprising strain 7D4C2, C. fermentans, C. galactitolivorans, and several unclassified strains, are chain-elongating bacteria that encode a highly conserved rBOX gene cluster. Caproiciproducens sp. 7D4C2 (DSM 110548) was studied here to understand n-caproate production better at mildly acidic pH within microbiomes and has the additional potential as a pure-culture production strain to convert sugars into n-caproate.

Direct Medium-Chain Carboxylic Acid Oil Separation from a Bioreactor by an Electrodialysis/Phase Separation Cell

Medium-chain carboxylic acids (MCCAs) are valuable platform chemicals and can be produced from waste biomass sources or syngas fermentation effluent through microbial chain elongation. We have previously demonstrated successful approaches to separate >90% purity oil with different MCCAs (MCCA oil) by integrating the anaerobic bioprocess with membrane-based liquid-liquid extraction (pertraction) and membrane electrolysis. However, two-compartment membrane electrolysis unit without pertraction was not able to separate MCCA oil. Therefore, we developed a five-compartment electrodialysis/phase separation cell (ED/PS). First, we tested an ED/PS cell in series with pertraction and achieved a maximum MCCA-oil flux of 1.7 × 10³ g d^-1 per projected area (m²) (19 mL oil d^-1) and MCCA-oil transfer efficiency [100% × moles MCCA-oil moles electrons^-1] of 74% at 15 A m^-2. This extraction system at 15 A m^-2 demonstrated a ∼10 times lower electric-power consumption (1.1 kWh kg^-1 MCCA oil) than membrane electrolysis in series with pertraction (9.9 kWh kg^-1 MCCA oil). Second, we evaluated our ED/PS as a stand-alone unit when integrated with the anaerobic bioprocess and demonstrated that we can selectively extract and separate MCCA oil directly from chain-elongating bioreactor broth with just an abiotic electrochemical cell. However, the electric-power consumption increased considerably due to the lower MCCA concentrations in the bioreactor broth compared to the pertraction broth.

Highly selective recovery of medium chain carboxylates from co-fermented organic wastes using anion exchange with carbon dioxide expanded methanol desorption

The aim of this work was to recover a mixture of carboxylates ranging from 2 to 7 carbon atoms using a strong anion exchange resin, followed by desorption with CO₂-expanded methanol. Medium chain carboxylates hexanoate and heptanoate adsorbed better than acetate, and the corresponding medium chain carboxylic acids desorbed easier than acetic acid. Consequently, hexanoate and heptanoate were concentrated up to 14.6 and 20.7 times, respectively. These findings will enable effective separation and purification of the produced carboxylic acids. Notably, the presence of inorganic ions in the sample, such as chloride, decreased the adsorption affinity compared to a synthetic mixture only of carboxylates.

Microbial electrosynthesis: Towards sustainable biorefineries for production of green chemicals from CO2 emissions

Decarbonisation of the economy has become a priority at the global level, and the resulting legislative pressure is pushing the chemical and energy industries away from fossil fuels. Microbial electrosynthesis (MES) has emerged as a promising technology to promote this transition, which will further benefit from the decreasing cost of renewable energy. However, several technological challenges need to be addressed before the MES technology can reach its maturity. The aim of this review is to critically discuss the bottlenecks hampering the industrial adoption of MES, considering the whole production process (from the CO₂ source to the marketable products), and indicate future directions. A flexible stack design, with flat or tubular MES modules and direct CO₂ supply, is required for site-specific decentralised applications. The experience gained for scaling-up electrochemical cells (e.g. electrolysers) can serve as a guideline for realising pilot MES stacks to be technologically and economically evaluated in industrially relevant conditions. Maximising CO₂ abatement rate by targeting high-rate production of acetate can promote adoption of MES technology in the short term. However, the development of a replicable and robust strategy for production and in-line extraction of higher-value products (e.g. caproic acid and hexanol) at the cathode, and meaningful exploitation of the currently overlooked anodic reactions, can further boost MES cost-effectiveness. Furthermore, the use of energy storage and smart electronics can alleviate the fluctuations of renewable energy supply. Despite the unresolved challenges, the flexible MES technology can be applied to decarbonise flue gas from different sources, to upgrade industrial and wastewater treatment plants, and to produce a wide array of green and sustainable chemicals. The combination of these benefits can support the industrial adoption of MES over competing technologies.

