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Residues generated during fruit processing constitute an abundant and underutilized biomass rich in bioactive compounds, pigments, structural polysaccharides, lipids, and fermentable carbohydrates. Although their potential for biorefinery applications is widely recognized, existing studies are often fragmented, focusing on isolated products, which limits a comprehensive understanding of integrated valorization strategies. To address this gap, this study presents an integrative review supported by bibliometric analysis to identify global research trends, dominant technological pathways, and key challenges associated with the use of fruit residues in biorefineries. The review covers technologies for extracting phenolic compounds, essential oils, pigments, and structural fibers, as well as lipid recovery, enzyme production, and biochemical routes for bioethanol, biohydrogen, and biogas generation. The review reveals that emerging technologies, such as pressurized fluid extraction, microwave-assisted extraction, and ultrasound-assisted extraction, enable efficient recovery of antioxidant compounds, high-purity pectin, and fermentable sugars, particularly when applied in sequential and integrated processing schemes. Bioethanol production is the most extensively investigated route, with yields strongly dependent on biomass composition and pretreatment strategies, identifying banana, cashew, apple, mango, coconut, and palm residues as promising feedstocks. In addition, biohydrogen production via dark fermentation and anaerobic digestion for biogas generation shows high technical feasibility, especially when integrated with upstream extraction steps. Overall, integrated valorization of fruit residues emerges as a key strategy to enhance economic performance and environmental sustainability in agro-industrial systems. Full article

Microbial components are part of the composition of all waste, including lignocellulosic biomass (e.g., agricultural, domestic, industrial, and municipal wastes) generated via human activities. If little attention is given to these wastes or if they are not adequately managed, they tend to end up in the environment (soil, water, and farmland), decomposing naturally through microbial activities, producing greenhouse gases, causing eutrophication, preventing sunlight penetration, and depleting oxygen in the water. Several treatment methods are applicable to these wastes. However, anaerobic digestion is presented as the best option to properly treat the waste. It is regarded as the best technique to achieve sustainable energy development in both developing and developed countries. During anaerobic digestion, the organic matter in the waste is converted via the concerted activities of microbes belonging to different trophic levels, in the absence of oxygen, to yield biogas (renewable energy), bio-fertiliser, and sanitisation of the waste, rendering it better and safer for human handling. Varying levels of loss of bacterial viability and their antibiotic-resistance genes are observed with this process, as bacteria differ in susceptibility to temperature, pH, nutrient scarcity, and the presence of antimicrobials. Anaerobic digestion of agricultural residues and the immediate processing (post-treatment) of the digestate help to stabilise the digestate, making it safe for land applications, tackling waste management, and protecting food chains from contamination, in addition to the environment. This review focuses on the anaerobic digestion of lignocellulosic biomass, yielding biogas as energy, alongside sanitising the wastes by inactivating microbial components found therein, therefore reducing the contamination potential of the effluent or digestate discharged from the biodigester following the process. Several findings registered by different researchers through different studies performed in different countries under different scenarios while employing varying methods have been assembled in a chronological fashion to emphasise similarities and divergences or variations that deepen knowledge pertaining to the significance of the anaerobic digestion process in terms of the microbial interactions responsible for producing energy, addressing sanitisation and hygiene crisis, and the post-treatment of the digestate to ensure its use as biofertiliser. In other words, it is a comprehensive review that synthesises knowledge from multiple fields covering comparative aspects of anaerobic digestion in terms of sanitation, hygiene, and energy production and consolidates it in a single document to present and address the problem of waste management through anaerobic digestion technology. Full article

