Syngas fermentation is a promising bioprocessing method that utilises autotrophic organisms to convert C1 gases, such as CO and CO2, into valuable chemicals, offering both environmental and economic benefits. Despite these advantages, the industrial application of gas fermentation remains limited owing to challenges in productivity associated with gas substrates. While previous studies have focused on optimizing reactor design, mass transfer, and growth medium as solutions to these specific challenges, the direct correlation between cell viability and productivity remains unexplored. To address this gap, this study investigates the viability of the acetogenic strain Eubacterium callanderi KIST612 and its impact on acetate production across various operational modes. Unlike conventional single-reactor systems, a dual-reactor strategy was implemented to enhance viable cell retention, leading to improved process efficiency. This approach significantly increased the total carbon conversion rate to 9.30mmolh-1 and the specific productivity of viable cells to 0.13g gcell-1h-1, ultimately achieving the highest acetate titer (34.4gL-1) with >53% cell viability. These findings represent a major advancement over previous studies, demonstrating that maintaining cell viability is critical for optimizing acetate productivity. By integrating viability control into process operations, this study presents a scalable and efficient strategy to enhance gas fermentation performance, improve substrate conversion efficiency, and expand biochemical production potential for industrial applications.

Many fermentation products inhibit their own microbial production, which complicates industrial-scale fermentation development for these products. When a product is volatile, this inhibition can be circumvented by removing product during fermentation through evaporation in a loop around the bioreactor. Microbes can survive this loop if its temperature is reduced using vacuum. Then, regrowing of microbes is not required. From a separation efficiency viewpoint, the evaporation loop should not use a single equilibrium stage, but a multistage vacuum distillation column. Such in situ product removal (ISPR) by vacuum distillation has hardly been recognized as an option, however. Costs for this product removal with subsequent purification are modest, even when product titers are low. A prerequisite is the use of advanced energy integration and heat pumping methods.

The butanediols (BDOs), 2,3-, 1,4- and 1,3-butanediol, are platform chemicals that are mainly produced from fossil hydrocarbons but may be obtained through fermentation. However, low product concentration, by-product formation and high boiling temperatures of BDOs hinder downstream processing and increase overall fermentation costs. This study increases the competitiveness of industrial biotechnology by designing a large-scale process (broth processing capacity of 160 ktonne/y) for the final purification of BDOs after fermentation (recovery >99 %). It includes an initial preconcentration step in a vacuum distillation column to remove most water and light impurities. The initial removal of most of the water and the use of a heat pump system allowed significant energy reduction. At the heart of the process is an integrated dividing-wall column that can efficiently purify BDO from the remaining light and heavy impurities. Moreover, a single process design was proven effective in purifying different BDOs to > 99.4 wt%. This was cost-effective (total purification costs of 0.208 – 0.243 $/kgBDO) and energy-efficient (with primary energy requirements of 1.854 – 2.176 kWthh/kgBDO). The proposed purification sequence can be used for each BDO type, which offers flexibility in developing sustainable bioprocesses for BDO production.

Alternative fermentation feedstocks such as ethanol can be produced from CO₂ via electrocatalytic processes that coproduce O₂. In this study, industrial-scale fermentation of ethanol with pure O₂ for single cell protein (SCP) production was studied using a modeling approach. This approach considered (i) microbial kinetics, (ii) gas–liquid transfer, and (iii) an exploration of potential operational constraints. The technical feasibility for producing up to 58 kt/y of SCP in a 600 m³ bubble column operating in continuous mode was assessed and attributed mainly to a high O₂ transfer rate of 1.1 mol/(kg h) through the use of pure O₂. However, most of the pure O₂ fed to the fermenter remains unconsumed due to the large gas flows needed to maximize mass transfer. In addition, biomass production may be hampered by high dissolved CO₂ concentrations and by large heat production. The model estimates a microbial biomass concentration of 114 g/kg, with a yield on ethanol of 0.61 g_x/g_ethanol (> 95% (Formula presented.)). Although the large predicted O₂ transfer capacity seems technically feasible, it needs further experimental validation. The model structure allows the analysis of alternative substrates in the same way as identifying the best carbon feedstock.