Medium-Chain fatty acids and long-chain alcohols production from waste activated sludge via two-stage anaerobic fermentation

Traditional bioenergy recovery in the form of short chain fatty acids (SCFAs) from waste activated sludge (WAS) is generally limited by economic unattractiveness and complexity of products separation. Herein, a novel biotechnology process of two-stage anaerobic fermentation for converting the WAS into high energy density, easy-separated medium chain fatty acids (MCFAs) and long-chain alcohols (LCAs) was evaluated. In this process, the WAS was first converted to WAS alkaline fermentation liquid (WASAFL), serving as electron acceptors (EAs) and inoculum, then adding ethanol as electron donor (ED) for chain elongation (CE). The co-production of MCFAs and LCAs during CE were studied under three different ED to EA ratios, i.e., 3:1, 4:1 and 5:1. Experimental results demonstrated that when the ratio of ED to EA increased from 3:1 to 5:1, the production of MCFA and LCAs respectively increased from 5.57 ± 0.17 and 2.58 ± 0.18 to7.67 ± 0.48 and 4.21 ± 0.19 g COD/L. A similar observation was made in the total product electron efficiency, increasing from 59.9% to 72.1%. However, the highest total product selectivity (i.e., 68.0%) and highest products production yield (i.e., 59.77%) were not achieved at the ED to EA ratio of 5:1 due to toxicity caused by higher accumulation of n-caproate. The kinetic analysis further confirmed that high ratio of ED to EA induced improvement in product maximum yield, production rate for both MCFAs and LCAs. Microbial community analysis indicated that Clostridium, Caproiciproducens, Acinetobacter, Exilispira, and Oscillibacter were clearly enriched in the CE reactor and had positive correlation with MCFAs and LCAs production.

Zerovalent Iron Effectively Enhances Medium-Chain Fatty Acids Production from Waste Activated Sludge through Improving Sludge Biodegradability and Electron Transfer Efficiency

A novel zerovalent iron (ZVI) technique to simultaneously improve the production of medium-chain fatty acids (MCFAs) from waste activated sludge (WAS) and enhance WAS degradation during anaerobic WAS fermentation was proposed in this study. Experimental results showed that the production and selectivity of MCFAs were effectively promoted when ZVI was added at 1-20 g/L. The maximum MCFAs production of 15.4 g COD (Chemical Oxygen Demand)/L and MCFAs selectivity of 71.7% were both achieved at 20 g/L ZVI, being 5.3 and 4.8 times that without ZVI (2.9 g COD/L and 14.9%). Additionally, ZVI also promoted WAS degradation, which increased from 0.61 to 0.96 g COD/g VS when ZVI increased from 0 to 20 g/L. The microbial community analysis revealed that the ZVI increased the populations of key anaerobes related to hydrolysis, acidification, and chain elongation. Correspondingly, the solubilization, hydrolysis, and acidification processes of WAS were revealed to be improved by ZVI, thereby providing more substrates (short-chain fatty acids (SCFAs)) for producing MCFAs. The mechanism studies showed that ZVI declined the oxidation-reduction potential (ORP), creating a more favorable environment for the anaerobic biological processes. More importantly, ZVI with strong conductivity could act as an electron shuttle, contributing to increasing electron transfer efficiency from electron donor to acceptor. This strategy provides a new paradigm of transforming waste sludge into assets by a low-cost waste to bring significant economic benefits to sludge disposal and wastewater treatment.

