Search Results (105)

In the context of the global shift toward sustainable energy systems and circular economy strategies, anaerobic digestion is a well-established biotechnology for the valorisation of organic residues, the production of biogas as a renewable energy carrier, and the generation of nutrient-rich digestate. This study aimed to evaluate the stability and methane yield of a two-stage co-digestion process using a substrate mixture developed based on the actual feedstock composition of a full-scale agricultural biogas plant. The daily feed mixture consisted of maize silage (8.2%), sugar beet pulp (4.9%), cellulose pulp (6.6%), distillery stillage (38.6%), corn syrup (6.4%), cattle slurry (22.5%) and sterilised animal by-products (12.8%). Digestate was recirculated separately as part of the reactor operation. Laboratory-scale experiments were conducted in a two-stage anaerobic digestion system operated at 42 °C and 50 °C. The physicochemical properties of the substrates and digestate were determined, biogas quantity and composition were monitored, and the microbial community structure was assessed using 16S rRNA gene amplicon sequencing. The process remained stable throughout the experimental period, with pH values ranging from 8.23 to 8.53, alkalinity between 2600 and 2940 mg CaCO₃/dm³, and a VFAs/alkalinity ratio of 0.17–0.93. Despite ammonium nitrogen concentrations reaching 4346 mg N-NH₄⁺/L, no clear concurrent reduction in gas or methane production was observed. Methane accounted for approximately 70–80% of the biogas produced. The overall specific methane yield reached 346.4 NL CH₄ kg⁻¹ VS added. 16S rRNA gene amplicon sequencing revealed a diverse microbial community containing taxa previously associated with hydrolysis, fermentation and syntrophic interactions, including Proteiniphilum and Syntrophaceticus. The results demonstrate stable process performance and methane production in this site-specific laboratory-scale case study based on the feedstock composition and process configuration of a full-scale agricultural biogas plant. Full article

Rapid population growth, urbanization, and industrialization are continuously increasing global energy demand while intensifying climate change associated with fossil fuel consumption. In this context, renewable energy production from organic waste has gained increasing attention as a sustainable and environmentally friendly strategy. Anaerobic digestion (AD) offers significant potential for simultaneous waste stabilization and biomethane generation. However, many previous studies investigating lignocellulosic or nutrient-rich substrates have relied on physical, chemical, or thermal pretreatment methods to enhance biodegradability, despite their additional operational costs, energy consumption, and environmental impacts. Therefore, developing low-cost and pretreatment-free co-digestion strategies remains an important research need. This study investigated the biomethane production potentials of untreated chicken manure (CM) and duckweed (Lemna minor-LM) collected from the final sedimentation tanks of wastewater treatment plants under mono-digestion and co-digestion conditions. The study hypothesized that rapidly growing and widely available LM biomass could enhance methane production without requiring pretreatment. Among all reactors, CM0.75 (75% of the total TS derived from CM and 25% from LM and inoculum) achieved the highest performance with a cumulative biogas production of 5350 mL (74.2% of CH4) and a methane yield of 327 mL CH4/g VS, while mono-digestion of CM resulted in the lowest methane yield of 104 mL CH4/g VS. The results demonstrated that LM biomass naturally proliferating in wastewater treatment plants can be directly utilized as an effective co-substrate to improve biomethane production from poultry wastes. The proposed approach provides a cost-efficient, eco-friendly, and circular-economy-oriented alternative by eliminating the need for pretreatment while simultaneously valorizing problematic biomass generated in wastewater treatment facilities. Full article