Energy recovery technology is becoming a crucial part of modern approaches that address decarbonization, efficiency, and transitioning into a circular economy. In addition, apart from its advancements in efficiency and environmental benefits, its progress appears to be progressively limited due to its maturity and increasing complexity. In this case, innovation that focuses solely in the firm appears ineffective because more and more important knowledge in terms of innovation in processes and environmental aspects is becoming and remaining outside of organizational boundaries. In this paper, open innovation will be explored in its function as a structural innovation method of advancing energy recovery technology. The paper employs the narrative literature review of peer-reviewed literature indexed in the Scopus database to explore the implications of the outside-in model of open innovation, the inside-out model of open innovation, and the coupled model of open innovation with respect to the primary recovery processes of energy such as combustion, gasification, pyrolysis, anaerobic digestion, and landfill gas recovery. The literature incorporates findings about the implications of knowledge inflows and outflows with respect to the mentioned energy recovery processes. The results show that open innovation efficacy strongly varies according to the degree of technological maturity and performance issues, in that outside-in open innovation tends to be very effective in mature and semi-mature technology sectors, where incremental improvements in efficiency require specialized knowledge outside the industry, while coupled open innovation is crucial for addressing system-wide issues in areas such as emissions, regulatory compatibility, and infrastructure integration, while inside-out innovation is largely a means of facilitating technology dissemination and standardization once a degree of technological maturity had been realized. This study, through the association of selective open innovation practices with corresponding energy recovery technology and challenges, aims to provide a more nuanced perspective on the assistive potential of collaborative innovation in effecting sustainable development in energy recovery technology. Full article

Whey and its permeates constitute highly organic, low-alkalinity dairy streams whose management remains suboptimal in many processing facilities. This narrative review integrates recent evidence on the anaerobic digestion (AD) of whey, linking substrate composition and biodegradability with microbial pathways, inhibition mechanisms, biogas quality, and techno-economic and environmental feasibility in industrial settings. Data for sweet whey, acid whey, and their permeates are synthesized, with emphasis on operational windows, micronutrient requirements, and co-digestion or C/N/P/S balancing strategies that sustain resilient methanogenic communities. Options for biogas conditioning and upgrading towards combined heat and power, boiler applications, and compressed or liquefied biomethane are examined, and selection criteria are proposed based on impurity profiles, thermal integration, and methane-recovery performance. Finally, critical R&D gaps are identified, including mechanistic monitoring, bioavailable micronutrition, modular upgrading architectures, and the valorization of digestate as a recovered fertilizer. This review provides an integrated framework to guide the design and operation of technically stable, environmentally verifiable, and economically viable whey-to-biomethane schemes for the dairy industry. Full article

The rapid increase in organic waste generation poses significant environmental challenges and highlights the limitations of conventional waste management practices. In this context, black soldier fly (Hermetia illucens) larvae (BSFL) have emerged as a promising biological tool for valorizing organic residues within circular bioeconomy frameworks. This review provides an integrated analysis of BSFL-based bioconversion systems, focusing on the biological characteristics of BSFL, suitable organic waste streams, and the key process parameters influencing waste reduction efficiency, larval biomass production, and frass (the residual material from larval bioconversion) yield. The performance of BSFL in converting organic waste is assessed with emphasis on substrate characteristics, environmental conditions, larval density, and harvesting strategies. Environmental and economic implications are discussed in comparison with conventional treatments such as landfilling, composting, and anaerobic digestion. Special attention is given to the nutritional composition of BSFL and the valorization of larvae as sustainable protein and lipid sources for animal feed and emerging human food applications, while frass is highlighted as a nutrient-rich organic fertilizer and soil amendment. Finally, current challenges related to scalability, safety, regulation, and social acceptance are highlighted. By linking waste management, resource recovery, and sustainable protein production, this review clarifies the role of BSFL and frass in resilient and resource-efficient food and waste management systems. Full article

Anaerobic digestate (AD) has the potential to partially replace inorganic fertiliser, containing readily available nitrogen and other macro- and micronutrients. However, these properties vary with the feedstock. The objectives of this study were to analyse the chemical composition of AD materials and measure their effects on plant growth and greenhouse gas emissions. Anaerobic digestate came from a conventional reactor using vegetable waste and maize as feedstock (‘food AD’) and from a biogas system on a smallholder dairy farm using manure feedstock (‘manure AD’). Undigested cattle slurry (‘manure slurry’) and a complete mineral fertiliser were used as controls. These were applied to wheat plants grown in a glasshouse. Wheat grown with the food AD had a higher yield than the complete mineral fertiliser control, even when applied at a lower rate of nitrogen. Wheat grown with both the food AD and manure AD had macronutrient concentrations equal to or higher than the complete mineral fertiliser treatment. Furthermore, the wheat P concentration was significantly greater with the manure AD treatment, which was unrelated to a biomass dilution effect. However, food AD caused high ammonia emissions, and residual methane was emitted with manure AD, indicating incomplete digestion in the latter. Optimal yields and reduced greenhouse emissions were obtained with mixtures of AD and mineral fertiliser in a 1:1 ratio, indicating the potential to greatly reduce the costs and environmental impact of fertiliser application. Full article