Review on the production of medium and small chain fatty acids through waste valorization and CO2 fixation

The developing approaches in the recovery of resources from biowastes for the production of renewable value-added products and fuels, using microbial cultures as bio-catalyst have now became promising aspect. In the path of anaerobic digestion, the microorganisms are assisting transformation of a complex organic feedstock/waste to biomass and biogas. This potentiality consequently leads to the production of intermediate precursors of renewable value-added products. Particularly, a set of anaerobic pathways in the fermentation process, yields small-chain fatty acids (SCFA), and medium-chain fatty acids (MCFA) via chain elongation pathways from waste valorization and CO₂ fixation. This review focuses on the production of SCFA and MCFA from CO₂, synthetic substrates and waste materials. Moreover, the review introduces the metabolic engineering of Escherichia coli and Saccharomyces cerevisiae for SCFAs/MCFAs production. Furtherly, it concludes that future critical research might target progress of this promising approach as a valorization of complex organic wastes.

Electro-membrane separations in biotechnology

Membrane processes are of crucial importance for downstream processing in biotechnology. This is due to their selectivity and the mild operating conditions, enabling to extract target products without damages caused by overheating and chemical agents. Besides the most spread membrane processes like ultrafiltration and reverse osmosis, electrodialysis is very important for removal and extraction of electrically charged products, i. e. anions of organic acids, some antibiotics, etc. The electrodialysis process can be organized in batch or continuous mode. On the other hand, in the electro-crossflow filtration, the transport of target solutes across the membrane is guided by two main driving forces, the transmembrane pressure and the electric potential. This combination enables various possibilities for more selective and efficient downstream processing in biotechnology. This chapter provides a brief overview of recent achievements of electrodialysis in selected bioproducts separations and recovery. A special focus, including original experimental data, is then given to electro-filtration, which is a powerful tool creating new opportunities for performing separations on the basis of both electric charge and particle size differences.

Microbial electrosynthesis from CO2: Challenges, opportunities and perspectives in the context of circular bioeconomy

Recycling CO₂ into organic products through microbial electrosynthesis (MES) is attractive from the perspective of circular bioeconomy. However, several challenges need to be addressed before scaling-up MES systems. In this review, recent advances in electrode materials, microbe-catalyzed CO₂ reduction and MES energy consumption are discussed in detail. Anode materials are briefly reviewed first, with several strategies proposed to reduce the energy input for electron generation and enhance MES bioeconomy. This was followed by discussions on MES cathode materials and configurations for enhanced chemolithoautotroph growth and CO₂ reduction. Various chemolithoautotrophs, effective for CO₂ reduction and diverse bioproduct formation, on MES cathode were also discussed. Finally, research efforts on developing cost-effective process for bioproduct extraction from MES are presented. Future perspectives to improve product formation and reduce energy cost are discussed to realize the application of the MES as a chemical production platform in the context of building a circular economy.

Unveiling the mechanisms of medium-chain fatty acid production from waste activated sludge alkaline fermentation liquor through physiological, thermodynamic and metagenomic investigations

Effective sludge treatment with bioenergy production is attracting increasing interests as large quantities of waste activated sludge (WAS) are produced during the wastewater treatment. In this study, a new biotechnical process for converting the WAS alkaline fermentation liquor (WASAFL) into valuable, easy-separated medium chain fatty acids (MCFAs) through chain elongation (CE) was investigated, which may provide a new insight into sludge treatment. In the process, ethanol was served as the electron donor (EDs) and WASAFL were main electron acceptors (EAs). The MCFAs productions were investigated under three different ED to EA ratios (i.e., 1:2, 1:1 and 2:1). The result showed that MCFAs production was increased from 2.88 ± 0.01 to 5.28 ± 0.18 g COD/L with the increase of ED to EA ratio. However, the highest MCFA selectivity was achieved at 72.9% when the ED to EA ratio was 1:1. The decrease in the selectivity at high ED:EA ratio is mainly due to the production of higher alcohol (i.e., n-butanol and n-hexanol). The thermodynamic analysis confirmed all CE processes for MCFAs production from WASAFL were exothermic reactions, with the spontaneity and energy release of the reactions increased with the ethanol level. The microbial community analysis showed that the relative abundances of Clostridium, Oscillibacter, Leptolinea and Exilispira were positively correlated with the MCFAs production. The metagenomic analysis suggested that both the reverse β-oxidization pathway and fatty acid biosynthesis pathway contributed to the CE process in the studied system. The functional enzymes were mainly associated within Clostridium, with Clostridium Kluyveri, Clostridium botulinum and Clostridium magnum being likely the key species responsible for the CE process.