Anaerobic digestion (AD) plays an important role in the circular bioeconomy by converting organic waste into renewable methane and nutrient-rich fertilizer. However, consistent, high-quality biomethane production is hindered by four main factors: hydrolysis limitations, fluctuating feedstock quality, microbial instability, and the high cost/energy demand of purification. This review explores three key areas that improve biomethane production: (i) process intensification (pretreatments and advanced reactors), (ii) microbial regulation through additives, and (iii) biogas upgrading for pipeline use. Anaerobic digestion can be greatly improved by combining thermal or hybrid pretreatments, staged digestion, high-solids technology, and electrochemical systems. These methods speed up hydrolysis and help the system handle higher amounts of organic material more effectively. However, actual performance benefits depend on specific substrate characteristics, heat integration, and control complexity. Optimizing the C:N ratio, buffering capacity, and trace-element supplementation, while simultaneously diluting toxic inhibitors, makes co-digestion an effective and adaptable approach to enhancing anaerobic digestion processes. Additives like carbon, iron nanoparticles, enzymes, and buffers can optimize digestion, but their performance is highly dependent on dosage and substrate. Additionally, they lack validation in long-term, industrial-scale applications. Conventional physicochemical techniques continue to be standard for generating high-quality biomethane, but biological methanation and microalgal systems are playing a growing role in integrating Power-to-Gas technology and using CO₂ efficiently. Critical research needs to focus on four areas: (1) standardized reporting metrics, (2) AI-enabled monitoring and control, (3) coupled techno-economic and life-cycle analysis (TEA-LCA), and (4) long-term pilot or full-scale validation. Overall, comprehensive optimization of the entire flow is more effective than improving isolated parts. Full article

Polyetherimide (Ultem-1000) hollow-fiber membranes were developed for biogas purification with emphasis on the relationship between spinning conditions, membrane morphology, gas transport properties, and module performance. Hollow fibers were prepared from dope solutions based on dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP) at different conditions, followed by post-treatment with 1 and 3 wt.% silicone solution in n-heptane to reduce nonselective defects and improve selectivity toward the intrinsic behavior of dense PEI films. SEM analysis revealed that DMF-based fibers formed a more open, macrovoid-rich structure, whereas NMP-based fibers exhibited a more homogeneous sponge-like morphology with a better-defined selective layer. DMF-based fibers experienced faster demixing, which promoted macrovoid formation, increased pore connectivity of the substructure, lowered mass transfer resistance, and at the same time increased the probability of nonselective pathways and defect-related loss of selectivity. This structural evolution was reflected in gas transport properties: untreated DMF fibers showed high mixed-gas permeance but limited selectivity, while NMP fibers demonstrated lower permeance and selectivity values closer to those of the dense film. Silicone post-treatment significantly improved separation performance, with 3 wt.% coating being markedly more effective than 1 wt.% coating. The best compromise between permeance and selectivity was achieved for the DMF-based fibers treated with 3 wt.% silicone, which exhibited CO₂ and H₂S permeances of 39.4 and 47.12 GPU, respectively, together with selectivity values of 22.4, 26.8 and 20.2 for CO₂/CH₄, H₂S/CH₄ and CO₂/N₂. A membrane module containing 500 fibers was studied during the quasi-real biogas upgrading. With increasing stage-cut, the CH₄ concentration in the retentate increased from ~74 to 96 mol.%, while CO₂ decreased from ~21 to 2 mol.%. The results demonstrate that structure control combined with silicone post-treatment is an effective strategy for producing PEI hollow fibers suitable for simultaneous methane enrichment and removal of acid impurities from biogas. Full article

Biochar (BC) and hydrochar (HC) are carbon-rich materials derived from organic wastes through pyrolysis/gasification and hydrothermal carbonization (HTC), respectively, offering promising pathways for waste valorization and resource recovery within a circular economy framework. Owing to their porous structure and surface functionality, these materials have gained attention as additives in anaerobic digestion (AD), where they may enhance the microbial activity, improve the buffering capacity, and facilitate direct interspecies electron transfer (DIET), resulting in greater process stability and higher methane (CH₄) yields. This study evaluated the effect of BC and HC derived from aloe vera leaves and algae on methane production during the AD of food waste (FW). Batch experiments were conducted under mesophilic conditions (37 °C) for 60 days, using a 1:1 inoculum-to-substrate ratio and a dosage of 10 g L⁻¹ of each carbonaceous material. The results show that adding BC increased cumulative biogas production by 10–14%, while HC led to an 18–35% increase compared with the control. Additionally, dissolved chemical oxygen demand (d-COD) removal improved by up to 30%, and volatile solids (VSs) removal rose by 31% in the FW and aloe HC reactors, highlighting the greater enhancement potential of HCs in methane production. Overall, the findings demonstrate that integrating carbonaceous materials derived from organic residues into AD systems can enhance bioenergy recovery while reducing environmental impacts, supporting more sustainable and circular waste-to-energy processes. Full article