This study investigated the effects of microwave (MW) pre-treatment (45 kJ total irradiated microwave energy) and magnetic nanoparticles (MPs) on the anaerobic digestion (AD) of meat-processing sludge, integrating biokinetic modeling with dielectric parameter measurements. Five different sludge variants were examined: native (non-treated control); MP-only control; microwave pre-treated sludge, and MW + MP combination with the nanoparticles either retained in the fermentation medium or removed prior to anaerobic digestion. Cumulative biomethane production was evaluated using the modified Gompertz, Logistic, and Weibull models, and key kinetic parameters (maximum achievable methane yield, maximum rate of product formation, and λ-values) were compared across the different treatments. The results revealed that the highest production rate, along with the highest biomethane potential, could be achieved when combining MW treatment with magnetic nanoparticles which were retained in the fermentation medium during AD. Based on the biokinetic analysis, this combined method increased biomethane potential by 52% to 390 mL CH₄/gVS and maximum methane production rate by 85% to 37 mL CH₄/gVS/day compared to the untreated control. The measurement of relative permittivity (${\epsilon }^{\prime }$) exhibited progressive changes during digestion, and the maximum rate of change in ${\epsilon }^{\prime }$ strongly correlated with the maximum methane production rate across all samples (R² > 0.98). These results highlight the potential of microwave–metal oxide nanoparticle pre-treatment for process enhancement and to demonstrate the suitability of dielectric parameter measurement as a rapid, non-invasive indicator of biochemical activity during anaerobic digestion. Full article

The study comprises an in-depth characterization of compositional groups of the liquid by-products obtained from the pyrolysis of swine manure at 500 °C, with the aim of providing an alternative and efficient approach for the valorisation of this waste stream, alongside with the production of biogas and char, the latter of which can be further converted into activated carbon. Two samples were considered: de-watered cake and solid product from anaerobic digestion of swine manure. Biocrude oils were fractionated into weak acidic, strong acidic, alkaline and neutral oil fractions. Subsequently, the neutral oil fraction was separated into paraffinic–naphthenic, slightly polar and polar fractions. All fractions were analyzed by GC–MS. The major identified compositional groups were: (i) for de-watered cake: steroids (40.7%), fatty acids, FAs (23.7%) and n-alkenes/n-alkanes (23.3%); (ii) for solid product from anaerobic digestion: FAs (31.0%), phenols/methoxy phenols (26.6%), n-alkenes/n-alkanes (10.8%) and steroids (10.6%). A variety of short-chain FAs (i.e., linear saturated, mono- and di-unsaturated, cis (i-), trans (ai-), isoprenoid, phenyl alkanoic, amongst others) and methyl esters (FAMEs) were identified as well. FA distribution, nC₁₂–nC₂₀, was similar for both manures studied with nC₁₆ and nC₁₈ as major compounds. FAMEs (nC₁₄–nC₂₈, with even carbon number dominance) in the slightly polar fraction of both samples were accompanied by considerable amounts of oleic (nC_18:1) and linoleic (nC_18:2) acids, and corresponding methyl esters. Hydrocarbons, i.e., n-alkenes/n-alkanes, were in the range of nC₁₅–nC₃₄, with nC₁₈ maximizing. Anaerobically digested manure has resulted in (i) an increase in the portion of longer homologues of hydrocarbons and FAMEs and (ii) the appearance of new FAs series of long chain members nC_22:1–nC_26:1, ω-9. The comprehensive analysis of the biocrude oils obtained from the slow pyrolysis of swine manure indicates their potential for use as biodiesel additives or as feedstock to produce value-added materials. Full article