Concentrating lactate-carbon flow on medium chain carboxylic acids production by hydrogen supply

Upgrading lactate/carbohydrate-rich waste biomass into medium-chain carboxylic acids (MCCAs) by chain elongation (CE) technology exhibits economic and environmental benefits. However, the largely dispersive lactate-carbon-flow decreases MCCAs yield. This work discovered appropriate H₂ supply could significantly reduce lactate-carbon-flow loss and improve MCCAs production (∼1.65 times) when the system is not operated according to well-defined operating conditions, and revealed corresponding mechanism. Hydrogen (H₂) supply largely enhanced electron efficiency and electron transfer capacity, and H₂ could reduce propionate (from competing acrylate pathway, which should be prevented, but when not possible, the carbon recovery from propionate is possible) to propanol, which was used as electron donor to elongate acetate and propionate. Moreover, H₂ could react with CO₂ (from CE process) to sequentially generate acetate and ethanol, which further contributed to caproate/caprylate generation. Comparing with non-H₂-supplemented test, the lactate-carbon-flow used for MCCAs production was enhanced by ∼28.4% after H₂ supply, and Clostridium spp. were the key discriminative microorganisms.

Synergetic alginate conversion by a microbial consortium of hydrolytic bacteria and methanogens

Sludge, of which alginate-like biomaterial is a major organic component, is an increasing environmental problem. Thus, efficient anaerobic degradation of alginate provides a new method for sludge utilization. In this study, anaerobic alginate hydrolytic bacteria (AHB) were proposed to enrich with methanogens synergetically to reduce the inhibition of intermediate metabolites. The COD of produced methane reached 80.7 ± 1.9% (n = 4) of initial alginate COD. After considering the microbial growth (8%-18% of COD), a good COD balance indicated that alginate was fully consumed and the main final metabolites were methane and CO₂. Methanogenesis could promote alginate conversion by AHB. The enriched bacteria for alginate degradation in this study were different from that of former known AHB. The metabolic pathway of alginate degradation was revealed by metagenomics, in which oligo-alginate lyase was detected in twelve bacteria, and typical carbon metabolic pathways to convert alginate to methane were identified. More studies of bacterial isolation and biofuel production are still needed in the future.

Enhanced Anaerobic Mixed Culture Fermentation with Anion-Exchange Resin for Caproate Production

The bioproduction of caproate from organic waste by anaerobic mixed culture is a very attractive technology for upgrading low-grade biomass to a high-value resource. However, the caproate production process is markedly restricted by the feedback inhibition of caproate. In this study, four types of anion-exchange resin were investigated for their enhancing capability in caproate fermentation of anaerobic mixed culture. The strong base anion-exchange resin D201 showed the highest adsorption capacity (62 mg/g), selectivity (7.50), and desorption efficiency (88.2%) for caproate among the test resins. Subsequently, the optimal desorption temperature and NaOH concentration of eluent for D201 were determined. The adsorption and desorption efficiency of D201 remained stable during eight rounds of the adsorption–desorption cycle, indicating a satisfactory reusability of D201. Finally, performances of caproate fermentation with and without resin adsorption for carboxylate were evaluated. The results demonstrated that the final concentration of caproate was improved from 12.43 ± 0.29 g/L (without adsorption) to 17.30 ± 0.13 g/L (with adsorption) and the maximum caproate production rate was improved from 0.60 ± 0.01 g/L/d to 2.03 ± 0.02 g/L/d. In the group with adsorption, the cumulative caproate production was increased to 29.10 ± 0.33 g/L broth, which was 134% higher than that of the control group. Therefore, this study provides effective approaches to enhance caproate production.

