Biomethane production by typical straw anaerobic digestion: Deep insights of material compositions and surface properties

Genome-centric metagenomics revealed functional traits in high-solids anaerobic co-digestion of restaurant food waste, household food waste and rice straw

High-solids anaerobic co-digestion (HS-AcoD) of food waste (FW) and other organic wastes is an effective option to improve the biogas production and system stability compared to mono-digestion. However, the clean and sustainable HS-AcoD strategy for FW and associated microbial functional traits have not been well explored. Here, HS-AcoD of restaurant food waste (RFW), household food waste (HFW) and rice straw (RS) were performed. Results showed that the maximum synergy index (SI) of 1.28 were achieved when the volatile solids ratio of RFW, HFW and RS was 0.45:0.45:0.1. HS-AcoD alleviated the acidification process by regulating metabolism associated with hydrolysis and volatile fatty acids formation. The synergistic relationship between syntrophic bacteria and Methanothrix sp., and the enhanced metabolic capacity associated with the acetotrophic and hydrogenotrophic pathways dominated by Methanothrix sp., provided a further explanation of the synergistic mechanism. These findings advance the knowledge about microbial mechanisms underlying the synergistic effect of HS-AcoD.

Recent progress and challenges in biotechnological valorization of lignocellulosic materials: Towards sustainable biofuels and platform chemicals synthesis

Lignocellulosic materials (LCM) have garnered attention as feedstocks for second-generation biofuels and platform chemicals. With an estimated annual production of nearly 200 billion tons, LCM represent an abundant source of clean, renewable, and sustainable carbon that can be funneled to numerous biofuels and platform chemicals by sustainable microbial bioprocessing. However, the low bioavailability of LCM due to the recalcitrant nature of plant cell components, the complexity and compositional heterogeneity of LCM monomers, and the limited metabolic flexibility of wild-type product-forming microorganisms to simultaneously utilize various LCM monomers are major roadblocks. Several innovative strategies have been proposed recently to counter these issues and expedite the widespread commercialization of biorefineries using LCM as feedstocks. Herein, we critically summarize the recent advances in the biological valorization of LCM to value-added products. The review focuses on the progress achieved in the development of strategies that boost efficiency indicators such as yield and selectivity, minimize carbon losses via integrated biorefinery concepts, facilitate carbon co-metabolism and carbon-flux redirection towards targeted products using recently engineered microorganisms, and address specific product-related challenges, to provide perspectives on future research needs and developments. The strategies and views presented here could guide future studies in developing feasible and economically sustainable LCM-based biorefineries as a crucial node in achieving carbon neutrality.

Recent Update on anaerobic digestion of paddy straw for biogas production: Advancement, limitation and recommendations

At present, development and production of advanced green energy sources are highly demanded, and this may offer a clean and sustainable environment to our modern society. In this reference, biogas is emerging as a promising green energy source and seems to have high potential to replace fossil-fuel based energy sources in the coming future. Further, lignocellulosic biomass (LCB) based biogas production technology has been found to be highly promising owing to several advantages associated therewith. Rich inorganic content, renewable nature, huge availability and low-cost are the key beneficial factors of LCB-based feedstock l to produce biogas. Among the varieties of LCB, paddy straw is one of the most demanding feedstocks and is highly rich in organic compounds that are imperative to producing biogas. Nevertheless, it is noticed that paddy straw as a waste material is usually disposed-off by direct burning, whereas it exhibits low natural digestibility due to the presence of high lignin and silica content which causes severe environmental pollution. On the other hand, paddy straw can be a potential feedstock to produce biogas through anaerobic digestion. Therefore, based on the current ongoing research studies worldwide, this review evaluates the advancements made in the AD process. Meanwhile, existing limitations and future recommendations to improve the yield and productivity of the biogas using paddy straw have been discussed. The emphasis has also been given to various operational parameters developments, related shortcomings, and strategies to improve biogas production at pilot scale.

Removal Ability of Bacillus licheniformis on Waxy Cuticle on Wheat Straw Surface

The outermost surface of wheat straw (WS) is covered with hydrophobic lipophilic extracts and silica, which affects follow-up processes such as impregnation pretreatment of pulping and papermaking. In this study, a strain named Bacillus licheniformis (B. licheniformis) was screened from the black liquor of papermaking, which was used to explore the effect of its treatment on the waxy cuticle of WS. Scanning electron microscope-energy dispersive spectroscopy (SEM-EDS) showed that the B. licheniformis had a certain destructive effect on the outer surface of WS and the content of Si on the outer surface decreased by 80%. The results of FTIR and X-ray photoelectron spectroscopy (XPS) displayed that the wax composition on the outer surface of WS decreased and the fiber structure inside appeared. The mechanical properties of paper demonstrated that the treated WS is still feasible in this field and the content of Si in the black liquor is reduced by 33%. Therefore, the WS treated by B. licheniformis can destroy the waxy cuticle on its outer surface and improve the wettability of WS. It provides a new idea to alleviate the “Si interference” problem of alkali recovery in WS traditional pulping and papermaking.