To mitigate the emission of greenhouse gases (GHG) from fossil fuel combustion, biomass-to-power development via biochemical or thermochemical pathways has been recognized as a sustainable route for advancing towards a society based on a circular bioeconomy. The key differences between these pathways lie in operating temperature, process design capacity, feedstock characteristics and primary products. The biochemical route focuses on specific biofuels (e.g., biogas), and the thermochemical route often offers broader energy forms like heat and electricity. This perspective paper updates Taiwan’s achievements of its installed capacity and power (electricity) generation over a period of five years (2020–2024) under regulatory promotion that echoes official policies for sustainable development goals (SDGs) and 2050 carbon neutrality. Furthermore, the challenges of the biomass-to-power development in Taiwan (especially biogas-to-power systems) are addressed in the present study. These key issues include biomass resource, promotion incentives, stationary air pollution, site land use requirements and units for meeting performance durability requirements. Based on installed capacity, the main findings showed that biomass-to-power systems using biochemical routes (i.e., anaerobic digestion) in Taiwan showed an increasing trend, as well as increasing results for those using thermochemical routes (direct combustion, gasification). Furthermore, the data on total power generation indicated an upward trend from 201.7 Gigawatt-hour (GWh) in 2021 to 237.7 GWh in 2024, regardless of the kind of route used, whether biochemical or thermochemical. In conclusion, biomass-to-power systems have provided sustainable waste management and a circular bioeconomy model in Taiwan, which can be linked to the targets of sustainable development goals (SDGs) like SDG-7 (i.e., affordable and clean energy) and SDG-12 (i.e., responsible consumption and production). Full article

Agricultural production residues are an easily accessible raw material for energy recovery in a circular economy. Therefore, the possibility of biogas production from herb processing waste, namely common thyme (Thymus vulgaris L.), peppermint (Mentha × piperita L.), curled mint (Mentha crispa L.), and currants (woody stems and leaves), was investigated. In this study, the evaluation of the natural biodegradability of plant waste under conditions typical for an agricultural biogas plant was consciously carried out without the application of pre-treatment processes (shredding, steam hydrolysis, chemical treatment) to facilitate the methane fermentation process. The average values of biogas production efficiency ranged from 75 to 320 m³/mg DM for herb species and from 152 to 209 m³/mg DM for currant varieties under normal conditions. As part of laboratory tests, the elemental composition, i.e., C, H, N, S, O, was determined. Moreover, the analysis showed the energy potential of the tested waste in thermochemical processes (combustion). Garden thyme residues have particularly high energy potential, as indicated by the high calorific value, low nitrogen and sulfur content, and low ash content. Full article

This study evaluated low-cost food waste anaerobic digestion (FWAD) designed for African urban informal settlements, where electricity and process control are limited. Eight small-scale reactors were operated under varying mixing, pH control, and temperature conditions to assess the feasibility of stable operation with minimal input. Results showed no significant difference in methane yield between continuously mixed and minimally mixed (48-hourly) systems, nor between reactors with continuous pH dosing and those adjusted every 48 h (ANOVA p > 0.05 for all comparisons). The highest mean methane yield, 0.267 L CH₄ g VS⁻¹, was achieved by the minimally mixed reactor with 48-hourly pH control at 30 °C, while the controlled reactor at 37 °C produced a comparable 0.247 L CH₄ g VS⁻¹. Total methane production was similar at both temperatures, although gas generation was faster during the first 24 h at 37 °C. Compared to gas recovery achieved by extended batch operation following semi-continuous feeding, 58–73% of total methane was produced within the 48-h cycle, suggesting conversion could increase by 30–40% with extended liquid retention. Microbial analyses showed compositional differences but consistent performance, indicating functional redundancy within the microbial consortia. These results confirm the capacity of FWAD for stable, efficient biogas production without continuous energy input. Full article

While the technological foundation for sludge valorization (anaerobic digestion and pyrolysis) is mature, a significant disconnect exists between traditional research and the advanced application of artificial intelligence. This study identifies that Machine Learning (ML) remains in a peripheral position, representing an untapped frontier for achieving predictive and circular systems. The methodology involved a quantitative bibliometric analysis of 190 Scopus-indexed documents (2005–2025). We analyzed indicators of scientific production, collaboration, and thematic evolution using Bibliometrix and VOSviewer 1.6.20. The results reveal a rapidly growing research field, predominantly led by Chinese institutions. The temporal analysis projects a productivity peak around 2033. Core topics include established technologies like anaerobic digestion and pyrolysis. However, network and keyword analyses reveal an emerging trend toward hydrothermal processes and, crucially, the early incorporation of ML. However, ML still occupies a peripheral position within the main scientific discourse, highlighting a gap between traditional research and the advanced application of artificial intelligence. The study systematizes existing knowledge and demonstrates that, although the technological foundation is mature, the deep integration of ML represents the future frontier for achieving sludge valorization systems that are more predictive, efficient, and aligned with the principles of the circular economy. Full article