This study evaluated thermal and thermo-alkaline post-treatment of digested cattle manure (DCM) as a strategy to increase methane recovery and improve the flexibility of biogas systems within hybrid renewable energy alternatives. A 10 L mesophilic CSTR was operated for 311 days, producing lignin-rich digestate that was subjected to a statistically designed range of post-treatment conditions varying temperature (50–90 °C), pH (8–12), and contact time (6–24 h). Biomethane potential assays and lignocellulosic fractionation were used to determine changes in solubilization, biodegradability, and methane production kinetics. Thermal treatment provided modest improvements, reaching 84 mg SCOD g⁻¹ PCOD solubilization and a 26 mL CH₄ g⁻¹ COD increase in methane yield. Thermo-alkaline treatment produced substantially higher enhancements, with the most severe condition (90 °C-pH 12–24 h) achieving 493 mg SCOD g⁻¹ PCOD solubilization, 66% removal of structural carbohydrates, and a 60.2 mL CH₄ g⁻¹ COD increase in methane yield, corresponding to a 16% rise in biodegradability and a twofold increase in methane production rate. Gompertz modeling indicated accelerated kinetics and minimal lag time. A strong linear correlation (R² = 0.90) between severity index and solubilization supported predictable scalability. These results demonstrate that thermo-alkaline hydrolysis can significantly enhance post-digestion methane recovery and strengthen the role of agricultural biogas in integrated renewable energy systems. The techno-economic analysis revealed that, despite higher operating costs for thermo-alkaline post-treatment than for the control, the main drivers are chemical costs and the price of renewable energy, and thus the application of post-treatment as a sustainable solution for animal manure treatment will likely improve as renewable energy prices increase in the future. Full article

The environmental crisis and the growing need to reduce solid waste make it imperative to adopt integrated, scientifically sound, and environmentally friendly solid waste management practices in order to ensure a sustainable future. This study presents an alternative waste management proposal in accordance with the standards set out in the European Waste Directive (Directive 2018/850/EC) in order to lessen greenhouse gas emissions. The primary objective is to develop a circular waste management system that uses waste as feedstock for the production of biofuel in order to meet Catalonia’s energy needs and, at the same time, reduce its environmental footprint. Waste that is highly biodegradable and rich in organic matter cannot be disposed of in landfills, according to order TED/834/2023, and is therefore used to produce biogas through anaerobic digestion (AD) or to produce compost. In addition, gas emissions from landfills, which are rich in methane, are also collected and used for biogas production. Plans for biogas production at landfills and at an anaerobic digestion biogas plant, and for compost production from organic waste, were implemented using SuperPro Designer simulation software. The research has shown that this approach to solid waste management offers positive results in terms of energy due to biogas production, in terms of the environment due to waste reduction and compost production, and in terms of the economy due to a 25% increase in the efficiency of the biogas plant. Full article

Anaerobic digestion (AD) is a key technology for energy recovery in wastewater treatment plants, converting organic matter into methane-rich biogas. However, its efficiency is constrained by slow reaction rates, particularly during hydrolysis and methanogenesis, necessitating large reactor footprints for effective sludge digestion. Alternative AD configurations for process intensification present a promising solution to address these limitations by altering the design and operational setup of the AD process. In this review, key configuration-based AD intensification strategies were systematically analyzed, including recuperative thickening, single-stage thermophilic AD, acid/gas two-stage AD, temperature-phased AD, and multi-stage AD systems. The mechanisms, governing factors, efficiency gains, and scalability of these technologies were critically examined. These configurations demonstrated substantial improvements in methane production rates, process intensification, and the removal of solids and organics. Single-stage thermophilic and cascade AD technologies showed the highest potential for full-scale implementation, supported by successful real-world applications. Conversely, recuperative thickening exhibited promising results at lab and pilot scales but remains limited by its lower technology readiness level. Furthermore, the integration potential of such alternative systems with other intensification technologies was explored, highlighting synergistic opportunities for further optimization. This review provides critical insights into means to intensify AD process through alternative process configurations, offering a comprehensive guide for their application in biogas upgrading. It also identifies key challenges and outlines actionable steps to advance these systems toward widespread adoption in full-scale AD operations. Full article

