Improved efficiency of anaerobic digestion through direct interspecies electron transfer at mesophilic and thermophilic temperature ranges

Direct interspecies electron transfer (DIET) in microbial communities plays a significant role in improving efficiency of biomethane production from anaerobic digestion. In this study, the impacts of conductive graphene on mesophilic and thermophilic anaerobic digestion (MAD and TAD) were comparatively assessed using the model substrate ethanol. The maximum electron transfer flux for graphene-based DIET was calculated at mesophilic and thermophilic temperatures (35 °C and 55 °C). Biomethane potential results showed that the addition of graphene (1.0 g/L) significantly enhanced biomethane production rates by 25.0% in MAD and 26.4% in TAD. The increased biomethane production was accompanied with enhanced ethanol degradation. The theoretical calculation for maximum DIET flux showed that graphene-based DIET in MAD (76.4 mA) and TAD (75.1 mA) were at the same level, which suggests temperature might not be a significant factor affecting DIET. This slight difference was ascribed to the different Gibbs free energy changes of the overall DIET reaction (CH3CH2OH + 1/2CO2 → 1/2CH4 + CH3COO- + 5H+) in MAD and TAD. Microbial analysis revealed that the dominant microbes in response to graphene addition were distinctly different between MAD and TAD. The results indicated that the bacteria of Levilinea dominated in MAD, while Coprothermobacter dominated in TAD. The abundance of archaeal Methanobacterium decreased, while Methanosaeta increased with increasing temperature

Improving gaseous biofuel production from seaweed Saccharina latissima: the effect of hydrothermal pretreatment on energy efficiency

Marine macroalgae (seaweed) is a promising feedstock for producing biohydrogen and biomethane via dark fermentation and anaerobic digestion, respectively. However, one of the limiting steps in the biological process is the conversion of polymeric carbohydrates into monomeric sugars. Here hydrothermal pretreatments were assessed for hydrolysis and subsequent production of biohydrogen and biomethane from the brown seaweed Saccharina latissima. The solubilization of S. latissima improved with increasing temperatures from 100 to 180 °C, resulting in a maximum yield of 0.70 g soluble chemical oxygen demand/gram volatile solid (sCOD/g VS); equivalent to an increase of 207.5% compared with untreated seaweed. However, the yield of the derived monomeric sugar mannitol peaked at 140 °C and decreased with increasing temperatures, likely due to production of fermentative inhibitors. Microstructural characterization revealed that the algal structure was significantly damaged, and the major chemical groups of carbohydrates and proteins were weakened after pretreatment. Regardless of hydrothermal temperatures, biohydrogen yield only slightly increased in dark fermentation, while biomethane yield significantly increased from 281.4 (untreated S. latissima) to 345.1 mL/g VS (treated at 140 °C), leading to the sCOD removal efficiency of 86.1%. The maximum energy conversion efficiency of 72.8% was achieved after two-stage biohydrogen and biomethane co-production. In comparison, considering the energy input for pretreatment/fermentation/digestion, the highest process energy efficiency dropped to 37.8%. Further calculations suggest that a significant improvement of efficiency up to 56.9% can be achieved if the heat from pretreatment can be recovered

Enhanced dark hydrogen fermentation of Enterobacter aerogenes/HoxEFUYH with carbon cloth

Long-range extracellular electron transfer through microbial nanowires is critical for efficient bacterial behaviors. The application of carbon cloth on the dark hydrogen fermentation using transgenic Enterobacter aerogenes (E. aerogenes/HoxEFUYH) was first proposed to enhance hydrogen production from glucose. Scanning electron microscopy images showed that the microbial nanowires between E. aerogenes/HoxEFUYH cells almost vanished due to the presence of carbon cloth. Approximately 59.1% of microorganisms concentrated in biofilms on the surface of carbon cloth, which probably promoted the intercellular electron transfer. The results from Fourier transform infrared spectra and Excitation Emission Matrix spectra indicated that carbon cloth biofilms primarily included polysaccharide and protein. Moreover, the fluorophore of biofilms (88.1%) was much higher than that of supernatant (11.9%). The analysis of soluble metabolic degradation byproducts revealed that carbon cloth selectively enhanced the acetate pathway (C6H12O6+2H2O→2CH3COOH+2CO2+4H2), but weakened the ethanol pathway (C6H12O6→2C2H5OH+2CO2). With 1.0 g/L carbon cloth, the hydrogen yield increased by 26.6% to 242 mL/g, and the corresponding peak hydrogen production rate increased by 60.3%

