The required characteristics of ensiled crops used as a feedstock for biogas production: a review

Valorization of crop residues and animal wastes: Anaerobic co-digestion technology

To switch the over-reliance on fossil-based resources, curb environmental quality deterioration, and promote the use of renewable fuels, much attention has recently been directed toward the implementation of sustainable and environmentally benign 'waste-to-energy' technology exploiting a clean, inexhaustible, carbon-neutral, and renewable energy source, namely agricultural biomass. From this perspective, anaerobic co-digestion (AcoD) technology emerges as a potent and plausible approach to attain sustainable energy development, foster environmental sustainability, and, most importantly, circumvent the key challenges associated with mono-digestion. This review article provides a comprehensive overview of AcoD as a biochemical valorization pathway of crop residues and livestock manure for biogas production. Furthermore, this manuscript aims to assess the different biotic and abiotic parameters affecting co-digestion efficiency and present recent advancements in pretreatment technologies designed to enhance feedstock biodegradability and conversion rate. It can be concluded that the substantial quantities of crop residues and animal waste generated annually from agricultural practices represent valuable bioenergy resources that can contribute to meeting global targets for affordable renewable energy. Nevertheless, extensive and multidisciplinary research is needed to evolve the industrial-scale implementation of AcoD technology of livestock waste and crop residues, particularly when a pretreatment phase is included, and bridge the gap between small-scale studies and real-world applications.

A systematic review of the design considerations for the operation and maintenance of small-scale biogas digesters

This review investigates small-scale biogas digesters' design and construction considerations to address biogas digesters' failures shortly after installation. The frequent failures of small-scale or household biogas digesters negatively affect its adoption as a clean domestic cooking fuel in developing countries, affecting the achievement of Sustainable Development Goal (SDG) 7. The study considered Scopus database-indexed peer-reviewed journals published between 2000 and 2022. Selected papers focussed on real-time monitoring, stirring mechanisms, and temperature control systems based on predefined inclusion and exclusion criteria with initial search results of 4751 documents, narrowing to 55 papers. The PRISMA 2020 statement was adopted to conduct the study. The study highlights the importance of incorporating a real-time monitoring system as a design factor in small-scale biogas digesters for successful operation and maintenance. The study's findings may be helpful to practitioners, policymakers, and researchers promoting sustainable energy and waste management solutions in low-resource settings.

Enhanced silage pretreatment improving the biochemical methane potential of Miscanthus sinensis

The choice of silage additives is an important factor for the storage of silage. One standard ensiling method and two enhanced ensiling methods (using natural silage, silage with mixed lactic acid bacteria, and silage with acetic acid, respectively) were carried out on Miscanthus sinensis. To determine the effects of these different methods, the biochemical methane potential (BMP) was determined. The results revealed that ensiling with acetic acid was the best method among the three ensiling methods. Acetic acid could quickly reduce the pH of the system to inhibit the growth of harmful bacteria. The rate of loss of dry matter was 0.92% when acetic acid was added, and the cumulative methane production was 149.6 mL·g^-1 volatile solids. From an analysis of correlations between the properties and BMP of silage, the contents of acetic acid and total volatile fatty acids were significantly correlated with the BMP. This study provides a theoretical basis for improving the BMP of M. sinensis and achieving better effects of silage.

Improving the storage of cover crops by co-ensiling with different waste types: Effect on fermentation and effluent production

Cover crops harvested at a low maturity stage generally have a high moisture content, which may generate energy losses during silage storage via effluent production and undesirable fermentations. This paper investigates the use of different waste types as absorbent co-substrates to be added before ensiling. The relation between the absorbent water holding capacity and silage effluent volume was first studied to find an effective parameter to prevent effluent production. Effluent retention was found to be proportional to the absorbent loading and water holding capacity (r2 = 0.98) and up to 90 % of effluent production was avoided when compared to control (295 l.t-1). The impact of different co-substrates (including bio-waste and manures) on overall ensiling performances was then investigated at an optimized absorbent loading. All co-substrates allowed a total effluent retention while a 76 l.t-1 effluent volume was reported for the control. The silage fermentation was modified or mostly unchanged depending on the co-substrate chemical and microbial properties and different metabolic pathways were observed (e.g. homolactic or butyric fermentation). In most conditions, the methane potential of the crop was efficiently preserved over a storage of 60 days. Co-ensiling was shown to be a relevant silage preparation method for biogas production.