Due to climate change and increasing droughts, wastewater treatment and water reuse are gaining importance. Yet, the state-of-the-art bubble-aerated membrane bioreactor (BA-MBR) faces competitiveness challenges due to its high energy use and maintenance requirements, especially at small scale. This study investigates a novel membrane-aerated MBR (MA-MBR) that integrates membrane aeration and filtration to reduce energy consumption and system footprint, enabling resource-efficient non-potable reuse. The MA-MBR treated greywater for domestic reuse and achieved stable chemical oxygen demand (COD) removal efficiencies up to 95 % at high loading rates (up to 4 g L⁻¹ d⁻¹) and produced effluent with biological oxygen demand (BOD₅) values below 5 mg L⁻¹, meeting stringent reuse standards. Biomass dynamics revealed two distinct forms: biofilm on aeration membranes and flocs in suspension. Coarse bubble scouring facilitated biofilm detachment, enabling solid retention time (SRT) control. Oxidation-reduction potential (ORP) was linked to the biomass detachment efficiency, with negative ORP reducing mixed liquor suspended solids (MLSS) after scouring 5–10 times compared to operation at positive ORP. Reattachment of flocs reduced MLSS levels by 90 % within 60 min. A 25 % lower transmembrane pressure (TMP) in the MA-MBR compared to the BA-MBR after 72 h indicated lower fouling rates. Microbial communities were distinctly different between biofilm and flocs, especially under negative ORP conditions. These findings suggest the MA-MBR as low-footprint, low-fouling alternative for carbon removal from wastewaters with relatively high COD/N-ratios, and may improve resource efficiency for non-potable water reuse, for instance in decentralized source-separation applications.

Anticipating the occurrence and effects of mass transport limitations during fermentation scale-up is essential for commercialization, as heterogeneities might affect microorganisms. Tools like Computational Fluid Dynamics (CFD) aid this analysis but are computationally intensive, limiting design space exploration and consequently, fermentation optimization. Compartment models (CMs) based on CFD simulations offer an affordable alternative but require CFD recalibration with changing geometries or operating conditions, restricting their usage in optimization.
In this work, we introduce a hybrid machine-learning-aided compartment model (ML-CM) that accounts for flow pattern dynamics upon changes in both volume and stirring speed in a stirred tank bioreactor. The ML-aided dynamic compartment model (dyn-CM) enabled the spatiotemporal study of a process in 1/500th of the fermentation simulation time, maintaining reasonable accuracy. This method facilitates fed-batch fermentation modeling, process optimization, and scale-up effect analysis with modest computational resources, supporting reactor design and operational improvements within a defined operating space.

Microbial electrosynthesis (MES) is a novel carbon utilisation technology aiming to contribute to a circular economy by converting CO₂ and renewable electricity into value-added chemicals. This study presents a cradle-to-gate life cycle assessment (LCA) of hexanoic acid (C6A) production using MES, comparing this production with alternative technologies. It also includes a cradle-to-grave LCA for potentially converting C6A into a neat sustainable aviation fuel (SAF). On a cradle-to-gate basis, MES-based C6A exhibits a carbon footprint at 5.5 t CO₂eq/tC6A, similar to fermentation- and plant-based C6A. However, its direct land use is more than one order of magnitude lower than plant-based C6A. On a cradle-to-grave basis, MES-based neat SAF emits 325 g CO₂eq/MJ neat SAF, which is significantly higher than the counterparts from currently certified routes and conventional petroleum-derived jet fuel. However, its negligible indirect land use change emissions might potentially make it competitive against neat SAFs originating from first-generation biomass.

Combining intermittent renewable electricity (IRE) with carbon capture and utilisation is urgently needed in the chemical sector. In this context, microbial electrosynthesis (MES) has gained attention. It can electrochemically produce hexanoic acid, a value-added chemical, from CO₂. However, there is a lack of understanding regarding how the intermittency of renewable electricity could impact the design of a MES plant. We studied this using Aspen Plus models. A MES plant that was powered by constant grid electricity could operate from 100% down to 70% of its nominal capacity, at which point the heat exchangers and the internal geometrical design of the distillation towers became bottlenecks. The levelised production cost of hexanoic acid (LPC_C6A) was estimated at 4.0 €/kg. Switching to IRE supply increased LPC_C6A to 5.3 €/kg (for wind electricity) and 4.7 €/kg (for hybrid renewable electricity). A battery energy storage system (BESS) was deployed. The lowest LPC_C6A was found at a BESS installation of 29 GJ/h for wind electricity (5.1 €/kg) and at 12 GJ/h for hybrid renewable electricity (4.7 €/kg). In both situations, the volume flexibility of the MES plant was not improved. At the investigated market and operating conditions, coupling IRE to the MES plant was economically infeasible.