Deploying two-stage anaerobic process to co-digest greasy sludge and waste activated sludge for effective waste treatment and biogas recovery

High-strength waste activated sludge (WAS) and greasy sludge (GS) were largely generated from canned tuna processing. This study reports the performance of the two-stage anaerobic process for co-digesting WAS and GS. Various WAS:GS mixing ratios of 0:100, 10:90, 20:80, 30:70, 40:60, 50:50, 60:40, 70:30, 80:20, 90:10, and 100:00 (volatile solids (VS) basis) were investigated in batch acidogenic stage at ambient (30 °C ± 3 °C), 55 °C, and 60 °C temperatures. Subsequently, the effluents from the first stage were used to produce methane in the second methanogenic stage at an ambient temperature. The highest methane yield of 609 mL CH₄/g-VS_added was achieved using acidogenic effluents generated from a WAS:GS mixing ratio of 40:60 at an ambient temperature. The first-order kinetic constants (k) for the first (k₁) and second (k₂) stages were subsequently estimated to be 0.457 d^-1 and 0.139 d^-1, respectively. The obtained k constants were further used to predict the hydraulic retention time (HRT) for the two continuously stirred tank reactors (CSTR) in series. Consequently, the calculated 4-day HRT and 20-day HRT for 50-L CSTR₁ and 250-L CSTR₂, respectively, were used to operate the continuous two-stage process at an ambient temperature by feeding with a 40:60-WAS:GS mixing ratio. A satisfactory methane yield of 470-mL CH₄/g-VS along with 75% chemical oxygen demand (COD) removal was generated. Furthermore, the predicted methane yield of 450-mL CH₄/g-VS obtained from the simple kinetic CSTR model resembled the experimental yield with 96% accuracy. The obtained experimental results demonstrate that WAS and GS co-digestion could be successfully accomplished using a practical two-stage anaerobic process operated at an ambient temperature.

Fed-batch anaerobic digestion of raw and pretreated hazelnut skin over long-term operation

This study provided important insights on the anaerobic digestion (AD) of hazelnut skin (HS) by operating a fed-batch AD reactor over 240 days and focusing on several factors impacting the process in the long term. An efficient reactor configuration was proposed to increase the substrate load while reducing the solid retention time during the fed-batch AD of HS. Raw HS produced maximally 19.29 mL CH₄/g VS_add/d. Polyphenols accumulated in the reactor and the use of NaOH to adjust the pH likely inhibited AD. Maceration and methanol-organosolv pretreatments were, thus, used to remove polyphenols from HS (i.e. 82 and 97%, respectively) and improve HS biodegradation. Additionally, organosolv pretreatment removed 9% of the lignin. The organosolv-pretreated HS showed an increment in methane potential of 21%, while macerated HS produced less methane than the raw substrate, probably due to the loss of non-structural sugars during maceration.

A novel approach using protein-rich biomass as co-fermentation substrates to enhance phosphorus recovery from FePs-bearing sludge

A novel approach for the enhancement of phosphorus (P) recovery from Fe bound P compounds (FePs)-bearing sludge by co-fermentation with protein-rich biomass (PRB) is reported. Four PRBs (silkworm chrysalis meal, fish meal, corn gluten meal, and soya bean meal) were used for co-fermentation. The results revealed that PRBs with strong surface hydrophobicity and loose structure favored the hydrolysis and acidogenesis processes. Sulfide produced by PRB could react with FePs to form FeS and promote P release. Due to the neutralization of volatile fatty acids (VFAs) by a relatively high concentration of ammonia, the pH was maintained near neutral and thus prevented the dissolution of metal ions (e.g., Fe and Ca). This was beneficial to save the cost of subsequent P recovery and form high-purity struvite. Compared with the control, the soluble orthophosphate and VFAs increased by 88.3% and 531.3%, respectively, in the co-fermentation system with silkworm chrysalis meal. Cysteine was the important intermediate. The metagenomics analysis indicated that the gene abundances of phosphate acetyltransferase and acetate kinase, which were key enzymes in the acetate metabolism, increased by 117.7% and 52.2%, respectively. The gene abundances of serine O-acetyltransferase and cysteine synthase increased by 63.4% and 54.4%, respectively. Cysteine was primarily transformed to pyruvate and sulfide. This study provides an environment-friendly strategy to simultaneously recover P and VFAs resources from FePs-bearing sludge and PRB waste.