The Effect of Early and Delayed Harvest on Dynamics of Fermentation Profile, Chemical Composition, and Bacterial Community of King Grass Silage

The objective of this study was to assess the effect of harvesting time on the fermentation characteristics, chemical composition, and microbial community of king grass silage. King grass was harvested at three growth periods of 90 days (KN90S), 110 days (KN110S), and 130 days (KN130S); chopped into 2-3-cm particle size; and ensiled in polyethylene bags (20 × 30 cm). The fermentation quality and chemical composition of silages were analyzed after 1, 3, 7, 14, 30, and 60 days of ensiling. Bacterial community of silage ensiled for 60 days was profiled using next generation sequencing (NGS) technology. The KN110S showed the most extensive lactic acid (LA) fermentation during 7 days of fermentation compared to KN90S and KN130S. After 60 days of fermentation, the KN110S showed the lowest pH and the highest lactic acid content among the three treatments. The butyric acid and ammonia nitrogen contents of KN90S and KN130S were significantly greater than those of KN110S (p < 0.05). After a timespan of 60 days of ensiling, the bacterial community of king grass silage was predominantly populated by Proteobacteria in phylum level, whereas unclassified Enterobacteriaceae genus remained dominant in all silages. A higher relative abundance of Clostridium was observed in KN90S and KN130S, but not in KN110S, and greater abundance of Lactococcus appeared in KN110S and KN130S silages than KN90S. It is concluded that harvesting time had an important effect on the fermentation quality and microbial community of king grass silage.

Ensiling process for efficient biogas production from lignocellulosic substrates: Methods, mechanisms, and measures

Ensiling has been developed as mainstream technologies to preserve lignocellulose biomass for biogas production. However, the lack of general evaluation methods and process mechanism research hinders the understanding of its effectiveness. In this context, we reviewed existing studies and proposed some key considerations: (1) For assessing the ensiling process, determined dry matter contents should be corrected according to the volatilization loss in oven-drying method to obtain accurate storage loss and methane yield; (2) For comprehensive assessments, the trade-off between storage loss and enhanced biomethane yield should be evaluated from the entire-chain process; (3) The mechanism to enhance methane yield is primarily attributed to increased lignocellulosic biodigestibility through acid-based hydrolysis and biological degradation during ensiling; (4) Measures including co-storage, increasing buffering capacity, adjusting carbon/nitrogen ratio, and additives can be adopted to increase biogas production. The proposed methods, mechanisms, and measures (3Ms) could help initiate the specific quality criteria of biogas-oriented silages.

Ensilability of Biomass From Effloresced Flower Strips as Co-substrate in Bioenergy Production

Flower strips are grown to an increasing degree in order to enhance the ecological value of agricultural landscapes. Depending on their profitable life span and the crop sequence, the strips' biomass must be mulched after flowering to enable repeated tillage. A promising alternative is the use of the flower strips' biomass as a co-substrate for biomethanisation - thereby contributing to the climate-friendly generation of energy. This potential bioenergy substrate occurs only seasonally and is commonly produced only in limited quantities at a farm scale. To realize the additional benefit of flower strips as energy suppliers, stock piling of the strips' biomass is required. However, information about the ensilability of flower strip biomass is still rare. We conducted a 2-year study to analyze the ensilability of pure biomass from effloresced flower strips and mixtures of flower strip biomass with 33 and 67% whole crop maize, respectively. Ensiling took place in 3 l model silos at laboratory scale after chopping the substrate. Before ensiling several chemical characteristics of the biomass stock were determined to assess the substrate's biochemical ensilability potential (dry matter content, water-soluble carbohydrates, buffering capacity, nitrate content). The process-engineered ensiling success after 90 days was determined based on fermentation patterns. The ensilability potential of the pure flower strip substrates reached modest levels (fermentability coefficients according to Weißbach vary around the threshold of 45). Nevertheless, acceptable silage qualities were achieved under the laboratory conditions (pH ranging from 4.2 to 4.7). Compared to pure flower strip biomass, the addition of maize noticeably improved both the substrate's biochemical ensilability potential and the quality of real fermented silage. We conclude that a mixture of 33% biomass from flower strips with 67% whole crop maize can be regarded as a recommendable ratio if proper ensiling technology is applied.

