• Energy Research

Over the last decades, the use of mixed microbial communities has attracted increasing scientific attention due to their potential biotechnological applications in several emerging technological platforms such as the carboxylate, bioplastic, syngas and bio-electrochemical synthesis platforms. However, this increasing interest has not been accompanied by a parallel development of suitable cryopreservation techniques for microbial communities. While cryopreservation methods for the long-term storage of axenic cultures are well established, their effectiveness in preserving the microbial diversity and functionality of microbial communities has rarely been studied. In this study, the effect of the addition of different cryopreservation agents on the long-term storage of microbial communities at -80 °C was studied using a stable enrichment culture converting syngas into acetate and ethanol. The cryopreservation agents considered in the study were glycerol, dimethylsulfoxide, polyvinylpyrrolidone, Tween 80 and yeast extract, as well as with no addition of cryopreservation agent. Their effectiveness was evaluated based on the microbial activity recovery and the maintenance of the microbial diversity and community structure upon revival of the microbial community. The results showed that the commonly used glycerol and no addition of cryopreservation agent were the least recommendable methods for the long-term frozen storage of microbial communities, while Tween 80 and polyvinylpyrrolidone were overall the most effective. Among the cryoprotectants studied, polyvinylpyrrolidone and especially Tween 80 were the only ones assuring reproducible results in terms of microbial activity recovery and microbial community structure preservation.

Syngas fermentation allows for the conversion of wastes into useful commodity chemicals. To target higher value products, the conditions can be tuned to be favourable for both acetogenic and reverse beta-oxidation pathways and produce, in one stage, butyric and caproic acid. Studies in CSTR have shown the crucial role of pH, which must be low enough to allow for ethanol generation in the acetogenic step while avoiding the inhibition of reverse β-oxidation in acidic conditions. However, no studies have investigated the effect of pH in reactor configurations suitable for syngas fermentation (i.e., allowing for cell retention and exhibiting high mass transfer rates at low operating costs), such as Trickle Bed Reactors, TBR. In this study, two TBR were used to study the pH effect on the fermentation of syngas to produce C4 and C6 acids, using undefined mixed cultures. Five pH values were tested in the range 4.5–7.5, and pH 6 was found to be the most favourable for simultaneous production of C4 & C6 acids from syngas, which agrees with what was found in suspended growth systems. In addition, the highest titers in literature so far were achieved in the TRB. 16S rRNA analysis was performed showing Clostridium and Rummenliibacillus to be the key genus for the efficient process at pH 6. Finally, the experimental methodology followed, and data collected proved the robustness of mixed culture biofilm reactors in respect to pH changes, as the same reactor performance and bacterial community were achieved regardless of the operation history.

Biomethanation of biomass-derived syngas represents a promising bioenergy conversion technology that can be operated within integrated plants to deliver ancillary services such as carbon capture and storage (CCS), seasonal energy storage and fuel densification. In the present study, we developed a set of techno-economic process models considering syngas biomethanation as a core unit complemented by Power-to-Gas (PtG), pure oxygen compression, CCS, and biomethane liquefaction. Four different plant configurations and five operating modes with biomass inputs ranging between 8.4 and 60.2 MW were investigated overall, indicating biomethane yields between 0.16 and 0.48 m3 kg−1 (dry basis). An energy analysis demonstrated how intensive PtG operation delivers substantially higher biomethane cold gas efficiencies (44.4%) compared to operating modes without electrolysis (30%–34.8%). In fact, a small-scale PtG-biomethanation configuration (S-EL) delivers the lowest biomethane minimum selling price (MSP) of 1.63 € m−3. Under existing biomethane subsidies in Denmark and Italy, S-EL would achieve profitability only under stored electricity costs equivalent to approximately 50% of the current levelized cost of renewable electricity generation in the two countries, combined with maximum biomass costs of 50 and 30 € t−1, respectively. All other configurations suffer from high costs and efficiency limitations and would require subsidies equivalent to 126%–348% of the current natural gas consumer price to reach profitability. The study provides evidence to support an intensification of targeted policies that support multi-service plants, and it highlights the need for access to low electricity prices as well as the urgency for low-cost gasification technologies.

The present study focuses on the influence of substrate concentration on the fermentative hydrogen production from the sugars of sweet sorghum extract, in a continuous stirred tank bioreactor. The reactor was operated at a Hydraulic Retention Time of 12 h and substrate concentrations ranging from 9895 to 20990 mg/L, in glucose equivalents. The maximum hydrogen production rate and yield were obtained at the concentration of 17000 mg carbohydrates/L and were 2.93 ± 0.09 L H2 /L reactor /d and 0.74 ± 0.02 mol H2 / mol glucose consumed or 8.81 ± 0.02 LH2 / kg sweet sorghum, respectively. The main metabolic product at all steady states was butyric acid while ethanol production was high at high substrate concentrations. The experiments showed that the hydrogen productivity depends significantly on the initial carbohydrates concentration which also influences the distribution of the metabolic products.