The conversion of biogas into high-value chemicals for pharmaceutical, cosmetic, and nutraceutical markets offers an attractive alternative to conventional fossil-based production routes, enabling circular value chains with significant socio-economic impact. This study evaluated the valorization of biogas into osmolyte and carotenoid compounds with market prices ranging from 1000 to 7000 $·kg⁻¹. Specifically, an algal–methanotrophic co-culture operated under saline conditions, preventing external microbial contamination and stimulating osmolytes and carotenoids, was assessed for its capacity to simultaneously remove methane (CH₄) and carbon dioxide (CO₂), with efficiencies of 92 and 89%, respectively. while producing ectoine, hydroxyectoine, lutein, β-carotene, and astaxanthin. Shotgun metagenomic analyses identified the key microorganisms driving the process, predominantly alkaliphilic and halophilic green algae (Chlorella, Dunaliella) and cyanobacteria (Leptolyngbya), and halotolerant methanotrophs (Methylotuvimicrobium) and methylotrophs (Methylophaga). Metagenomics further revealed the presence of key metabolisms related to C1 utilization and biosynthetic genes associated with carotenoid and osmolyte production, confirming the metabolic potential of the consortium to convert biogas-derived carbon directly into high-value compounds. Overall, these results demonstrate the feasibility of an efficient, biologically driven bio-platform capable of transforming greenhouse gas-rich waste streams into economically relevant bioactive molecules, contributing to global priorities in sustainable biomass-to-biochemical innovation. Full article

The global output of organic solid residues (e.g., crop straw) is substantial, creating an urgent sustainability need for low-impact pathways that avoid open burning or disposal while recovering renewable energy. Dry anaerobic digestion (AD) offers a water-saving, high-solids valorization route for straw-rich substrates, but its deployment is often constrained by acidification that suppresses methanogenesis, reducing reliability and limiting practical adoption. Here, at laboratory scale, we formulated a co-digestion substrate dominated by wheat straw (50%) with swine manure and household organic waste, and evaluated whether co-supplementation of trace metals (Fe, Ni, Co) can enhance process stability and energy recovery, thereby strengthening the sustainability of high-solids straw treatment. System performance was assessed by pH, biogas production, volatile fatty acids (VFAs), functional genes, and microbial community profiles to elucidate micronutrient effects and microbial responses. Micronutrient addition stabilized pH (minimum 6.5) and enhanced biogas output. Specific yields in the supplemented digester were 260.64 ± 11.83 mL g⁻¹ TS and 319.89 ± 14.27 mL g⁻¹ VS, compared with 220.31 ± 9.45 mL g⁻¹ TS and 270.33 ± 11.72 mL g⁻¹ VS in the control; cumulative gas production was higher by 18.33%. Community analyses showed marked enrichment of Methanosarcina, increasing from 7.28% on day 10 to 44.00% on day 30. Molecular ecological network analysis indicated a transition from a sparse, fragmented configuration to a highly connected, centralized one: the number of nodes decreased from 74 to 70; the number of edges increased from 46 to 223 (a 4.85-fold rise); network density increased from 0.0170 to 0.0923; mean degree increased from 1.24 to 6.37; the number of modules declined from 39 to 5; and the proportion of positive versus negative links shifted from 85%/15% to 70%/30%, evidencing stronger interspecies coupling and functional robustness. Consistently, methyl-coenzyme reductase subunit A gene copy numbers were about 1.60-fold higher on day 30 and about 1.51-fold higher on day 50 than in the control. Overall, Fe-Ni-Co co-supplementation enhances methane potential and suppresses acidification in straw-rich dry anaerobic digestion, providing a low-input and practical strategy to stabilize high-solids systems. By improving microbial robustness, this approach enables efficient renewable energy recovery with reduced water demand and lower risk of process failure, thereby supporting scalable straw valorization and advancing circular bioeconomy pathways for agricultural and organic solid residues. Full article