Isobutanol is a highly attractive renewable alternative to conventional fossil fuels, with superior fuel properties as compared to ethanol and 1-butanol. Even though the isobutanol production by fermentation has significant potential, complex downstream processing is limiting the wide-spreading of this technology. Accordingly, this original research significantly contributes to the advancement in industrial biofuel production by developing two eco-efficient downstream processes for the industrial-scale recovery of isobutanol (production capacity 50 ktonne_IBUT/y), from a highly dilute fermentation broth (>98 wt% water). Vacuum distillation and a novel hybrid combination of gas stripping and vacuum evaporation were coupled with atmospheric azeotropic distillation to recover over 99.9 % of isobutanol as a high-purity product (100 wt%). Advanced heat pumping and heat integration techniques were further implemented to allow the complete electrification of these recovery processes. Furthermore, implementation of these techniques significantly decreased total annual costs (0.131–0.161 $/kg_IBUT), reduced energy requirements (0.488–0.807 kW_eh/kg_IBUT) and lowered CO₂ emissions (0.303–0.449 kg_CO2/kg_IBUT), resulting in highly competitive purification processes. In addition to efficiently recovering isobutanol, the designed downstream processes provide the potential to enhance the fermentation process by recycling all present microorganisms and reducing water demand. Therefore, the results of this original research substantially contribute to the advancement in industrial biotechnology and the wide-spreading of biofuel production.

Isopropanol-butanol-ethanol (IBE) fermentation is a superior biofuel production technology as compared to acetone-butanol-ethanol (ABE) fermentation due to the better fuel properties of the obtained products. However, low product concentrations, thermodynamic constraints and the presence of microorganisms lead to complex downstream processing that limits the competitiveness of this biofuel production method. Thus, this original research proposes a novel thermally self-sufficient and eco-efficient downstream process for industrial-scale recovery after IBE fermentation (74 ktonne/y capacity), from a highly dilute broth (>97 wt% water). Gas stripping and heat pump-assisted vacuum evaporation were implemented to separate valuable products from most of the broth. Furthermore, an advanced highly integrated heat pump-assisted azeotropic dividing-wall column was designed to recover high-purity (99 wt%) butanol biofuel and isopropanol – ethanol fuel supplement (89 wt%). The proposed purification process recovers over 99 % of biofuel products in a cost-effective (0.130 $/kg_IBE) and energy-efficient way (0.673 kW_eh/kg_IBE) while allowing full recycle of biomass and most of the separated water. Besides improving yield by continuously recovering the inhibitory products, fermentation can be further enhanced by avoiding biomass loss and reducing water requirements. Lastly, the implemented energy-saving techniques ensure complete electrification of the proposed IBE recovery process. Therefore, the original results of this research study significantly contribute to the development of sustainable biofuel production processes.

Carbon-based products are essential to society, yet producing them from fossil fuels is unsustainable. Microorganisms have the ability to take up electrons from solid electrodes and convert carbon dioxide (CO₂) to valuable carbon-based chemicals. However, higher productivities and energy efficiencies are needed to reach a viability that can make the technology transformative. Here, we show how a biofilm-based microbial porous cathode in a directed flow-through electrochemical system can continuously reduce CO₂ to even-chain C2–C6 carboxylic acids over 248 days. We demonstrate a threefold higher biofilm concentration, volumetric current density, and productivity compared with the state of the art. Most notably, the volumetric productivity (VP) resembles those achieved in laboratory-scale and industrial syngas (CO-H₂-CO₂) fermentation and chain elongation fermentation. This work highlights key design parameters for efficient electricity-driven microbial CO₂ reduction. There is need and room to improve the rates of electrode colonization and microbe-specific kinetics to scale up the technology.

CO2 electroreduction driven by renewable energy is a promising technology for defossilizing the chemical industry, but intermittency challenges its operation. This work aims to understand the impacts of intermittency on the design, volume flexibility, and scheduling of a microbial electrosynthesis (MES) plant that converts CO2 to hexanoic acid. A battery and a storage tank were considered to buffer the intermittency. Explorative case studies showed that batteries were economically unfavorable. Restricted by the downstream processing (DSP) flexibility, a storage tank with optimized size combined with optimal scheduling, under the assumed conditions in this work, improved the plant’s volume flexibility only by 10%. The carbon footprint became 3 times lower when switching from grid to renewable electricity, but the levelized production cost of hexanoic acid increased. Hence, coupling with renewable electricity was not economically but environmentally favorable. Developing more flexible DSP technologies or synthesizing higher-purity chemicals are needed to enhance MES’s attractiveness.