How to optimise photosynthetic biogas upgrading: a perspective on system design and microalgae selection

Photosynthetic biogas upgrading using microalgae provides a promising alternative to commercial upgrading processes as it allows for carbon capture and re-use, improving the sustainability of the process in a circular economy system. A two-step absorption column-photobioreactor system employing alkaline carbonate solution and flat plate photobioreactors is proposed. Together with process optimisation, the choice of microalgae species is vital to ensure continuous performance with optimal efficiency. In this paper, in addition to critically assessing the system design and operation conditions for optimisation, five criteria are selected for choosing optimal microalgae species for biogas upgrading. These include: ability for mixotrophic growth; high pH tolerance; external carbonic anhydrase activity; high CO2 tolerance; and ease of harvesting. Based on such criteria, five common microalgae species were identified as potential candidates. Of these, Spirulina platensis is deemed the most favourable species. An industrial perspective of the technology further reveals the significant challenges for successful commercial application of microalgal upgrading of biogas, including: a significant land footprint; need for decreasing microalgae solution recirculation rate; and selecting preferable microalgae utilisation pathway

Effects of pre-treatment and biological acidification on fermentative hydrogen and methane co-production

A sequential two-stage process comprising biological acidification followed by anaerobic digestion was proposed to enhance gaseous biofuel production from the mixture of rice residue and micro-algae after thermo-chemicial hydrolysis. The maximum specific hydrogen yield of 223.1 ± 8.8 mL/g volatile solids (VS) and production rate of 10.4 ± 0.4 mL/g VS/h were achieved from hydrothermal acid pre-treated biomass during biological acidification. Increase in hydraulic retention time of biological acidification from 12 to 144 h significantly affected the distribution of solubilised metabolic products and led to improved biological acidification rates (BARs) from 15.5% to 78.5%. Compared with single stage anaerobic digestion, the first stage acidification phase led to reductions in the lag-phase time and peak time of anaerobic digestion in such a two-stage process. The maximum specific methane production rate of 2.2 ± 0.03 mL/g VS/h was achieved with a deep acidification of 144 h yielding a BAR of 78.5%. Increasing the length of time in biological acidification from 12 to 144 h contributed to improved energy conversion efficiency of 25.4%–64% after 120 h of anaerobic digestion. These results demonstrate that biological acidification is feasible to improve bioenergy recovery in two-stage fermentation

Improving gaseous biofuel yield from seaweed through a cascading circular bioenergy system integrating anaerobic digestion and pyrolysis

Advanced biofuels include biomass sources free from land use such as seaweed. Seaweed biomethane may contribute significantly to a climate-neutral transport future; however, seaweed has limited biodegradability via anaerobic digestion (AD). To address this issue, the authors proposed a cascading circular bioenergy system incorporating pyrolysis (Py) for production of biochar, syngas and bio-oil, with the primary use of biochar in AD to promote biomethane production through direct interspecies electron transfer. The feasibility of the proposed AD-Py system was demonstrated by integrating a seaweed-based AD and a residue-based Py system to enhance advanced biofuels production. The AD results showed biochar achieved comparable performances to high-cost graphene in terms of enhancing biomethane production from seaweed. When digesting Laminaria digitata (common kelp), optimal biochar addition at 1/4 (biochar mass: volatile solid of seaweed) increased biomethane yield by 17% and peak production rate by 29% with accelerated volatile fatty acids conversion during AD. When digesting Saccharina latissima (sugar kelp), biomethane yield increased by 16% with optimal biochar addition. A mass and energy balance analysis indicated that processing 1.000 t of Laminaria digitata in AD, combustion of syngas and surplus biochar (in excess of biochar added in AD) from Py of 1.254 t forest residue and 0.078 t dried digestate could fulfil all the heat demand for the integrated AD-Py system. The process integration increased biomethane yield by 17% and bio-oil yield by 10%. Furthermore, a 26% decrease in digestate mass flow could be achieved, thereby reducing the demand for agricultural land for digestate application

Boosting biomethane yield and production rate with graphene: the potential of direct interspecies electron transfer in anaerobic digestion