The untreated discharge of dairy industry wastewater, characterized by high organic and nutrient loads, poses a severe eutrophication threat, leading to oxygen depletion and the disruption of aquatic ecosystems, which necessitates advanced treatment strategies. Anaerobic digestion (AD) represents an effective and sustainable alternative, converting organic matter into biogas while minimizing sludge production and contributing to Circular Economy strategies. This study investigated the effects of fat concentration and operational temperature on the anaerobic digestion of dairy effluents. Three types of effluents, skimmed, semi-skimmed, and whole substrates, were evaluated under mesophilic 35 °C and thermophilic 55 °C conditions to degrade substrates with different fat content. Low-fat effluents exhibited higher COD removal, shorter lag phases, and stable activity under mesophilic conditions, while high-fat substrates delayed start-up due to accumulation of fatty acids and brief methanogen inhibition. Thermophilic digestion accelerated hydrolysis and methane production but demonstrated increased sensitivity to lipid-induced inhibition. Kinetic modeling confirmed that the modified Gompertz model accurately described mesophilic digestion with rapid microbial adaptation, while the Cone model better captured thermophilic, hydrolysis-limited kinetics. The thermophilic operation significantly enhanced methane productivity, yielding 105–191 mL CH₄ g⁻¹VS compared to 54–70 mL CH₄ g⁻¹VS under mesophilic conditions by increasing apparent hydrolysis rates and reducing lag phases. However, the mesophilic process demonstrated superior operational stability and robustness during start-up with fat-rich effluents, which otherwise suffered delayed methane formation due to lipid hydrolysis and volatile fatty acid (VFA) inhibition. Overall, the synergistic interaction between temperature and fat concentration revealed a trade-off between methane productivity and process stability, with thermophilic digestion increasing methane yields up to 191 mL CH₄ g⁻¹ VS but reducing COD removal and robustness during start-up, whereas mesophilic operation ensured more stable performance despite lower methane yields. Full article

The biogas dry reforming reaction offers a promising route for syngas production while simultaneously mitigating greenhouse gas emissions. Membrane reactors have proven to be an excellent option for hydrogen production and separation in a single unit, where conversion and yield can be enhanced over conventional processes. In this study, a Pd/YSZ membrane integrated with a Ru/CeO₂ catalyst was evaluated for biogas reaction under varying operating conditions. The selective removal of hydrogen through the palladium membrane improved reactant conversion and suppressed side reactions such as methanation and the reverse water–gas shift. Experiments were performed at temperatures ranging from 500 to 600 °C, pressures of 1–6 bar, and a gas hourly space velocity (GHSV) of 800 h⁻¹. Maximum conversions of CH₄ (43%) and CO₂ (46.7%) were achieved at 600 °C and 2 bar, while the maximum hydrogen recovery of 78% was reached at 6 bar. The membrane reactor outperformed a conventional reactor, offering up to 10% higher CH₄ conversion and improved hydrogen production and yield. Also, a comparative analysis between Ru/CeO₂ and Ni/Al₂O₃ catalysts revealed that while the Ni-based catalyst provided higher CH₄ conversion, it also promoted methane decomposition reaction and coke formation. In contrast, the Ru/CeO₂ catalyst exhibited excellent resistance to coke formation, attributable to ceria’s redox properties and oxygen storage capacity. The combined system of Ru/CeO₂ catalyst and Pd/YSZ membrane offers an effective and sustainable approach for hydrogen-rich syngas production from biogas, with improved performance and long-term stability. Full article