Abstract: Syngas fermentation to ethanol has reached industrial production. Further improvement of this process would be aided by quantitative understanding of the influence of imposed reaction conditions on the fermentation performance. That requires a reliable model of the microbial kinetics. Data were collected from 37 steady states in chemostats and from many batch experiments that use Clostridium authoethanogenum. Biomass-specific rates from CO conversion experiments were related to each other according to simple reaction stoichiometries and the Pirt equation, with only the ratio of ethanol to acetate production remaining as degree of freedom. No clear dependency of this ratio on dissolved concentrations, such as CO or acetic acid concentration, was found. This is largely caused by the lack of knowledge about the dependency of the CO uptake rate (and hence all other rates) on the CO concentration. This knowledge gap is caused by a lack of dissolved CO measurements. For dissolved H₂, a similar gap applies. Modelling H₂ consumption adds more degrees of freedom to the system, so that more structured experiments with H₂ is needed. The inhibition of gas consumption by acetate and ethanol is partly known but needs further study. Key points: • Set of Clostridium autoethanogenum syngas fermentation data from chemostats. • Unstructured kinetic models can relate most biomass-specific rates to dilution rates. • Lack of dissolved gas measurements limits deeper understanding.

Even though industrial biotechnology is successfully used for the production of some chemicals, for many other chemicals it is not yet competitive with conventional petrochemical production. Usually, fermentation as well as downstream processing requires improvement. Downstream processing has to deal with low product concentrations, microorganisms, impurities and thermodynamic constraints (e.g., azeotropes), which often makes it very challenging and expensive, especially on a large scale. However, downstream processing of biochemicals has not attracted as much attention as upstream fermentation processes. In that context, this perspective paper offers a lightly referenced scholarly opinion about the downstream processing performance of different bioalcohols after fermentation. Due to the stronger toxicity effects on microbes, the achievable concentrations of monohydric aliphatic alcohols in the fermentation broth decrease with the increasing chain length. Specifically, the concentrations used here are 6.14, 5.00, 1.61 and 0.24 wt% of ethanol, isopropanol, isobutanol and hexanol, respectively. More dilute fermentation broths lead to more complex recovery processes. According to our previous work, the total purification costs increase from 0.080 USD kg−1 for ethanol, 0.109 USD kg−1 for isopropanol and 0.161 USD kg−1 for isobutanol to 0.529 USD kg−1 for hexanol. A similar trend is noticeable for the energy usage (0.960, 1.348, 2.018 and 3.069 kWthh kg−1, respectively) and the related CO2 emissions (0.164, 0.221, 0.449 and 0.555 kgCO2 kg−1, respectively). This work shows that advanced separation and purification based on process intensification principles are crucial for overall efficient production processes. The achievable product concentration in the fermentation broth – and not so much the alcohol chain length – has the biggest influence on the performance of downstream processing. Therefore, simultaneous development of both upstream and downstream processing is necessary to ensure the competitiveness and viability of industrial fermentation processes. © 2024 The Author(s). Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).

Pass-through distillation (PTD) is a novel separation technology that can effectively overcome challenges related to using vacuum distillation in bio-based processes (defined temperature limit for evaporation that might result in very low condensation temperature). This method allows evaporation and condensation to be performed at different pressures by decoupling them using an absorption-desorption loop with an electrolyte absorption fluid. This original paper presents a process design for large-scale bioethanol recovery from fermentation broth (production capacity ~100 ktonne/y) by PTD. The flexibility of the novel PTD technology is expanded by combining it with heat pumps (PTD-HP) and multi-effect distillation (PTD-MED). Total cost and energy requirements for the recovery of high-purity bioethanol (99.8 wt%) are respectively 0.122 $/kgEtOH and 1.723 kWthh/kgEtOH for PTD-HP, and 0.131 $/kgEtOH and 1.834 kWthh/kgEtOH for PTD-MED, proving the effectiveness of the newly designed recovery processes for concurrent alcohol recovery and fermentation.

Distillation is the most used separation technology at industrial-scale, but using distillation in bio-based processes (e.g. fermentation processes to produce bioethanol) is quite challenging when mild temperatures are needed to keep the microbes alive. Vacuum distillation can be used to perform evaporation at low temperatures, but setting a low distillation pressure fixes also the condensation temperature to very low values that may require expensive refrigeration. Pass-through distillation (PTD) is an emerging hybrid separation technology that effectively combines distillation with absorption in a sorption-assisted distillation process that decouples the evaporation and condensation steps. This is achieved by inserting between the evaporation and condensation steps an absorption-desorption loop that passes through the component to be separated and allows the use of different pressures and types of heating and cooling utilities. This paper is the first to present the process design and rigorous simulation (implemented in Aspen Plus) of a new pass-through distillation process for bioethanol (∼100 ktonne/y plant capacity), proving its effectiveness in concurrent alcohol recovery and fermentation (CARAF). Combining PTD with heat pumps leads to low recovery costs of 0.122 $/kg_EtOH and energy requirements of only 1.723 kW_thh/kg_EtOH. Alternatively, combining PTD with multi-effect distillation resulted in 0.131 $/kg_EtOH recovery costs and 1.834 kW_thh/kg_EtOH energy intensity.