Caproic acid production from acid whey via open culture fermentation - Evaluation of the role of electron donors and downstream processing

The objective of this study was to investigate the potential of supplementing ethanol and lactic acid as electron donors in reverse β-oxidation for short chain carboxylic acids chain elongation during anaerobic fermentation of acid whey. Best results were achieved when lactic acid was added at concentration of 300 mM. It resulted in medium chain carboxylic acids (MCCAs) concentration of 5.0 g/L. In the trials with ethanol addition, the overall yield was 20% lower. Subsequently liquid-liquid extraction with ionic liquids (ILs) was investigated as a potential purification method of caproic acid. The most promising, with respect to recovery of caproic acid, was piperazinium IL [C₁C₁C₁₀Ppz][NTF₂], however, the selectivity was only 0.39. Less effective [C₁C₁C₆Ppz][NTF₂] recovered 85.9% of caproic acid while reaching a higher selectivity of 0.53. Technoeconomic model revealed that to meet the conservative value of $2.25 per kg of caproic acid, the downstream processing should not exceed $0.65 per kg.

Recent trends in integrated bioprocesses: aiding and expanding microbial biofuel/biochemical production

Microbial biosynthesis of fuels and chemicals represents a promising route for their renewable production. Product toxicity, however, represents a common challenge limiting the efficacy of this approach. Integrated bioprocesses incorporating in situ product separation are poised to help address this intrinsic problem, but suffer their own unique shortcomings. To improve and expand the utility of this versatile bioprocessing strategy, recent innovations have focused on developing more effective separation materials and novel process configurations, as well as adapting designs to accommodate semi-continuous modes of operation. As a result, integrated bioprocesses are finding new applications to aid the biosynthesis of an ever-growing list of bioproducts. Emerging applications, meanwhile, are exploring the further expansion of such designs to interface microbial and chemical catalysts, leading to new and versatile routes for the one-pot synthesis of an even greater diversity of renewable products.

Medium chain carboxylic acids production from waste biomass: Current advances and perspectives

Alternative chemicals to diverse fossil-fuel-based products is urgently needed to mitigate the adverse impacts of fossil fuel depletion on human development. To this end, researchers have focused on the production of biochemical from readily available and affordable waste biomass. This is consistent with current guidelines for sustainable development and provides great advantages related to economy and environment. The search for suitable biochemical products is in progress worldwide. Therefore, this review recommends a biochemical (i.e., medium chain carboxylic acids (MCCAs)) utilizing an emerging biotechnological production platform called the chain elongation (CE) process. This work covers comprehensive introduction of the CE mechanism, functional microbes, available feedstock types and corresponding utilization strategies, major methods to enhance the performance of MCCAs production, and the challenges that need to be addressed for practical application. This work is expected to provide a thorough understanding of the CE technology, to guide and inspire researchers to solve existing problems in depth, and motivate large-scale MCCAs production.

Medium Chain Carboxylic Acids from Complex Organic Feedstocks by Mixed Culture Fermentation

Environmental pressures caused by population growth and consumerism require the development of resource recovery from waste, hence a circular economy approach. The production of chemicals and fuels from organic waste using mixed microbial cultures (MMC) has become promising. MMC use the synergy of bio-catalytic activities from different microorganisms to transform complex organic feedstock, such as by-products from food production and food waste. In the absence of oxygen, the feedstock can be converted into biogas through the established anaerobic digestion (AD) approach. The potential of MMC has shifted to production of intermediate AD compounds as precursors for renewable chemicals. A particular set of anaerobic pathways in MMC fermentation, known as chain elongation, can occur under specific conditions producing medium chain carboxylic acids (MCCAs) with higher value than biogas and broader applicability. This review introduces the chain elongation pathway and other bio-reactions occurring during MMC fermentation. We present an overview of the complex feedstocks used, and pinpoint the main operational parameters for MCCAs production such as temperature, pH, loading rates, inoculum, head space composition, and reactor design. The review evaluates the key findings of MCCA production using MMC, and concludes by identifying critical research targets to drive forward this promising technology as a valorisation method for complex organic waste.