Impact of substrate digestibility on microbial community stability in methanogenic digestors: The mechanism and solution

This study investigated the temporal dynamics of digestion efficiency and community stability in digesters fed with waste activated sludge (WAS), straw (STR-AD), food waste (FW-AD) and mixture of straw-and-food waste (STR-FW-AD). Results showed that carbon removals of recalcitrant substrates (i.e., 48.2 ± 3.9% in WAS-AD and 57.8 ± 4.9% in STR-AD) were lower than that of labile substrates (i.e., 70.7 ± 4.0% in FW-AD). Nonetheless, carbon removal of recalcitrant substrates was largely improved through co-digestion (70.3 ± 3.2% in STR-FW-AD). In contrast to monopoly communities (e.g., the highly enriched Paludibacter) fed with the labile substrates, recalcitrant substrates supported highly diverse communities. Accordingly, the medians of negative/positive cohesions of communities in WAS-AD, STR-AD, STR-FW-AD and FW-AD decreased from 0.86 to 0.63, suggesting their decreasing community stability. Microbial source tracking analyses showed the major contribution of the STR-AD community to the co-digestion community. This study provided unprecedented mechanistic insight into stability improvement of substrate co-digestion on the methanogenic digestion microbiome.

Hydrothermal carbonization of garden waste by pretreatment with anaerobic digestion to improve hydrohcar performance and energy recovery

Sustainable and resourceful utilization of garden waste with high lignocellulosic content remains a huge challenge, anaerobic digestion (AD) and hydrothermal treatment provide prospective technologies with achieving environmental and economic benefits. In this study, a 7-28 d AD was provided as a biomass pretreatment means and combined with hydrothermal carbonization (HTC) to treat three typical garden wastes (leaves, branches, grass). The results showed that AD pretreatment could effectively change the surface composition and structure properties of the feedstocks and thus modulating the properties of the hydrochar downstream. Compared to the unpretreatment samples, the specific surface area (SSA), higher heating value (HHV), energy density and nutrient elements (P and K) of hydrochar obtained by AD pretreatment were significantly improved and enriched, respectively. Specifically, the highest HHV of hydrochar obtained from leaves, branches, and grass were 25.71, 25.63, and 23.81 MJ/kg, which obtained with 21, 14, and 7 d of AD pretreatment respectively. The P contents of hydrochar of leaves and grass pretreated with AD for 14 and 7 d were 205% and 15% higher than those without AD pretreatment, respectively. Additionally, in this coupled system, the biomass energy recovery of 90.2% (78.2% biochar and 12.0% CH₄) was achieved on leaves pretreated with AD for 21 d. Energy recovery of 81.2% (66.8% biochar, 14.4% CH₄) and 71.3% (39.7% biochar, 31.6% CH₄) was obtained by 14 d of AD pretreatment on branches and grass, respectively. Thus, this study enhances energy utilization efficiency and reduces secondary waste generation, providing valuable new insights into AD coupled with HTC technology.

Multi-fractal structure features of corn stalks and their correlation with pretreatment homogeneity and efficacy

Lignocellulose biomass is a natural porous medium with fractal characteristics, which tend to be distinct after certain pretreatment and relational with processing effects. In this work, multi-scale fractal features of corn stalks after steam explosion pretreatment were systematically characterized employing digital image processing and mercury intrusion porosimetry methodologies. The macroscopic surface fractal features (D_s: 2.8278 ∼ 2.8523) and microscopic pore fractal features (D_p: 2.3003 ∼ 2.8867) presented same variation tendency with pretreatment severity, revealing the self-similarity of processing results of corn stalks with the scale. In association with pretreatment homogeneity and efficacy, the decrease in fractal dimensions corresponded to morphologically homogeneous and structurally complex samples with preferable auto-hydrolysis degree of structural components, which led to high reactivity with enzymes. Quantitatively, there were strongly linear correlation between fractal dimensions and enzymatic digestibility with r² > 0.95. Fractal dimension was expected to theoretically guide the rational evaluation, prediction and promotion of the key pretreatment technique in biorefinery.

