Search Results (428)

The aim of this study was to evaluate the effect of Sida hermaphrodita harvest timing on biomass composition, properties, and methane fermentation performance. In addition, an energy–economic assessment was performed for biomass obtained at different stages of the growing season. The economic assessment assumed CHP electrical and thermal efficiencies of 38% and 47%, electricity and heat prices of 0.18 and 0.05 EUR/kWh, respectively, month-specific agrotechnical costs, and OPEX equal to 30% of total energy revenue. The biomass exhibited clear seasonal changes, transitioning from a material with high bioavailability during the summer period to a structurally more recalcitrant substrate in the autumn and winter months, as indicated by increasing lignification and fibrous fraction contents. The highest CH₄ production yields, ranging from 300 to 320 mL/g VS, and maximum production rates of up to 33.5 mL/g VS·d were obtained between June and August. In December, the CH₄ yield decreased to 180 ± 9 mL/g VS, accompanied by a substantial deterioration in kinetic performance. Despite the relatively stable theoretical methane potential, which ranged from 405 to 430 mL/g VS, its conversion efficiency declined from 77.1% in the summer period to 41.9% in the winter period. Regression analysis confirmed the key influence of the C/N ratio and total solids content, with model fits reaching R² values of 0.74–0.80, while the structure of lignocellulosic complexes had a less pronounced but still relevant effect. The maximum CH₄ production per unit cultivation area, approaching 3380 m³/ha, was achieved in July–August, reflecting a balance between high specific methane yield and biomass productivity. At the same time, the results demonstrated that the maximum biomass yield did not translate into the highest energy and economic performance. The highest net economic return, 1789 ± 330 EUR/ha, was obtained in July, despite biomass yield being 13.6% higher in September. These findings indicate a seasonal decoupling between biomass yield and energy performance, highlight biomass quality as a critical determinant of anaerobic digestion efficiency, and support harvest-date optimization as a low-cost strategy for the practical use of S. hermaphrodita in agricultural biogas plants. Further long-term continuous and semi-continuous studies are required to validate process stability and performance under industrial operating conditions. Full article

The growing demand for more sustainable energy alternatives has increased interest in microalgae-based fuels, promising options due to their high biomass productivity, carbon dioxide assimilation capacity, and potential for cultivation using wastewater. However, despite this strong theoretical basis, the industrial consolidation of these routes remains limited, revealing a persistent gap between scientific progress and the technological maturity achieved across specific biofuel pathways. This review examines the current state of the main microalgae-to-fuel conversion routes, emphasizing their technological readiness, proximity to industrial application, and the factors underlying their uneven progress. It also discusses the main opportunities and challenges associated with these biofuels, including cultivation performance, photobioreactor limitations, process intensification, scale-up, integration with waste streams, and downstream processing and product recovery requirements. Overall, this review identifies three main insights. First, the industrial viability of microalgae-based fuels depends less on maximizing individual conversion yields than on overcoming systemic bottlenecks across the production chain. Second, wet biomass conversion routes, particularly hydrothermal liquefaction and anaerobic digestion, appear more compatible with current industrial constraints. Third, lipid- and carbohydrate-based fuels remain more limited by biomass production costs and downstream processing requirements. Accordingly, further progress will depend on process intensification, integrated biorefineries, and technological innovations capable of simultaneously improving productivity, economic viability, and environmental performance. Full article

High-value components in the organic fraction of both municipal and industrial waste are still underused. In fact, there are two components in organic matter with high energy and emission value: carbohydrates (up to 46%) and fats (3.9–25%). The technological potential of using an integrated sequential biorefinery route, including lipid extraction for HVO/SAF, carbohydrate fermentation for bioethanol, and pyrolysis for renewable hydrogen generation, is not fully exploited. The objective of this work is to propose an approach based on a systematic six-step engineering methodology to determine the feasibility of its recovery. This integrated strategy achieves an attractive economic performance, with payback periods between 1.97 and 3.00 years, significantly outperforming traditional waste-to-energy models such as anaerobic digestion or composting. While current green hydrogen production costs range from USD 4.28 to USD 6.86, our model positions urban waste as a competitive feedstock for energy transition, achieving a selling price of 4.84 EUR/kg at midpoint values. For the remaining organic matter, a definitive thermal barrier for the 100% removal of microplastics is proposed, to prevent them from reaching agricultural soils. At the same time, efficient waste characterization, aligned with the European RED III directive, will allow the identification of high-value products and the application of the best available techniques for their extraction and use. Full article