Inhibitory effects of free propionic and butyric acids on the activities of hydrogenotrophic methanogens in mesophilic mixed culture fermentation

The aim of this work was to study the inhibitory of free propionic acid (FPA) and free butyric acid (FBA) on enriched hydrogenotrophic methanogens. It demonstrated that concentrations of FPA and FBA were correlated well with the specific methanogenic activity. Coenzyme M concentrations also agreed well with the trends of FPA and FBA. Two fators of C_50% (concentration at 50% inhibition) and C_RC (recoverable concentration from inhibition) were used to quantitively analyze the inhibitory order using the former result of free acetic acid (FAA) and the results of FBA and FPA. The order according to C_50% was FAA (5.2 mM) > FBA (8.3 mM) > FPA (8.5 mM), while for C_RC it was FPA (9.3 mM) > FAA = FBA (13.5 mM). After comparing with literatue, it suggests that the toxicities of these three organic acids are similar. Thus, accumulating free organic acid offers a cost-effective method to inhibit methanogenesis.

Interfacing anaerobic digestion with (bio)electrochemical systems: Potentials and challenges

For over a century, anaerobic digestion has been a key technology in stabilizing organic waste streams, while at the same time enabling the recovery of energy. The anticipated transition to a bio-based economy will only increase the quantity and diversity of organic waste streams to be treated, and, at the same time, increase the demand for additional and effective resource recovery schemes for nutrients and organic matter. The performance of anaerobic digestion can be supported and enhanced by (bio)electrochemical systems in a wide variety of hybrid technologies. Here, the possible benefits of combining anaerobic digestion with (bio)electrochemical systems were reviewed in terms of (1) process monitoring, control, and stabilization, (2) nutrient recovery, (3) effluent polishing, and (4) biogas upgrading. The interaction between microorganisms and electrodes with respect to niche creation is discussed, and the potential impact of this interaction on process performance is evaluated. The strength of combining anaerobic digestion with (bio)electrochemical technologies resides in the complementary character of both technologies, and this perspective was used to distinguish transient trends from schemes with potential for full-scale application. This is supported by an operational costs assessment, showing that the economic potential of combining anaerobic digestion with a (bio)electrochemical system is highly case-specific, and strongly depends on engineering challenges with respect to full-scale applications.

Higher Substrate Ratios of Ethanol to Acetate Steered Chain Elongation toward n-Caprylate in a Bioreactor with Product Extraction

Syngas fermentation to ethanol and acetate has recently been coupled to microbial chain elongation to produce medium-chain carboxylates, including n-caproate and n-caprylate. These medium-chain carboxylates are relatively hydrophobic, and thus easier to extract from solution than miscible ethanol. Here, we examined the effect of 11 different ethanol-to-acetate substrate ratios (ranging from 1.8 to 275 g COD g COD^-1 [1.2 to 183 mol mol^-1]) on directing chain elongation toward n-caprylate in a 0.7-L upflow anaerobic filter with product extraction. During an eight-month operating period, we monitored the performance and characterized the microbiome composition of this chain-elongating bioreactor. We also developed a thermodynamic model to predict the favorability of n-caprylate production at different substrate ratios. As predicted by our model, higher ethanol-to-acetate substrate ratios fed to our bioreactor led to higher specificities for n-caprylate production. We observed that feeding primarily ethanol to the bioreactor (i.e., ethanol-to-acetate substrate ratio of 275 g COD g COD^-1) resulted in the highest specificity for n-caprylate, but the n-caprylate production rate decreased at this high ratio, resulting in lower conversion efficiencies. Thus, care should be taken not to overload the system with primarily ethanol as the substrate and to lower the organic loading rate.

