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Co-digestion has been considered a promising method to improve methane yield. The effect of the proportion of dominant substrate on the performance and microbial community of anaerobic digestion of Pennisetum hybrid (PH) and livestock waste (LW) was investigated. An obvious synergistic effect was obtained with an increase of 15.20%-17.45% in specific methane yield compared to the predicted value. Meanwhile, the dominant substrate influenced the relational model between methane yield enhancement rate and mixture ratio. For the LW-dominant systems, a parabolic model between enhancement rate and mixture ratio was observed with a highest value of 392.16 mL/g VS achieved at a PH:LW ratio of 2:8. While a linear pattern appeared for PH-dominant systems with the highest methane yield of 307.59 mL/g VS. Co-digestion selectively enriched the relative abundance of Clostridium_sensu_stricto_1, Terrisporobacter, Syntrophomonas, Methanosarcina and Methanobacterium, which boosted the performance of hydrolysis, acidogenesis, acetogenesis and methanogenesis processes., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

In this work, the impact of initial pH on the production of volatile fatty acids (VFAs) of hybrid Pennisetum was investigated. The batch experiments were conducted under six distinct beginning pH at a mesophilic temperature. Initial pH had an obvious effect on VFA yield and composition, and severe alkaline circumstances (pH in  = 11.0) could boost VFA production and acetic acid selectivity. The highest VFAs yield and acetate proportion were obtained when the initial pH was 11.0, with 518 ± 29 mg g -1 VS and 92%. Furthermore, microbial community analysis showed that alkaliphilic acetogenic anaerobe such as Amphibacillus, Tissierella, and Natronincola were the dominant species when the initial pH was 11.0. The Amphibacillus is the main hydrolysis bacterium under these conditions because of its high ability for xylan degradation at pH 9.0-10.0. Because of the increased VFA yield and superior acetic acid selectivity, the results suggest that adjusting the initial pH to 11.0 in batch mode would be possible for scaling-up purposes., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

The effect of additives on the silage quality, microbial community, and anaerobic digestion performance of Pennisetum purpureum with high moisture content was studied. The sample treated with a mixed additive had best silage quality with the lowest pH and highest lactic acid/acetic acid ratio. Different additives influenced the dominant desirable bacteria. Correspondingly, Enterobacter was the dominant bacterial genus for sample with non-fermentative additives, whereas for the samples with fermentative or mixed additives, both Enterobacter and Lactobacillus had high relative abundance. The parameters of NH 3 -N, hemicellulose and lactic acid were positively correlated with the specific methane yield, while the lignin content was inversely correlated with the specific methane yield. The higher specific methane yield of 293.81 ± 0.15-334.69 ± 22.75 mL/g VS was obtained for samples treated with fermentative additive. Therefore, the mixed additive and fermentative additive are recommended for the silage of material with high-moisture content to improve the silage quality and methane yield., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