Storage of livestock effluents represents a major source of ammonia (NH₃) emissions into the atmosphere. Reducing ammonia volatilisation is essential to improve the efficiency of nutrient use, mitigate air pollution, and enhance the overall sustainability of livestock production systems aligned with the principles of the circular economy. Therefore, identifying efficient and sustainable mitigation strategies is crucial. Conventional floating covers are commonly used to reduce emissions; however, they present limitations in terms of management, durability, and cost. This study proposes a novel approach by comparing traditional floating cover materials like straw and light expanded clay with biochar as an innovative and sustainable mitigation strategy to reduce ammonia volatilisation from the liquid fraction of buffalo digestate obtained from an anaerobic digestion plant in southern Italy. All cover materials were applied at a uniform thickness of 2 cm under laboratory conditions using a dynamic chamber technique. Additionally, a cost analysis was performed considering the material purchase cost for an average storage tank of 700 m² and two hypothetical reduction efficiencies (50% and 70%). Results indicated that biochar was the most effective cover, achieving a 67% reduction in ammonia emissions compared with the uncovered control. Light expanded clay exhibited the lowest efficiency, likely due to its insufficient sealing capacity at the applied thickness. From an economic perspective, biochar becomes increasingly competitive when emission reduction efficiency is accounted for, owing to its favourable physical–chemical properties. The results highlight the potential of using biochar as a sustainable, circular strategy for mitigating atmospheric emissions and improving nitrogen conservation, while also creating opportunities for its subsequent reuse in agriculture as a soil amendment. Full article

The article presents an energy, economic and environmental (3-E) analysis of a reference wastewater treatment plant (WWTP) with a capacity of 200,000 population equivalent (PE). The analysis includes sewage sludge treatment, anaerobic digestion (AD), combined heat and power (CHP), and mono-incineration of solar-dried sludge. The specific investment cost for the reference WWTP is 435 €/PE. Annual costs for operation and maintenance are estimated at 26 €/(PE·y) and the energy costs are 5 €/(PE·y). The annual energy demands are 32 kWh_el/(PE·y) of electricity and 14 kWh_th/(PE·y) of thermal energy for digesters’ heating. For a specific sludge quantity of 20 kg_DS/(PE·year), the biogas production is 245 Nm³/t_DS or 5 m³/(PE·y). Biogas-driven CHP supplies 10.3 kWh/(PE·year) of electricity and 14.7 kWh/(PE·year) of thermal energy, which meets 30% of the electrical demand and 100% of the thermal energy demand. Total (capital and operation) costs of sludge mono-incineration are evaluated at 300 €/t_DM or 6 €/PE. The heating value of digested and solar-dried sludge is 2 kWh/kg_WM. The total cost of the solar drying system is 30 €/PE while the sludge solar drying rate is 370 kg_DM/(m²·y). The environmental analysis showed that the on-site carbon footprint of the reference WWTP is 50 kg_CO2eq/(PE·y), with the largest contributions arising from N₂O emissions during wastewater treatment, CO₂ from sludge mono-incineration, and CO₂ from biogas combustion in the CHP unit. Full article

Photosynthetic biogas upgrading (PBU) using microalgae is a promising biological approach for converting raw biogas into biomethane while recovering nutrients and fixing part of the biogenic CO₂ into algal biomass. Unlike conventional physicochemical technologies, which mainly separate CO₂ from CH₄, PBU can combine gas upgrading with wastewater or digestate treatment, nutrient recycling, and biomass production. This review assesses the current state of PBU technology, with particular emphasis on high-rate algal ponds, absorption columns, and closed photobioreactors. It examines the main operating parameters that control gas–liquid mass transfer, carbonate buffering, and photosynthetic activity, including the liquid-to-gas ratio, pH, alkalinity, temperature, light regime, light intensity, and gas retention time. Special attention is given to the combined effects of the L/G ratio, pH, and alkalinity, as these parameters strongly influence CO₂ absorption, CH₄ enrichment, and O₂ contamination of the upgraded gas. The use of wastewater or anaerobic digestate instead of synthetic growth media is identified as an important sustainability advantage, particularly at wastewater treatment plants with existing anaerobic digestion and nutrient-rich side streams. However, digestate use may also create operational challenges related to turbidity, ammonium inhibition, solids, and variable composition. Available studies indicate that PBU may reduce operating costs and greenhouse gas emissions under favorable conditions while creating additional value from algal biomass. Nevertheless, wider deployment is still limited by high land requirements, seasonal variability, O₂ contamination, biomass harvesting, and limited evidence from large-scale systems. Future development should therefore focus on improved oxygen management, more efficient reactor designs, nanoparticle-assisted enhancement of photosynthetic activity, better integration with wastewater treatment, and AI-supported monitoring and control to improve process stability and support scale-up. Full article