Anaerobic digestion (AD) provides significant environmental benefits by converting livestock manures, such as cattle manure (CM) and pig manure (PM), into biogas and nutrient-rich digestate, supporting circular economy principles. However, challenges arise when feedstock overload disrupts microbial balance, leading to reduced methane (CH₄) yields and process instability. This study examined the performance of AD using CM and PM with gradually increasing organic loading rates (OLR). At steady state, CH₄ yields were 120.32 mL-CH₄/g VS for CM and 229 mL-CH₄/g VS for PM. The lower yield for CM is attributed to its high cellulose and hemicellulose content, which exceeds 50% and is difficult to degrade. In contrast, PM showed more efficient carbohydrate degradation, resulting in higher CH₄ production. Key methanogens, including Methanocorpusculum, Methanosaeta, Methanosarcina, Methanobacterium, and Methanospirillum, were present in both reactors. Metagenomic analysis revealed that pathways for degrading cellulose and hemicellulose were poorly represented in CM, while PM exhibited enhanced total volatile fatty acid metabolism. This study offers valuable insights into the metabolic pathways associated with CM and PM in anaerobic digestion. Full article

In this study, a novel activated carbon (AC) (AC-Ba(OH)₂) was synthesized through a three-step process (including hydrothermal carbonization (at 250 °C), alkali activation by Ba(OH)₂, and pyrolysis (at 850 °C)), from Plane tree fruits (PTFs). By using various experimental methods for material characterization, it was established that the resulting material possesses a variety of oxygen functional groups, rich in alkaline earth oxides (BaO/CaO), SiO₂, consisting of graphitized carbon with graphene structures. A detailed kinetic and thermodynamic analysis of AC-Ba(OH)₂ thermal restoring was also carried out. Thermodynamic analysis revealed the existence of a true thermodynamic compensation effect (TCE) during restoration. Restoration was controlled by entropy, where experimental temperatures are above the iso-entropic temperature, i.e., the temperature where contributions of enthalpy and entropy to activation free energy are balanced. Kinetic modeling has shown that restoration allows carbon material to be significantly modified by removing oxygen-containing groups via diffusion, changing active sites on the surface, and preparing material for catalyst support. CaO and SiO₂ act as catalysts, while BaO alters graphene surface properties. Isothermal prediction tests have shown an extremely high long-term stability of modified AC-Ba(OH)₂, supporting an elevated activity, selectivity, and lifetime, as well. The restoring process resulted in an energy consumption of 0.762 kWh, which is equivalent to the reactivation of AC with a lower specific surface area. Manufactured AC and its thermally modified counterpart can be used as both a catalyst support and catalyst for reforming processes, such as methanol synthesis, biogas purification, and dry reforming of methane. Full article

Sheep manure and beet waste (the uneatable leaf part of the beet) are promising feedstock for biogas production due to their abundance and organic richness. However, their high lignocellulosic content reduces anaerobic digestibility and controls methane yield. This study investigates the effect of various pretreatment strategies, namely physical, thermal, and combined physical–thermal methods, on the Biochemical Methane Potential (BMP) of sheep manure and beet waste. Batch anaerobic digestion experiments were conducted under mesophilic conditions, with BMP values recorded for each treatment. The highest BMP for sheep manure, 125 Nml CH₄/g VS, was achieved using combined physical and thermal pretreatment. This approach enhanced methane production by 16%, 25%, and 60% compared to physical pretreatment (PP) alone, thermal pretreatment (TP) alone, and no pretreatment, respectively, while the one BMP for beet waste is 80 Nml CH₄/g VS and obtained with thermal pretreatment. To predict BMP outcomes, three machine learning approaches are applied, namely Linear Regression (LM), Random Forest Regression (RFR), and Gradient Boosting Machine (GBM), using digestion time (N days), total solids (Ts), volatile solids (Vs), pretreatment type, and biomass type. The variance analysis confirmed that the interaction between pretreatment and biomass type significantly improved model performance. While diagnostic checks revealed non-linear patterns limiting the linear model, ensemble methods achieved stronger results. The RFR model explained 79.5% of the variance with a Root Mean Square Error (RMSE) of about 15.7, whereas the GBM model achieved the lowest RMSE of 5.05. GBM captures complex non-linear interactions. In addition, variable importance analyses identified digestion time, solid content, and pretreatment as the most influential factors for methane yield, with the combined chemical and physical pretreatment producing the highest biogas outputs. These findings underscore the potential of advanced machine learning models, particularly GBM (Gradient Boosting Machine), for optimizing anaerobic digestion strategies and maximizing biogas recovery from sheep manure and beet waste. Full article