Upgrading liquor-making wastewater into medium chain fatty acid: Insights into co-electron donors, key microflora, and energy harvest

Ethanol and lactate are considered suitable electron donors (EDs) for chain elongation (CE); however, their respective shortcomings still limit the substrate conversion ratio and medium chain fatty acid (MCFA) production. To address this limitation, different EDs and electron acceptors (EAs) were combined to compare their CE performances, and to investigate whether the combination of ethanol and lactate could further enhance the MCFA production based on the complementary characteristics of ethanol and lactate. The results verified, for the first time, ethanol and lactate as the co-EDs formed a cooperative relationship to largely promote the conversion of substrates into MCFA. The co-EDs of ethanol and lactate stimulated the transformation of dispersive lactate-carbon flux from the competing acrylate pathway into n-heptylate. Additionally, the coexisting by-products (H₂ and CO₂) from ethanol and lactate also contributed to the supererogatory MCFA generation. The key microbial taxa that distinguished the co-EDs from their single action were the preponderant species from class Negativicutes and family Ruminococcaceae. In addition, the co-EAs of acetate, n-butyrate, and n-caproate also promoted MCFA generation. Low concentration of n-caproate could be directly elongated into n-caprylate, while n-caproate concentration exceeding the toxic limit was unsuitable as an EA. This research provided a guide for substrate selection and collocation for CE technology. Chinese liquor-making wastewater (CLMW) was subsequently used as a substrate for MCFA production since it contains abundant lactate, ethanol, and short-chain fatty acids. In this study, a MCFA selectivity of 80.34 ± 5.26%, a slightly higher selectivity which is in the range of previously reported ones, was obtained. This study paves a way for the sustainable development of Chinese liquor industry by recycling the high-output CLMW into MCFA.

Prospects in bioelectrochemical technologies for wastewater treatment

Bioelectrochemical technologies are emerging as innovative solutions for waste treatment, offering flexible platforms for both oxidation and reduction reaction processes. A great variety of applications have been developed by utilizing the energy produced in bioelectrochemical systems, such as direct electric power generation, chemical production or water desalination. This manuscript provides a literature review on the prospects in bioelectrochemical technologies for wastewater treatment, including organic, nutrients and metals removal, production of chemical compounds and desalination. The challenges and perspectives for scale-up were discussed. A technological strategy to improve the process monitoring and control based on big data platforms is also presented. To translate the viability of wastewater treatment based on bioelectrochemical technologies into commercial application, it is necessary to exploit interdisciplinary areas by combining the water/wastewater sector, energy and data analytics technologies.

Bioaugmentation with Clostridium tyrobutyricum to improve butyric acid production through direct rice straw bioconversion

One-pot bioconversion is an economically attractive biorefinery strategy to reduce enzyme consumption. Direct conversion of lignocellulosic biomass for butyric acid production is still challenging because of competition among microorganisms. In a consolidated hydrolysis/fermentation bioprocessing (CBP) the microbial structure may eventually prefer the production of caproic acid rather than butyric acid production. This paper presents a new bioaugmentation approach for high butyric acid production from rice straw. By dosing 0.03 g/L of Clostridium tyrobutyricum ATCC 25755 in the CBP, an increase of 226% higher butyric acid was yielded. The selectivity and concentration also increased to 60.7% and 18.05 g/L, respectively. DNA-sequencing confirmed the shift of bacterial community in the augmented CBP. Butyric acid producer was enriched in the bioaugmented bacterial community and the bacteria related to long chain acids production was degenerated. The findings may be useful in future research and process design to enhance productivity of desired bio-products.

Electrobiorefineries: Unlocking the Synergy of Electrochemical and Microbial Conversions

An integrated biobased economy urges an alliance of the two realms of "chemical production" and "electric power". The concept of electrobiorefineries provides a blueprint for such an alliance. Joining the forces of microbial and electrochemical conversions in electrobiorefineries allows interfacing the production, storage, and exploitation of electricity as well as biobased chemicals. Electrobiorefineries are a technological evolution of biorefineries by the addition of (bio)electrochemical transformations. This interfacing of microbial and electrochemical conversions will result in synergies affecting the entire process line, like enlarging the product portfolio, increasing the productivity, or exploiting new feedstock. A special emphasis is given to the utilization of oxidative and reductive electroorganic reactions of microbially produced intermediates that may serve as privileged building blocks.