Biomass valorization plays a vital role in achieving carbon neutrality and circular economy frameworks. Owing to its carbon-rich structure, biomass represents a promising feedstock to produce bio-based hydrocarbons via biological and thermochemical pathways. While biological conversion routes have been extensively studied, their deployment at commercial scale is constrained by high capital costs and low product yields. In contrast, thermochemical conversion technologies are increasingly being explored as viable large-scale biomass valorization routes. This review presents a comprehensive assessment of thermochemical pathways, with particular emphasis on hydrothermal liquefaction (HTL). The review identifies hydrothermal liquefaction (HTL) as a strategically advantageous route for wet and heterogeneous biomass valorization, due to simultaneous yields of liquid biocrude, and solid hydrochar. The review emphasizes the application of biocrude upgradation processes like hydrodeoxygenation under biphasic solvent systems using sulfided NiMo and CoMo catalysts. Further, the review also establishes hydrochar as a tunable functional material rather than a mere byproduct for applications in fields of energy production, soil amendment, and heterogeneous catalysis. The review article examines technology readiness levels of different biomass valorization techniques, and suggests that while combustion, anaerobic digestion, torrefaction, and transesterification are commercially mature, HTL and carbon capture utilization and storage (CCUS)-integrated fuel synthesis pathways remain at intermediate readiness. Additionally, the review carries out an in-depth study on artificial intelligence and machine learning (AI and ML) applications in biomass valorization, where it observes that Tree-based ensemble models, particularly Random Forest and XGBoost, show strong performance for several HTL prediction tasks, while Gaussian Process Regression and neural network–Bayesian optimization approaches provide additional advantages for uncertainty estimation and process-level optimization. Finally, the future research opportunities in biomass valorization and AI/ML application in HTL-process optimization have been identified for improving the bio-based fuel production techniques. Full article

Volatile fatty acids (VFAs) are essential precursors in chemical synthesis for various chemicals, polymers, pharmaceuticals, and fragrance compounds. Acidogenic anaerobic digestion (or arrested methanogenesis) is a promising method to stabilize organic wastes and convert them to value-added products such as VFAs. However, the VFAs’ accumulation could in turn suppress the fermentation process through product inhibition and limit the titer of VFA in the digestate. Therefore, in situ separation and recovery of VFAs from the fermentate is crucial to constructing an effective continuous VFA-producing system. Recent research has been dedicated to addressing these issues and advancing the utilization of biobased VFAs, particularly through process-intensified strategies employing novel green solvents such as natural deep eutectic solvents. Furthermore, in situ conversion of VFAs into esters is another potential strategy for VFA removal. However, VFA esterification in an aqueous medium is challenging due to the abundant water driving the reaction toward hydrolysis. Recent advances in free or immobilized enzyme catalysis in solvents have demonstrated improved ester yield by providing a hydrophobic space for the esterification reaction in aqueous solution. In this review, we present an overview of critical aspects on the state-of-the-art of green solvent-based process intensification strategies, including feedstock selection and pretreatment, operating condition optimization, advances in membrane- and solvent-based recovery methods, and biocatalytic in situ esterification. Lastly, we provide perspectives toward cost-effective, continuous, high-solid, environmental-benign, and industrial-relevant VFA production applications. Full article

This study investigates the integration of bioeconomy principles in the Greek livestock sector, framing the transition from conventional farming toward a circular bioeconomy as a strategy for resource conservation and reduced environmental pressure. It assesses farmers’ awareness of bioeconomy principles, the adoption of circular practices, and the associated economic and conservation-related performance. Data were collected through a structured questionnaire administered to 383 livestock farmers across the main livestock-producing regions of Greece and analyzed using descriptive statistics and multiple regression. Although respondents show substantial awareness, adoption remains incomplete, mainly because of high initial capital costs and insufficient financial incentives. Farmers implementing circular strategies reported gains in resource-use efficiency, waste minimization, and the conservation of soil, water, and biodiversity, particularly reduced greenhouse-gas emissions, while public subsidies and fiscal incentives emerged as the principal drivers of adoption. In applied terms, support should be prioritized for capital-intensive investments such as anaerobic digestion, manure and nutrient recovery, and water reuse, and the awareness–adoption gap is best closed through targeted subsidies and training. The findings offer concrete guidance for conservation-oriented agri-environmental policy supporting the green transition of livestock farming in Greece. Full article