To enhance the biodegradability and methane production of hybrid Pennisetum, a pretreatment method with high selectivity for lignin removal, namely sodium chlorite/acetic acid (SCA) pretreatment, was examined in this work. Results showed that SCA pretreatment can selectively remove lignin with minimal impact on cellulose and hemicellulose. After up to 200 min of SCA treatment, 79.4% of lignin was removed and over 90% of the holocellulose was retained. The physicochemical changes after pretreatment were analyzed by confocal laser scanning microscopy, X-ray diffractometer and Fourier transform infrared spectroscopy, showing that the majority of lignin was removed from secondary cell walls and cell middle lamella while the chlorite-resistant lignin remained in the cell corner. Lignin removal significantly enhanced the biodegradability from 59.6% to 86.4% and increased methane production by 38.3%. Energy balance showed that SCA pretreatment was efficient to increase the energy output of hybrid Pennisetum., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Alkaline pretreatment with NaOH was used to improve methane yield from Pennisetum Hybrid. The pretreatments were carried out with different NaOH solutions (2-8% w/w) at three temperatures (35, 55 and 121 °C) for different periods of time (24, 24 and 1 h). All treated and untreated Pennisetum Hybrid were digested under mesophilic conditions (37 °C) to biogas, significant effects of the pretreatments on the yield of methane were observed. Results showed the modified Gompertz equation was reliable (determination coefficients (R 2 ) greater than 0.96) to describe the kinetic behavior of anaerobic digestion of Pennisetum Hybrid. The best result, obtained by the treatment at 35 °C 2% NaOH for 24 h, resulted in the methane yield of 301.7 mL/g VS, corresponding to 21.0% improvement in the methane yield. Compositional, SEM, XRD and FTIR analysis confirmed that lignin removal, structural modification and cellulose crystalline variation were responsible for the improvement., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Pretreatment methods play an important role in the improvement of biogas production from the anaerobic digestion of energy grass. In this study, conventional thermal and microwave methods were performed on raw material, namely, Pennisetum hybrid, to analyze the effect of pretreatment on anaerobic digestion by the calculation of performance parameters using Logistic function, modified Gompertz equation, and transference function. Results indicated that thermal pretreatment improved the biogas production of Pennisetum hybrid, whereas microwave method had an adverse effect on the performance. All the models fit the experimental data with R (2) > 0.980, and the Reaction Curve presented the best agreement in the fitting process. Conventional thermal pretreatment showed an increasing effect on maximum production rate and total methane produced, with an improvement of around 7% and 8%, respectively. With regard to microwave pretreatment, maximum production rate and total methane produced decreased by 18% and 12%, respectively.

BACKGROUND: A leach‐bed reactor (LBR) with leachate recirculation is a suitable technology for lignocellulosic biomass anaerobic digestion. In this study, the effect of percolate reflux strategies (continuous recirculation and intermittent recirculation) on methane production from Pennisetum hybrid, a type of energy crop, was investigated using a two‐layered LBR. RESULTS: Compared to a solid‐state anaerobic digestion batch test, LBRs with percolate recirculation provide a stable process with a short lag phase by avoiding inhibition of volatile fatty acid accumulation and inoculation effect. The results also indicated that continuous reflux led to a lower accumulative methane yield (56.20 L kg−1 VS) compared to intermittent reflux (78.50 L kg−1 VS), resulting from washing effect and poorer distribution on the lower layer. According to microbial analysis, Bacteroidetes, Firmicutes, Proteobacteria and Synergistetes were the most predominant bacterial phyla in each LBR test sample and aceticlastic methanogen (at least >55%) plays a predominant role for methane production in all samples. CONCLUSIONS: The results indicate that leachate recirculation could provide stable conditions for methane yield; however, two‐layer reactor designs which cannot guarantee the desired distribution in the lower layer result in a low methane yield. Suitable design of LBRs to maintain a well‐distributed reflux leachate should be further investigated. © 2019 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

In this study, the links between performance and microbial communities were investigated with municipal solid waste (MSW) at two feedstock ratios and eight organic loading rates (OLRs). The co-digestion systems were stably operated at OLRs of 2.0–6.0 g VS/(L·d). The performance of the co-digestion system varied with the feedstock ratio. Compared with the 50:50 (hybrid Pennisetum:MSW) system, the 75:50 system, GM31, obtained increases of 1.93–17.68% and 0.29–23.29% for the specific biogas and methane yields, respectively. Whereas a shift in bacterial and methanogen communities occurred as the operating conditions changed, particularly with OLR variations. The genera Saccharofermentans, Prevotella, Clostridium, Syntrophomonas, and Proteiniphilum became the dominant bacteria for the conversion of carbohydrates and nitrogen compounds as the OLR increased. Meanwhile, a shift from acetoclastic to hydrogenotrophic or multifunctional methanogens was observed. [ABSTRACT FROM AUTHOR]