Carbon Footprint Assessment of a Novel Bio-Based Composite for Building Insulation

This research explores the carbon removal of a novel bio-insulation composite, here called MycoBamboo, based on the combination of bamboo particles and mycelium as binder. First, an attributional life cycle assessment (LCA) was performed to define the carbon footprint of a European bamboo plantation and a bio-insulation composite, as well as its ability to remove CO2 along its lifecycle at a laboratory scale. Secondly, the Global Worming Potential (GWP) was estimated through a dynamic LCA with selected end-of-life and technical replacement scenarios. Finally, a building wall application was analyzed to measure the carbon saving potential of the MycoBamboo when compared with alternative insulation materials applied as an exterior thermal insulation composite system. The results demonstrate that despite the negative GWP values of the biogenic CO2, the final Net-GWP was positive. The technical replacement scenarios had an influence on the final Net-GWP values, and a longer storage period is preferred to more frequent insulation substitution. The type of energy source and the deactivation phase play important roles in the mitigation of climate change. Therefore, to make the MycoBamboo competitive as an insulation system at the industrial scale, it is fundamental to identify alternative low-energy deactivation modes and shift all energy-intensity activities during the production phase to renewable energy.

Links between carbon/nitrogen ratio, synergy and microbial characteristics of long-term semi-continuous anaerobic co-digestion of food waste, cattle manure and corn straw

Links between carbon/nitrogen (C/N) ratio, synergy and microbial characteristics of anaerobic co-digestion of food waste (FW), cattle manure (CS) and corn straw (CS) were investigated. Digesters with 100% CS, 25% FW + 75% CS, 25% CM + 75% CS suffered acid inhibition, in close association with unbalanced C/N and the resulting recessions of Syntrophomonadaceae and Methanosaeta. Co-digestion overcame C/N imbalance and achieved multiple synergies. Process performance had a positive correlation with Syntrophomonadaceae. Digester with 75% FW + 25% CS had most Syntrophomonadaceae (26.7%) and methane yield (467.3-507.6 mL/g VS) among co-digestion trials. Synergy was greater under higher load and exhibited a good correlation with C/N ratio. Co-digestion of FW, CM and CS (2:2:1) with suitable C/N ratio (20.79) obtained the greatest synergistic rate (14.6%). Unstable systems were improved by adjusting C/N ratio to 30 via urea, which stimulated Methanosarcina growth therefore enhanced methanogenic pathway diversity and ensured powerful methanogenic functions.

The biological and abiotic effects of powdered activated carbon on the anaerobic digestion performance of cornstalk

To comprehensively evaluate the biological and abiotic influence of powdered activated carbon (PAC) on the anaerobic digestion of cornstalk, mesophilic and thermophilic digestion were conducted. Adding PAC (10 g/L) under thermophilic system obtained the maximum cellulose degradation rate and methane yield (MY), which were 57.47% and 128.19 L/kg VS. However, adding same dose of PAC at mesophilic system decreased the MY by 8.16% while increased the cellulose degradation rate and methane production rate by 6.48% and 17.92%. Under mesophilic conditions, the enhancement of PAC was owing to the enrichment of cellulolytic microorganisms, improvement of the syntrophic process and direct interspecies electron transfer. The lower methane yield was attributed to the adsorption of carbon source by PAC and CH₄ consumption by Norank_c_Bathyarchaeia. The good performance of thermophilic system was owing to the lower adsorption capability of PAC, absence of Norank_c_Bathyarchaeia, and concentrated carbon flow to methane.

Study on the performance of HNO3-modified biochar for enhanced medium temperature anaerobic digestion of food waste

Biochar can help promote direct interspecies electron transfer (DIET) and increase methane production; the surface redox groups play a constructive role in these processes. This study attempted to improve the anaerobic digestion (AD) performance by modifying biochar with HNO₃ to increase its redox activity. A comparative experimental study, raw biochar (BC0) and biochar treated with HNO₃ for 6 h (BC6), were conducted to investigate the effect of HNO₃ treatment on the medium temperature AD performance of food waste. Both BC0 and BC6 can enhance CH₄ yield and facilitate the degradation of volatile fatty acids. The enhanced yield of CH₄ was 36% for BC0 and 90% for BC6, respectively. Biochar can also enhance methanogenesis, presumably owing to direct interspecific electron transfer (DIET). Compared with BC0, BC6 had a higher redox activity and a smaller conductivity. It was supposed that BC0 mediated DIET through its conductivity, whereas BC6 accelerated DIET by surface redox groups.