A major limitation for the wider use of membrane-based technologies is the presence of biofouling, which is related to the decline of permeate flux, as well as the associated energy and economic costs for the necessary cleaning. In this work, the interactions and compatibility of 28 common polymeric materials with 36 potential biofoulants (categorized in six groups) is examined, based on Hansen Solubility Parameters (HSPs). Also, a simple methodology is proposed for polymer screening and comparing the suitability of 28 polymers to be used as fabrication materials or coatings, aiming to produce membranes with lower biofouling potential. The methodology gives a score to each polymer based on its interaction with water and various foulants. The screening among the commonly used polymers showed that poly (vinyl alcohol) (PVOH) is a good selection for the manufacturing of membranes, or for effective surface coating to limit biofouling, when compared to the other candidate polymers. The case of PVOH material received the highest score (11.6), while other polymers ranked with lower scores (less than 10). Its physically cross-linked nature that arises from a strong self-association pattern may also be beneficial for biofouling mitigation, since it limits the available sites for interactions (e.g., through hydrogen bonds) with the potential foulant agents. Swelling experiments on the PVOH gels with real wastewater (produced after anaerobic digestion) support the predictions for lowering the biofouling potential. Full article

Tofu wastewater (TWW), characterized as a high-strength organic effluent with elevated chemical oxygen demand (COD) and low pH, presents significant environmental challenges, including eutrophication, soil degradation, and greenhouse gas emissions. Conventional disposal methods have proven inadequate in mitigating these risks; however, thermophilic anaerobic digestion (TAD) has emerged as a viable technology for bioenergy recovery. Nonetheless, TAD is impeded by rapid acidification, ammonia and hydrogen sulfide inhibition, and the accumulation of volatile fatty acids (VFAs). This review introduces nano-calcium-peroxide-modified biochar (nano-CaO₂/BC) as a multifunctional additive designed to establish an integrated framework for intervention, risk mitigation, and resource recovery. The proposed amendment synergistically combines the adsorptive and microbial-supportive properties of biochar with the controlled oxidative and alkaline characteristics of nano-CaO₂. Under thermophilic conditions, the slow hydrolysis of nano-CaO₂ generates transient microaerobic zones that enhance polymer hydrolysis, suppress ammonia (NH₃) and hydrogen sulfide (H₂S) formation, and facilitate the oxidation of inhibitory VFAs, concurrently releasing calcium hydroxide (Ca(OH)₂) for sustained pH buffering. Utilizing failure mode and effects analysis (FMEA) as a semi-quantitative assessment tool, the results indicate that the composite significantly reduces risk priority numbers associated with acidification, ammonia toxicity, and sulfide inhibition when compared with conventional TAD methods. The resultant digestates, which are enriched in nutrients and recalcitrant carbon, possess the potential to serve as valuable soil amendments, thereby contributing to a circular bioeconomy. A techno-economic assessment grounded in unit cost analysis suggests that positive net benefits may be realized through enhanced biogas recovery and the mitigation of environmental penalties. However, empirical validation at the pilot scale is essential to substantiate the projected performance. This review underscores critical knowledge gaps and proposes a systematic experimental framework aimed at translating the conceptual risk mitigation strategy into practical applications. Full article

Two-stage anaerobic digestion systems are extensively researched for enhancing process stability and phase separation when processing complex organic materials. Scaling from laboratory setups to pilot plants necessitates engineering modifications to ensure operational feasibility. In this study, a laboratory-scale system comprising a 100 L horizontal CSTR and a packed-bed reactor was scaled up 100-fold. The design separates solid and liquid retention times, with fibers retained in the first stage while liquids and volatile fatty acids flow into the second. Fiber retention in the lab was achieved using a 100 µm sieve dividing the CSTR into two chambers, allowing prolonged lignocellulosic degradation. During scale-up, a filtration and recirculation system was introduced, able to return the fibers to the first reactor through a 1000 µm edge-gap filter, which separates liquids for the second reactor and recycles undegraded fibers. An economic analysis indicated a scale-up exponent of 0.396, indicating that unit costs decrease with plant size and demonstrating economies of scale. Laboratory-based mass balance estimates biogas production at approximately 16.3 m³ daily at the pilot scale, equivalent to 90 kWh. The modular system aims to be transferred to small farms, promoting cost-effective biogas from manure and local residues to support decentralized renewable energy in agriculture. Full article