[Display omitted] • A static ultra-high pressure pretreatment method for AD has been developed. • Electron transfer capacity of system was analyzed by electrochemical test. • A better digestion performance obtained at CIP samples of Hybrid Pennisetum. • Higher accessibility between material and inoculum through CIP was proved by mapping. Ultra-high pressing pretreatment has been employed to effectively improve methane production from Hybrid Pennisetum. In this study, Hybrid Pennisetum was pretreated by cold isostatic pressing (CIP) to improve methane production. The pretreatment enhanced the accessibility between the substrate and inoculum. The decomposition of the compact biomass structure was analyzed through Fourier Transform Infrared Spectrometer (FT-IR) and X-ray Diffractometer (XRD). Moreover, the electron transfer capability in the system was evaluated by electrochemical analysis. The systematic experiment yielded the highest methane production at a pressure of 400Mpa, reaching 227.67 mL/g VS. The pretreatment changed the reducing sugar content (RS), pH, and volatile fatty acids (VFAs) in the AD system, which in turn increased the cumulative methane production. This study provides a promising platform for the development of an efficient methanogenic pretreatment. [ABSTRACT FROM AUTHOR]

• Novel early warning method for anaerobic digestion failure is presented. • Gibbs free energy can be used as warning indicator for anaerobic digestion failure. • Anaerobic digestion failure can accordingly be indicated up to 21 days in advance. • Thermodynamic early warning has the potential to be a universal method. To investigate whether thermodynamic calculations of anaerobic digestion processes can be applied to the early warning for unstable anaerobic digestion, a group of semi-continuous digesters fed with an energy crop (Hybrid Pennisetum) were operated via a step-wise increase in the organic load rates until overload occurred. Traditional early warning indicators, such as biogas production and content, pH, alkalinity, and volatile fatty acids as well as the methane/carbon dioxide (CH 4 /CO 2) and volatile fatty acid/alkalinity ratios, were regularly monitored during the process. The Gibbs free energy changes (Δ G) of the methanogenesis phases of valerate, butyrate, and propionate were calculated based on Nernst and Van't Hoff equations. The results demonstrate that Δ G of the three syntrophic methanogenesis phases can be used as an early warning indicator for unstable anaerobic digestion, indicating anaerobic digestion failure (ceased biogas production) up to 21 days in advance, that is, 1–8 days earlier than some other indicators. [ABSTRACT FROM AUTHOR]

• Hybrid Pennisetum was delignified by gamma-valerolactone/water pretreatment. • The solid fraction after pretreatment was used for anaerobic digestion. • Gamma-valerolactone/water (50/50) pretreatment at 150 °C was the optimal condition. • The lignin-rich liquid fraction was used for the synthesis of lignin nanoparticles. • Gamma-valerolactone/water could be recycled after lignin nanoparticles synthesis. This study is the first to integrate gamma-valerolactone/water (GVL/water) pretreatment with anaerobic digestion (AD) for biogas production and lignin nanoparticles (LNPs) synthesis. The hydrothermal treatment was conducted at 135 to 180 °C with GVL at 0 to 90%. After pretreatment, the compositions of hybrid pennisetum were changed with the removal of lignin, hemicellulose, and cellulose to different extent. Subsequent anaerobic digestion achieved a maximal specific methane yield of 228.00 ± 4.37 mL/g VS, compared with that at 165.11 ± 1.99 mL/g VS for the control. The highest actual methane yield (150 mL/g RM) was achieved by pretreatment with GVL/water (50/50) at 150 °C for 90 min. LNPs at 200 to 2000 nm were synthesized from the liquid waste with a yield at ~4 mg/mL. The mass balance of this integrated method was discussed. In general, the maximal valorisation of hybrid pennisetum was achieved based on a catalyst-free of GVL/water pretreatment. [ABSTRACT FROM AUTHOR]

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 NH 3 -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. Image 101499739 • We assessed optimal harvesting height of Pennisetum hybrid for methane production. • Stem had a greater effect on the silage quality of whole plant than the leaf. • Lactobacillus was the dominant bacterial genus in the silage samples. • A better digestion performance obtained at silage samples of 100 cm height plants. • Harvesting grass at 100 cm is optimal for biogas production and silage quality. [ABSTRACT FROM AUTHOR]