Propionic acid is a valuable platform chemical that is usually produced via fossil routes. As these are energy-intensive and eco-unfriendly processes, fermentative production of propionic acid is becoming more attractive. However, the complex downstream processing (due to low achievable product concentrations, high water content, presence of by-products and thermodynamic constraints), presents a potential barrier to scale-up this technology. This original research proposes a novel intensified large-scale (production capacity of ∼ 20 ktonne/y) process for the final recovery of propionic acid after the initial biomass and counterion removal. Vacuum distillation steps ensure the recovery of a high-purity succinic acid product (>99.9 wt%) and a water stream that may be recycled to the fermentation to reduce the fresh water requirements. The main unit that follows is a highly integrated heat pump-assisted dividing-wall column (DWC), which allows the effective separation of propionic (>99.9 wt%) and acetic acid (99.4 wt%) products. Alternatively, it can serve as a reactive DWC that performs the esterification and separation of methyl propionate (99.8 wt%), methyl acetate (91.0 wt%) as higher added value products, and water by-product. If needed, extractive distillation can be implemented additionally to recover methyl acetate at higher purity (99.9 wt%). Overall, both options are proven to be economically interesting (recovery costs of 0.399 – 0.469 $/kg considering the product price of 1.349 – 1.802 $/kg) and environmentally attractive (3.206 – 3.678 kWthh/kg).

PDO (1,3-propanediol) is a platform chemical that is obtained by petrochemical routes and by fermentation. The latter needs relatively complex downstream processing after fermentation, due to the modest concentration of the high-boiling product, and the presence of microorganisms and many impurities in the fermentation medium. This novel research proposes an original large-scale (production capacity of 23 – 32 ktonne/y) process for the final purification of PDO and dominant by-products, from the fermentation of glucose or glycerol. Following the initial microfiltration, diafiltration, ultrafiltration and ion exchange steps, heat pump-assisted vacuum distillation was implemented to remove most of the water from the broth. Afterwards, an advanced highly-integrated dividing-wall column was designed to allow the final purification of PDO (product purity > 99.9 wt%) from light and heavy by-products. Fermentation by-products that are present in significant amounts can also be recovered and valorized. Overall, the developed final purification processes demonstrate cost-effectiveness (0.150 – 0.274 $/kg) and energy-efficiency (1.258 – 3.175 kWthh/kg) which contributes to the competitiveness of the overall downstream processing (0.256 – 0.384 $/kg and 1.487 – 3.496 kWthh/kg).

Ethyl acetate is a platform chemical conventionally obtained through fossil fuel routes, but more recently its production by fermentation from carbohydrates has been scaled up to a pilot scale. Yet, the complexity of downstream processing (low product concentrations in liquid broth and in off-gas, azeotrope formation, and the presence of microorganisms) may complicate industrial application. This original theoretical study is the first to develop advanced downstream processing, based on process intensification principles, for large-scale recovery (~10 kton/year) of ethyl acetate after fermentation. To minimize product losses, ethyl acetate is separated from both the liquid broth and off-gas. The final purification is performed in a highly integrated azeotropic dividing-wall column. The economic and sustainability analysis shows that using refrigeration for initial product separation from the gas phase is more cost-effective (~0.61 $/kg) and less energy-intensive (2.20–2.40 kW_thh/kg) than compression combined with high-pressure condensation using chilled water (1.09 $/kg and 9.98 kW_thh/kg).

Microbial conversions are generally inhibited by their products. This product inhibition is particularly dependent on the product's hydrophobicity and on its acid dissociation behaviour, hence on pH. Dependence on the microbial strain is relatively modest according to many published data. Since product inhibition has various mechanistic backgrounds, one requires an empirical model for simulation of fermentation processes. For obtaining a modelling framework that is consistent for various bioreactor operation modes, literature data for the glucose to ethanol fermentation were used to elaborate a relatively simple case. The Pirt equation and hyperbolic substrate uptake equation were used with three parameters that depend linearly on the inhibiting product concentration. This model type should facilitate model-based optimization of bioreactor operation.