Effective management of organic wastes is essential for green and low-carbon development. Conventional technologies, including incineration, pyrolysis, hydrothermal carbonization (HTC), gasification, anaerobic digestion (AD), and composting, have supported waste reduction and basic resource recovery, but they remain limited in high-efficiency conversion and high-value utilization. This review comparatively evaluates these conventional routes together with advanced and intensified technologies, including microwave-assisted pyrolysis (MAP), plasma treatment, supercritical water gasification (SCWG), and flash joule heating (FJH), with emphasis on suitable feedstocks, performance characteristics, application boundaries, and integration potential. In general, wastes with high moisture content are more suitable for HTC, AD, and SCWG, whereas relatively dry wastes and wastes with high carbon content are more suitable for pyrolysis, gasification, plasma treatment, and FJH upgrading. The review also discusses representative integrated pathways, such as HTC-SCWG, pyrolysis and plasma coupling, AD and gasification coupling, and pyrolysis and FJH coupling, which may improve carbon conversion, broaden product portfolios, and reduce residual pollutants. However, large-scale implementation is still constrained by feedstock heterogeneity, heat and mass transfer limitations, catalyst deactivation, reactor corrosion, and system cost. Overall, no single technology is universally optimal; technology selection should depend on feedstock properties, moisture content, and target products. Full article

Biogas produced from the anaerobic digestion of organic matter holds much promise as a renewable energy source for decentralized systems. However, raw biogas contains substantial volumes of carbon dioxide, hydrogen sulfide, water vapor, and other trace impurities. These impurities can reduce the calorific value of biogas and limit its direct use for household energy needs. Purifying biogas to high-grade biomethane (≥95%) is therefore important to improve methane (CH₄) content and combustion characteristics. This is a guarantee of its safe utilization in domestic appliances, including cooking, heating, lighting, and electricity generation. This article reviews and evaluates novel approaches for upgrading raw biogas into high-purity biomethane that can offset natural gas in domestic applications. It further examines recent developments in conventional and innovative upgrading technologies such as water scrubbing, chemical scrubbing, pressure swing adsorption, membrane separation, cryogenic separation, and biological upgrading. Particular emphasis is placed on low-cost and small-scale solutions suitable for off-grid or mini-grid rural energy systems. Moreover, the role of process optimization, intelligent monitoring, and data-driven control methods in increasing CH₄ recovery and process efficiency is discussed. Despite their relatively high capital costs and energy needs, conventional technologies such as water scrubbing, pressure swing adsorption, and membrane technology continue to dominate biogas purification systems. The findings show that coupling advanced separation technologies, including cryogenic separation, biological upgrading, and hybrid technologies, with optimized process control can significantly improve CH₄ purity, save energy use, and enhance the overall consistency of biogas purification systems. These innovative strategies have strong potential to promote the full-scale adoption of biomethane as a clean, sustainable, and affordable energy source for decentralized applications, particularly in the developing world. Full article

The textile industry consumes a significant quantity of water and produces effluent containing water-soluble dyes and heavy metals such as Lead (Pb), Cadmium (Cd), Chromium (Cr), Copper (Cu), and Zinc (Zn), among others. Heavy metal contamination of water bodies and their impact on aquatic life, as well as on human health, is of prime importance. This review examined the potential of phytoremediation, a low-cost and eco-friendly process for removing contaminants from textile effluent. This review also investigated the impact of heavy metal toxicity on aquatic plants used for biogas production post phytoremediation application. This review evaluated textile effluent characteristics, efficiency evaluation of phytoremediation of textile wastewater, metal uptake mechanisms of aquatic plants, and anaerobic digestion processes with emphasis on Water hyacinth (Eichhornia crassipes), Duckweed (Lemna minor), and Water lettuce (Pistia stratiotes). The findings indicated that these aquatic plants possess immense potential for removing heavy metals and other impurities by employing phytoextraction and rhizofiltration methods. Their rapid growth rate makes them preferred candidates for anaerobic digestion. However, accumulation of heavy metals in plant tissues inhibits microbial activities during anaerobic digestion, resulting in fluctuations in biogas and methane production. Findings also showed that these aquatic plants are efficient in the removal of heavy metals in water while yielding considerable biomass that can be used to produce bioenergy through anaerobic digestion. However, the sequestration of heavy metals in plant biomass may affect the rate of methane generation efficiency. The findings of this review suggest that phytoremediation has promising potential for the recycling of textile wastewater and, when coupled with biogas production, contributes towards a circular bioeconomy, an approach that integrates closed-loop resource utilization with renewable biological systems to minimize waste. Full article