Synergetic effect of combined ensiling of freshly harvested and excessively wilted maize stover for efficient biogas production

This study investigated the synergetic effects of ensiling freshly harvested maize stover (FHM) and excessively wilted maize stover (EWM) on biogas production. FHM and EWM were mixed in various proportions to obtain dry matter (DM) contents of 30%, 35% and 40%. For reference, FHM alone was ensiled and stored in open-air. Successful storage performance was obtained by the ensiling treatments, and the organic matter loss of 1.1-2.2% was far lower than in open-air storage (63.1%). An initial water-soluble carbohydrate (WSC) of 5% DM is adequate for the combined ensiling of maize stover with the highest WSC degradation rate of 81.2%. Combined ensiling enhanced the activity of Weissella, a genus of heterofermentative lactic acid bacteria, under relatively high pH conditions. Therefore, the combined ensiling can preserve FHM and enhance the digestibility of EWM (theoretical specific methane yield increased 16.5%), which would be a promising storage strategy for efficient biogas production.

Carboxylic acid production from ensiled crops in anaerobic solid-state fermentation - trace elements as pH controlling agents support microbial chain elongation with lactic acid

BACKGROUND

For the production of carboxylic acid platform chemicals like medium-chain fatty acids (MCFA) by anaerobic fermentation, pH control is required. However, adding buffer solutions is ineffective in leach-bed reactors.

AIM

In order to increase the MCFA production by maize silage fermentation and to engineer the process we investigated the effect of solid alkaline iron and manganese additives on the process performance and microbial community dynamics.

RESULTS

Without additives, the pH dropped to 3.9 and lactic acid bacteria were favored. Total product yields of 207 ± 5.4 g organic acids (C₂-C₆) and alcohols per kg volatile solids were reached. The addition of trace elements increased the pH value and the product spectrum and yields changed. With a commercial iron additive, the product yields were higher (293 ± 15.2 g/kg_{volatile solids}) and supposedly clostridia used lactic acid for microbial chain elongation of acetic acid producing n-butyric acid. With the addition of pure Fe(OH)₃ or Mn(OH)₂, the total product yields were lower than in the other reactors. However, increased production of MCFA and the occurrence of distinct bacterial taxa (Lachnospiraceae, Ruminococcaceae and Megasphaera) related to this metabolic function were observed.

CONCLUSIONS

The application of alkaline trace metal additives as pH stabilizing agents can mitigate spatial metabolic heterogeneities when trace metal deficient substrates like specific crops or residues thereof are applied.

Effect of particle size reduction and ensiling fermentation on biogas formation and silage quality of wheat straw

The effect of ensiling fermentation and mechanical pretreatment on the methane yield of lignocellulosic biomass was investigated in order to determine the optimum pretreatment conditions for biogas production. Wheat straw was treated using the following techniques: mechanical disintegration by chopping and extruder-grinding to particle sizes of 2.0 and 0.2cm, respectively, and ensiling by 30% and 45% total solids with addition of enzymatic, chemical and biological silage additives individually and in combination. The total and volatile solid content, biochemical methane potential and products of silage fermentation of 32 variants were tested. The results indicate that the methane potential increased by 26% (from 179 to 244mLCH₄g^-1VS) by reducing particle size. The maximum methane potential of 275mLCH₄g^-1VS was obtained from silage with 30% total solids and extruder grinding. However, the effect of the addition of silage additives on the methane potential was limited.

Characteristics of on-demand biogas production by using sugar beet silage

On-demand electricity generation can be achieved by just-in-time biogas production instantly utilized in co-generation units. For this goal, easily degradable substrates like sugar beet silage have a high potential. Potential for on-demand biogas production from co-digestion of sugar beet silage (SS) with grass silage (GS) was evaluated in two experiments at organic loading rates (OLRs) of 1.5 kgVS m^-3 day^-1 and 2.5 kgVS m^-3 day^-1, respectively. Each experiment was fed with intermittent feeding system at 8 hrs interval at the same feedstock ratios (volatile solids based) of GS:SS-1:0, 3:1 and 1:3, respectively. Modelling by Gaussian equation was performed in order to understand the effects of SS on biogas production. Addition of sugar beet silage led to maximum biogas production within a short time, but it differed significantly depending on feedstock ratios and OLRs, respectively. At OLR 1.5 kgVS m^-3 day^-1, during mono fermentation of grass silage maximum biogas production rate of 0.27 l_N hr^-1 was reached at 2.74 hrs. Production rate did not change at feedstock ratio of GS:SS-3:1 but increased to 0.64 l_N hr^-1 at GS:SS-1:3 within a shorter time span (1.58 hrs). On the contrary, at OLR of 2.5 kgVS m^-3 day^-1 time span between feedstock input and maximum biogas production did not differ significantly (p > 0.05) among the reactors. Biogas production rates were 0.60 l_N hr^-1 within 2.27 hrs and 0.82 l_N hr^-1 within 2.30 hrs at GS:SS-3:1 and GS:SS-1:3, respectively. Surprisingly, there was no time lag between maximum biogas and methane production rates, irrespectively of OLR. This implies that once the whole microbial community is adapted to intermittent substrate input, the metabolic products are instantly utilized through the all steps of anaerobic substrate degradation. Applying this finding opens new perspectives for on-demand biogas energy production.