Nowadays, the global community faces a worldwide challenge concerning the minimization of carbon emissions. Significant efforts are being conducted to apply environmental-friendly processes, such as the production of succinic acid by utilizing waste residual streams through a biorefinery approach. In this context, this study aimed to upscale the fermentative production (batch mode) of succinic acid (SA) by Actinobacillus succinogenes 130Z from an industrial candy residue. Process performance (titer, productivity, yield) was assessed at different operating conditions such as CO2 headspace gas overpressure (0, 0.4, 1.0, and 1.4 atm), initial sugars concentration (about 70, 95, 117 g L−1), yeast (10, 15 g L−1) and MgCO3 (0, 10 g L−1) concentration in fermentation media using two different batches of an industrial candy residue batches. SA concentration varied in the range of 37.78 ± 0.02–39.94 ± 0.04 g L−1 both in lab and pilot trials with yields of up to 0.74 (g g−1). Finally, a successful downstream concept for SA purification (pre-purification, membrane filtration, ion-exchange, vacuum distillation, and crystallization) was investigated. High purity (up to 93.5 ± 1.8 %) and SA crystallization recovery (up to 86.7 ± 3.2 %) was achieved from the fermentation broths.

Abstract The syngas biomethanation process is a promising bioconversion route due to its high versatility, as it could be applied as a stand-alone technology, coupled to gasification plants, and integrated in anaerobic digestion or bioelectrochemical conversion systems. The biomethanation of syngas typically takes place through a rather complex network of interspecies metabolic interactions, which may vary significantly depending on the operating conditions applied and the diversity of microbial groups present. Despite there are several benefits derived from using microbial consortia, these also present challenges associated with limited process control and low product selectivity. To address the latter, the syngas biomethanation process carried out by mesophilic and thermophilic microbial consortia was modelled with the ultimate goal of studying possible catabolic route control strategies through the modulation of key operating parameters. The results showed that the thermophilic microbial consortium presented much higher apparent specific methane productivity (18.8 mmol/g VSS/d) than the mesophilic (4.6 mmol/g VSS/d) at an initial PCO of 0.2 atm, and that the difference increased with increasing initial PCO. This difference in productivity was found to derive from the catabolic routes used rather than the kinetic parameters of each microbial consortium. Additionally, the thermodynamic considerations included in the models revealed the possibility of controlling the catabolic routes used by each consortium through the modulation of the mass transfer and PCO2. Our results strongly indicate that modulating the PCO2 is a promising operational strategy for boosting the product selectivity towards CH4, the productivity of the system and the biomethane quality simultaneously.

Continuous glucose fermentation produces bioethanol at higher volumetric rates than conventional batch or fed-batch systems. The retention of yeast cells via ultrasonic sedimentation in a lab-scale fermenter allowed for shearless, continuous cell upconcentration, and consequently process intensification. The cell separation efficiency of the ultrasonic system was predicted with a Response Surface Model (RSM) developed for yeast cells based on the linear, mixed, and quadratic effects of the operating variables and flow rates (3 levels, 5 variables). The experiments for the RSM calibration were designed via a central composite design. The efficiency model was validated and showed dependency to the Biomass concentration, Power input, and Harvest rate (R2calibration = 0.92, R2prediction = 0.83). A lab-scale fermenter fed with yeast growth medium was operated at varying dilution rates (0.1 – 0.6 h−1) based on the RSM to maximize the cell retention efficiency (23%−90%). Yeast cell concentration in the fermenter reached up to 31.5 ± 0.7 g/L while it remained at around 3 g/L in the harvest stream for all the dilution rates tested. The ethanol concentration ranged between 17 and 23 g/L and reached high volumetric productivity (8.8 g/L/h). A control run without ultrasonic sedimentation led to the washout of biomass at a dilution rate of 0.6 h−1. Ultrasonic yeast sedimentation is a promising technology for cell retention and enhanced productivity in continuous fermentation processes.

The low ethanol tolerance of thermophilic anaerobic bacteria (<2%, v/v) is a major obstacle for their industrial exploitation for ethanol production. The ethanol tolerance of the thermophilic anaerobic ethanol-producing strain Thermoanaerobacter A10 was studied during batch tests of xylose fermentation at a temperature range of 50-70 degrees C with exogenously added ethanol up to approximately 6.4% (v/v). At the optimum growth temperature of 70 degrees C, the strain was able to tolerate 4.7% (v/v) ethanol, and growth was completely inhibited at 5.6% (v/v). A higher ethanol tolerance was found at lower temperatures. At 60 degrees C, the strain was able to tolerate at least 5.1% (v/v) ethanol. A generalized form of Monod kinetic equation proposed by Levenspiel was used to describe the ethanol (product) inhibition. The model predicted quite well the experimental data for the temperature interval 50-70 degrees C, and the maximum specific growth rate and the toxic power (n), which describes the order of ethanol inhibition at each temperature, were estimated. The toxic power (n) was 1.33 at 70 degrees C, and corresponding critical inhibitory product concentration (P(crit)) above which no microbial growth occurs was determined to be 5.4% (v/v). An analysis of toxic power (n) and P(crit) showed that the optimum temperature for combined microbial growth and ethanol tolerance was 60 degrees C. At this temperature, the toxic power (n), and P(crit) were 0.50, and 6.5% (v/v) ethanol, respectively. From a practical point of view, the model may be applied to compare the ethanol inhibition (ethanol tolerance) on microbial growth of different thermophilic anaerobic bacterial strains.