Interspecies electron transfer between bacteria and archaea plays a vital role in enhancing energy efficiency of anaerobic digestion (AD). Conductive carbon materials (i.e. graphene nanomaterial and activated charcoal) were assessed to enhance AD of ethanol (a key intermediate product after acidogenesis of algae). The addition of graphene (1.0 g/L) resulted in the highest biomethane yield (695.0 ± 9.1 mL/g) and production rate (95.7 ± 7.6 mL/g/d), corresponding to an enhancement of 25.0% in biomethane yield and 19.5% in production rate. The ethanol degradation constant was accordingly improved by 29.1% in the presence of graphene. Microbial analyses revealed that electrogenic bacteria of Geobacter and Pseudomonas along with archaea Methanobacterium and Methanospirillum might participate in direct interspecies electron transfer (DIET). Theoretical calculations provided evidence that graphene-based DIET can sustained a much higher electron transfer flux than conventional hydrogen transfer

Improving methane production from Pennisetum hybrid by monitoring plant height and ensiling pretreatment

The biomass of grass-based Pennisetum hybrid commonly use for a biogas production via anaerobic digestion. However, it is necessary to determine a method to optimize the plant harvest time for high biogas production. Moreover, ensiling of biomass in the presence of diverse microbes may offer a solution to improve biogas production. In this study, whole plant of Pennisetum biomass (including stems and leaves) was collected at different harvesting time (plant heights of 70, 100, 150 cm), and then comparatively assessed for further ensiling and biogas production. Compared to leaves, stems exhibited a significant linear relationship (R2 = 0.99) with whole plants in terms of ensiling quality (i.e. pH and NH3-N). Microbial analysis further revealed that Lactobacillus was the dominant bacterial genus during ensiling of stems and whole plants, with the highest relative abundance of 50.08% obtained at the height of 100 cm. Ensiling of biomass at a height of 100 cm achieved the best digestion performance, with the methane yields of 316 ± 20 mL/g VS for leaves, 361 ± 43 mL/g VS for stems, and 356 ± 28 mL/g VS for whole plants. A harvesting time at the plant height of 100 cm was the optimal from the silage quality and anaerobic digestion performance

Enhancing fermentative hydrogen production with the removal of volatile fatty acids by electrodialysis

A three-chamber electrodialysis bioreactor comprising fermentation, cathode and anode chambers was proposed to remove in situ volatile fatty acids during hydrogen fermentation. The electrodialysis voltage of 4 V resulted in a volumetric hydrogen productivity of 1878.0 mL/L from the fermentation chamber, which is 55.4% higher than that (1208.5 mL/L) of the control group without voltage applied. Gas production was not observed in the cathode and anode chambers throughout fermentation. By applying different voltages (0–6 V), the hydrogen content accumulated to 54.6%–84.7%, and it exhibited increases of 7.1%–66.4% compared with that of the control. Meanwhile, the maximum concentrations of acetate and butyrate in the fermentation chamber decreased to 10.3 and 13.1 mmol/L at a voltage of 4 V, respectively, which are 68.0% and 62.4% lower than that for the control

Graphene addition to digestion of thin stillage can alleviate acidic shock and improve biomethane production

Production of biomethane from distillery byproducts (such as stillage) in a circular economy system may facilitate a climate neutral alcohol industry. Anaerobic digestion (AD) of easily degradable substrates can lead to rapid acidification and accumulation of intermediate volatile fatty acids, reducing microbial activity and biomethane production. Carbonaceous materials may function as an abiotic conductive conduit to stimulate microbial electron transfer and resist adverse impacts on AD. Herein, nanomaterial graphene and more cost-effective pyrochar were comparatively assessed in their ability to recover AD performance after acidic shock (pH 5.5). Results showed that graphene addition (1.0 g/L) could lead to a biomethane yield of 250 mL/g chemical oxygen demand; this is an 11.0% increase compared to that of the control. The recovered process was accompanied by faster propionate degradation (CH3CH2COO– + 2H2O → CH3COO– + CO2 + 6H+ + 6e–). The enhanced performance was possibly ascribed to the high electrical conductivity of graphene. In comparison, pyrochar addition (1.0 and 10.0 g/L) did not enhance the biomethane yield, though it reduced the digestion lag-phase time by 18.1% and 12.2% compared to the control, respectively. Microbial taxonomy analysis suggested that Methanosarcina (81.5% in abundance) with diverse metabolic pathways and OTU in the order DTU014 (6.4% in abundance) might participate in direct interspecies electron transfer contributing to an effective recovery from acidic shock