Material and microbial changes during corn stalk silage and their effects on methane fermentation

Silage efficiency is crucial for corn stalk storage in methane production. This study investigated characteristics of dynamic changes in materials and microbes during the silage process of corn stalks from the initial to stable state. We conducted laboratory-scale study of different silage corn stalks, and optimized silage time (0, 2, 5, 10, 20, and 30days) for methane production and the endogenous microbial community. The volatile fatty acid concentration increased to 3.00g/L on Day 10 from 0.42g/L on Day 0, and the pH remained below 4.20 from 5.80. The lactic acid concentration (44%) on Day 10 lowered the pH and inhibited the methane yield, which gradually decreased from 229mL/g TS at the initial state (Day 0, 2) to 207mL/g TS at the stable state (Day 10, 20, 30). Methanosaeta was the predominant archaea in both fresh and silage stalks; however, richness decreased from 14.11% to 4.75%.

Co-Digestion of Sugar Beet Silage Increases Biogas Yield from Fibrous Substrates

This study tested the hypothesis that the easily degradable carbohydrates of the sugar beet silage (S) will improve the anaerobic digestion of grass silage (G) more profoundly compared to co-digestion of sugar beet silage with maize silage (M). M : S and G : S mixtures were tested in two continuous laboratory-scale AD experiments at volatile solid ratios of 1 : 0, 6 : 1, 3 : 1, and 1 : 3 at organic loading rates of 1.5 kgVS m⁻³ day⁻¹. While the sugar beet effects in mixtures with maize silage were negligible, co-digestion with grass silage showed a beneficial performance. There, the specific methane production rate was 0.27 l_N kg⁻¹VS h⁻¹at G : S ratio of 6 : 1 compared to G : S 1 : 0 with 0.14 l_N kg⁻¹VS h⁻¹. In comparison to G : S 1 : 0, about 44% and 62% higher biogas yields were obtained at G : S 6 : 1 and 3 : 1, respectively. Also, the highest methane concentration was found in G : S at ratio of 1 : 3. Synergistic increase of methane yield was found in co-digestion in both experiments, but higher effect was realized in G : S, independently of the amount of sugar beet silage. The findings of this study emphasize the improvement of AD of grass silage by even low addition of sugar beet silage.

Low-cost additive improved silage quality and anaerobic digestion performance of napiergrass

Effects of molasses-alcoholic wastewater on the ensiling quality of napiergrass were investigated at ambient temperature, and its anaerobic digestion performance was assessed at mesophilic temperature. Results showed that the molasses-alcoholic wastewater had positive effect on silage quality and anaerobic digestion performance. Lower pH values of 5.20-5.28, lower NH3-N contents of 32.65-36.60 g/kg and higher lactic acid contents of 56-61 mg/kg FM were obtained for the silage samples with molasses-alcoholic wastewater addition. Higher specific biogas yield of 273 mL/g VS was obtained for the sample with 11% molasses-alcoholic wastewater added. Therefore 11% molasses-alcoholic wastewater addition was recommended.

Correlation of geographical location with stable isotope values of hydrogen and carbon of fatty acids from New Zealand milk and bulk milk powder

The aim of this study was to investigate the correlation of δ²H and δ¹³C of bulk milk powder and milk powder fatty acids to their production region. A total of 46 milk powder samples from across New Zealand were collected and analyzed. Principal component analysis (PCA) showed that the δ²H and δ¹³C of four fatty acids (C4:0, C14:0, C16:0, C18:1) and bulk milk powder were found to be correlated with regional production area. Linear discriminant analysis (LDA) models were prepared using different combinations of bulk and fatty acid δ²H and δ¹³C. All models were effective in discriminating samples from the North and South Islands. The LDA model using just fatty acid δ²H and δ¹³C provided the best separation. Therefore, the isotopic composition of the aforementioned fatty acids can be utilized as a good biomarker in milk powder that conveys reliable isotopic information to track milk powders to their regional origin.