Flow diagram of mechanical pretreatment and anaerobic digestion of Hybrid pennisetum. • Effects of particle size reductions on digestion of Hybrid pennistum was studied. • Changes were observed in chemical composition and CH 4 yield of different particles. • The highest methane yield 291.9 ± 4.7 mL/g VS was obtained in a size below 0.380 mm. • Excessive size reduction didn't improve CH 4 yield but enhanced the AD efficiency. This study investigated the effect of particle size on the performance of anaerobic digestion of Hybrid Pennisetum. Hybrid Pennisetum was ground and sieved to provide different particle sizes between 0.180 and 1.000 mm. Characterization of the different particle sizes suggested that the composition of ground Hybrid Pennisetum altered-carbohydrate content decreased and lignin content increased with a decrease in particle size. The highest specific methane yield was 291.9 ± 4.7 mL CH 4 ·g−1 VS at a particle size of 0.250–0.380 mm and this value plateaued as the particle size was reduced to below 0.250 mm. Excessive size reduction did not improve the methane yield but did result in a reduction of digestion time by 28.6–35.7%. The net energy output from the process was calculated at 300 kWh/t VS. Therefore, grinding was proved to enhance the anaerobic fermentation efficiency and energy output of Hybrid Pennisetum. [ABSTRACT FROM AUTHOR]

To investigate iron enhanced anaerobic digestion performance, a survey was conducted to examine how iron powder (IP) affected the stability of a co-digestion reactor for kitchen waste and Pennisetum hybrid in a semi-continuous mode. In addition, the mechanisms underlying this process were analyzed according to various parameters and microbial communities. The results showed that the maximum stable operational organic loading rate (OLR) for the IP reactor increased by 50% compared to that of the control reactor, while the specific methane yields and volumetric methane yields increased by 4.8%− 106.8% and 6.3%− 105.9%, respectively, when OLRs were 1.0–3.0 g VS/(L·d). Meanwhile, selective enrichment of propionate-utilizing bacteria and hydrogenotrophic methanogens may have promoted the conversion of volatile fatty acids while increasing gene abundance via a methane-forming pathway. The addition of IP changed the dominant microorganisms related to the methanogenesis pathway. Additionally, the occurrence of interspecies formate transfer promoted anaerobic digestion stability in the IP reactor. [Display omitted] • The stably operational OLRs for IP-addition system increased by 50%. • An increase in the SMY and VMY appeared for the IP-addition system. • The relative abundance of propionate-utilizing bacteria was enhanced by IP. • The pathway of hydrogenotrophic methanogens and gene abundance were both promoted. • IP enhanced the interspecies formate transfer of syntrophic metabolism. [ABSTRACT FROM AUTHOR]

• CH 4 production from liquid hot water pretreated Hybrid Pennisetum was optimized. • CH 4 production increased by 32.9% at optimal pretreatment condition of 175 °C. • Theoretical energy conversion efficiency of 76.1% was achieved after pretreatment. • Process energy efficiency can be realized to 51.7% by pretreatment heat recovery. To overcome the inherent complexity of lignocellulosic biomass and improve its biological conversion efficiency, the central composite surface response methodology was used to optimize the process variables (temperature, reaction time and liquid-to-solid ratio) in liquid hot water pretreatment for biomethane production from the anaerobic digestion of Hybrid Pennisetum (a typical energy crop in southern China). Temperature, reaction time and water content were varied between 160–200 °C, 20–60 min and 60–90 mL respectively. The biomethane yields from pretreated material were considered as response variables to different processes conditions. Results showed untreated Hybrid Pennisetum had a biomethane potential of 218.6 L/kg volatile solid (VS) (corresponding to a biodegradability of 62.6%). The effect of liquid hot water pretreatment on the biomethane yield was distinct. Pretreatment severities less than 4.55 resulted in increased biomethane yields. However, further increasing the severity led to lower biomethane yields. The optimum biomethane yield (290.6 L/kg VS) was obtained from pretreated Hybrid Pennisetum at 175 °C for 35 min, with the highest biodegradability of 83.2%. Energy balance showed that, under the optimal pretreatment condition, the highest energy conversion efficiency of 76.1% was attained in theory. If the process energy inputs are included, a process energy efficiency of 51.7% could be realized with proper heat recovery. [ABSTRACT FROM AUTHOR]