Hydrogen and carbon dioxide mixed culture fermentation in a hollow-fiber membrane biofilm reactor at 25 °C

There have been no reports of H₂ and CO₂ mixed-culture fermentation (MCF) at 25 °C in a hollow-fiber membrane biofilm reactor (HfMBR). In this study, H₂ and CO₂ MCF were conducted in an HfMBR at 25 °C producing metabolites including acetate, ethanol, butyrate, and caproate. Compared to pure culture fermentation (i.e., Clostridium carboxidivorans P7), the MCF in HfMBR at 25 °C produced a higher concentration of caproate in this study (3.4 g/L in batch 1 and 5.7 g/L in batch 2). The dominant genera were Clostridium_sensu_stricto_12 and Prevotella_7. The caproate was more likely formed from the pathway of acetate and ethanol rather than via butyrate and ethanol. Since caproate is more valuable than acetate and low temperature fermentation consumes less energy, this process of H₂ and CO₂ MCF at 25 °C is appropriate for industrial application.

One-step production of C6-C8 carboxylates by mixed culture solely grown on CO

BACKGROUND

This study aimed at producing C6-C8 medium-chain carboxylates (MCCAs) directly from gaseous CO using mixed culture. The yield and C2-C8 product composition were investigated when CO was continuously fed with gradually increasing partial pressure.

RESULTS

The maximal concentrations of n-caproate, n-heptylate, and n-caprylate were 1.892, 1.635, and 1.033 mmol L^-1, which were achieved at the maximal production rates of 0.276, 0.442, and 0.112 mmol L^-1 day^-1, respectively. Microbial analysis revealed that long-term acclimation and high CO partial pressure were important to establish a CO-tolerant and CO-utilizing chain-elongating microbiome, rich in Acinetobacter, Alcaligenes, and Rhodobacteraceae and capable of forming MCCAs solely from CO.

CONCLUSIONS

These results demonstrated that carboxylate and syngas platform could be integrated in a shared growth vessel, and could be a promising one-step technique to convert gaseous syngas to preferable liquid biochemicals, thereby avoiding the necessity to coordinate syngas fermentation to short-chain carboxylates and short-to-medium-chain elongation. Thus, this method could provide an alternative solution for the utilization of waste-derived syngas and expand the resource of promising biofuels.

Integrating electrochemical, biological, physical, and thermochemical process units to expand the applicability of anaerobic digestion

Anaerobic digestion (AD) is a mature biotechnology-production platform with millions of installations at homes, farms, and industrial/municipal settings. Large-scale industrial, agricultural, and municipal waste-treatment systems may observe novel integration with electrochemical, biological, physical, and thermochemical process units to make AD more attractive. Without governmental subsidies, AD has often only a relatively low economic return or none at all. Diversification of products besides methane in biogas may help to change this. Here, several sections discuss different process units to: 1) upgrade biogas into biomethane; 2) convert carbon dioxide in biogas to more biomethane; 3) generate cooling power from process heat; 4) produce bio-crude oil (bio-oil) from organic matter; and 5) produce a liquid biochemical product from organic matter. This is not meant to be an exhaustive list, but rather a selection of particularly promising process units from a technological view, which are already integrated with AD or close to full-scale integration.

Esters production via carboxylates from anaerobic paper mill wastewater treatment

This paper describes a new option for integrated recovery and esterification of carboxylates produced by anaerobic digestion at a pH above the pK_a. The carboxylates (acetate, propionate, butyrate, valerate and lactate) are recovered using a strong anion exchange resin in the bicarbonate form, and the resin is regenerated using a CO₂-expanded alcohol technique, which allows for low chemicals consumption and direct esterification. Paper mill wastewater was used to study the effect of pH and the presence of other inorganic anions and cations on the adsorption and desorption with CO₂-expanded methanol. Calcium, which is present in paper mill wastewater, can cause precipitation problems, especially at high pH. Esters yields ranged from 1.08±0.04mol methyl acetate/mol of acetate_in to 0.57±0.02mol methyl valerate/mol of valerate_in.