Biologically removing vanadium(V) from groundwater by agricultural biomass

Vanadium (V) in groundwater can pose a serious threat on both environment and health. Agricultural biomass contains solid carbon source (SCS) and could be attractive for biologically removing V(V). For this purpose, cypress sawdust, corn cob and wheat straw were selected as SCSs to remove vanadate (NaVO₃). The experiments demonstrated a high efficiency of V(V) up to 98.6%, and the anaerobically biological reduction of V(V) to V(IV) by wheat straw was identified to be the best SCS by the spectrum analysis of XRD and FTIR. Along with increasing the fragment size of wheat straw, the V(V)-removal efficiency decreased, and the fragment size down to 1-3 mm was confirmed to have a significant bio-removal performance on V(V). Based on the analysis of 16s rRNA sequencing, the microbial abundance and diversity increased in the suspension liquid in the end, indicating that the microbial community could tolerate and/or detoxify V(V), besides degrading lignocellulosic materials.

A novel thermal pretreatment method called air frying improves the enzymatic saccharification effect of straw

Thermal pretreatment is a shared method to improve the efficiency of straw biochemical conversion. However, the process is often accompanied by problems such as the loss of carbon source. The objective of this study was to develop a novel thermal pretreatment method, called air frying, with less loss of biodegradable organic matter and favorable surface hydrophilicity. After pretreatment of two straws, the sugar yields were improved, and the improvement effect of corn stover was more significant. The total saccharification rate of corn stover increased from 31.31 ± 1.06% to 44.77 ± 1.23%, and the cellulose conversion rate increased from 44.10 ± 1.85% to 67.44 ± 2.37%. Functional groups with lower polarity on the surface were oxidized into the one with stronger polarity, so the hydrophilicity was enhanced. The surface roughness decreased, the surface tended to be flat, the original pores collapsed, and the average pore size increased, which was more conducive to the binding of enzyme and active site. The matching relationship between the optimal conditions and substrate, and the possibility of scaling up need further study.

A Review of Key Technologies and Trends in the Development of Integrated Heating and Power Systems in Agriculture

Petroleum agriculture, characterized by mechanization and chemistry, is developing rapidly in China. However, petroleum agriculture has not only brought food safety problems, but also caused great obstacles to the sustainable development of society. In view of the disadvantages of oil agriculture, we provide an upgrading plan for energy systems in agriculture. This work can help reduce carbon emissions and improve food security. We introduce the most advanced technologies in Chinese agricultural development and the technical scope includes new agricultural energy power generation, agricultural energy use and the safe operation of agricultural energy systems. We describe the detailed data of agricultural bioenvironmental and energy engineering to clarify the level of agricultural energy efficiency in China. The overall conclusion of this paper is that the deep integration of agriculture and energy internet has become the development trend of agricultural energy systems.

A specious relevance between theory and practice: Insights into temperature parameter and multi-phase strategy of anaerobic digestion of straw

Anaerobic digestion (AD) of straw is a highly complex and dynamic process. The temperature range of mesophilic (30-40 °C) and thermophilic (50-65 °C) are usually recommended in textbook notion. The two-phase strategy is usually applied based on the classical theory, including acidification-phase and methanation-phase. However, both the optimized temperature parameter and the enhanced multi-phase strategy solely focus on the local AD process. A specious relevance between theory and practice during AD process of straw has always been argued. Classical AD theory was not necessarily the sufficient approach to guide the anaerobic biological transformation of straw. More profound investigations of optimum temperature are still needed, uniquely synergistic mechanisms of functional microorganisms, as well as process stability, should be taken into account. Besides, additional research should focus on the matching between the physicochemical properties and process parameters/strategies choosing. A multi-stage operation strategy based on straw material composition is a potential operation approach to improve its efficiency. Furthermore, more comprehensive attention should be paid to the collaborative response mechanism by coupling substrate, temperature, and microbial in complex AD systems for straws.

Evolution of chemical functional groups during torrefaction of rice straw

The evolution of CHON functional groups during torrefaction of rice straw at 200-300 °C were investigated. The results showed that 300 °C was more suitable for rice straw torrefaction due to the ideal fuel ratio, energy densification, energy-mass co-benefit index and the significantly improved HHV of the torrefied products. The functional groups such as O-H, N-H, C-H, C = O in the solids decreased with rising temperature accompanied by the releases of H₂O, CH₄, CO₂, CO and NH₃, et al. At 300 °C, 40.04% of fuel-N was released in the form of NH₃, HCN, HNCO et al. due to the decomposition of N-A which was the overall N-functionality in the raw rice straw. It is worth noting that the absorbance of NH₃ and HCN has the same order of magnitude as CO. Therefore, the releases of N-containing gases should be highly concerned for the application of torrefaction technology from the environmental perspective.