This study evaluates the environmental outcomes of integrating anaerobic digestion (AD) with pyrolysis (Py) and hydrothermal carbonization (HTC) to treat cattle slurry and grass silage in an Irish agricultural context. A consequential life cycle assessment (CLCA) was carried out for six scenarios based on 1 t of feedstock (0.4:0.6 cattle slurry/grass silage on a VS basis): two standalone AD systems (producing bioelectricity and biomethane) and four integrated AD–Py/HTC systems with different product utilisation pathways. Across all impact categories, the integrated systems performed better than standalone AD. This improvement is mainly due to the surplus bioenergy (electricity, biomethane, hydrocarbon fuel, hydrochar) that replaces marginal fossil energy (hard coal, natural gas and heavy fuel oil), together with the displacement of mineral NPK fertilisers by digestate-derived biochar and HTC process water. Among the configurations, the AD–HTC bioelectricity scenario (S4) achieved the best overall performance, driven by higher hydrochar yields, a favourable heating value, and a lower pretreatment energy demand compared with Py-based options. Across the integrated scenarios, climate change, freshwater eutrophication, and fossil depletion impacts were reduced by up to 84%, 86%, and 99%, respectively, relative to the fossil-based reference system, while avoiding digestate and fertiliser application reduced terrestrial acidification by up to 74%. Overall, the results show that the cascading utilisation of digestate via AD–Py/HTC can simultaneously enhance bioenergy production and nutrient recycling, providing a robust pathway for low-emission management of agricultural residues. These findings are directly relevant to Ireland’s renewable energy and circular economy targets and are transferable to other livestock-intensive regions seeking to valorise slurry and grass-based residues as low-carbon energy and biofertiliser resources. Full article

Bio-based fertilisers (BBFs) derived from waste streams represent a transformative approach to sustainable agriculture, addressing the dual challenges of waste management and food security. This comprehensive review examines recent advances in BBF production technologies, nutrient recovery mechanisms, soil health impacts, and the benefits of a circular economy. This review, based on an analysis of peer-reviewed studies, demonstrates that BBFs consistently improve the physical, chemical, and biological properties of soil while reducing environmental impacts by 15–45% compared to synthetic alternatives. Advanced biological treatment technologies, including anaerobic digestion, vermicomposting, and biochar production, achieve nutrient recovery efficiencies of 60–95% in diverse waste streams. Market analysis reveals a rapidly expanding sector projected to grow from $2.53 billion (2024) to $6.3 billion by 2032, driven by regulatory support and circular economy policies. Critical research gaps remain in standardisation, long-term performance evaluation, and integration with precision agriculture systems. Future developments should focus on AI-driven optimisation, climate-adaptive formulations, and nanobioconjugate technologies. Full article

The increasing atmospheric concentration of carbon dioxide (CO₂) poses a critical threat to global climate stability, highlighting the need for efficient carbon capture technologies. While amine-based solvents such as monoethanolamine (MEA) are widely used for industrial CO₂ capture, they are subject to limitations such as high energy requirements for regeneration, solvent degradation, and environmental concerns. This study investigates potassium carbonate/bicarbonate system as an alternative solution for CO₂ absorption. The absorption mechanism and reaction kinetics of potassium carbonate in the presence of bicarbonates were reviewed. A rate-based model was developed in Aspen Plus, using literature kinetics, to simulate CO₂ absorption using 20 wt% potassium carbonate (K₂CO₃) solution with 10% carbonate-to-bicarbonate conversion under different industrial conditions. Three flue gas compositions were evaluated: cement industry, biomass combustion, and anaerobic digestion, each at 3000 m³/h flow rate. The simulation was conducted to determine minimum column height and solvent loading requirements with a target output of 90% CO₂ removal from the gas streams. Results demonstrated that potassium carbonate systems successfully achieved the target removal efficiency across all scenarios. Column heights ranged from 18 to 25 m, with molar K₂CO₃/CO₂ ratios between 1.41 and 4.00. The biomass combustion scenario proved most favorable due to lower CO₂ concentration and effective heat integration. While requiring higher column heights (18–25 m) compared to MEA systems (6–12 m) and greater solvent mass flow rates, potassium carbonate demonstrated technical feasibility for CO₂ capture. The findings of this study provide a foundation for technoeconomic evaluation of potassium carbonate systems versus amine-based technologies for industrial carbon capture applications. Full article