Search Results (243)

The energetic valorization of digestate obtained from anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) was investigated via pyrolysis in a bench-scale rotary kiln. The mass rate of dried digestate to the rotary kiln pyrolyzer was fixed at 500 gr/h. The effect of the pyrolysis temperature was investigated at 600, 700, and 800 °C. The pyrolysis products, char, oil, and gas, were quantified and chemically analyzed. It was observed that with the increase in the temperature from 600 to 800 °C, the char decreased from 60.3% to 52.2% and the gas increased from 26.5% to 35.3%. With the aim of increasing the methane production and methane concentration in syngas, the effect of CaO addition to the pyrolysis process was investigated at the same temperature, too. The mass ratio CaO/dried digestate was set at 0.2. The addition of CaO sorbent has a clear effect on the yield and composition of pyrolysis products. Under the experimental conditions, CaO was observed to act both as a CO₂ sorbent and as a catalyst, promoting cracking and reforming reactions of volatile compounds. In more detail, at the investigated temperatures, a net reduction in CO₂ concentration was observed in syngas, accompanied by an increase in CH₄ concentration. The gas yield decreased with the CaO addition because of CO₂ chemisorption. The oil yield decreased as well, probably because of the cracking and reforming effect of the CaO on the volatiles. A very promising performance of the CaO sorbent was observed at 600 °C; at this temperature, the CO₂ concentration decreased from 32.2 to 13.9 mol %, and the methane concentration increased from 16.1 to 29.4 mol %. At the same temperature, the methane production increased from 34 to 63 g/kg_digestate. Full article

A three-year optimization study was conducted at a mechanical biological treatment plant with the aim of enhancing organic fractions recovery from mechanically separated fine fractions (MSFF) of residual waste using a screw press. The study aimed to optimize key operating parameters for the employed screw press, such as pressure, liquid-to-MSFF, feeding quantity per hour, and press basket mesh size, to enhance volatile solids and biogas recovery in the generated press water for anaerobic digestion. Experiments were performed at the full-scale facility to evaluate the efficiency of screw press extraction with other pretreatment methods, like press extrusion, wet pulping, and hydrothermal treatment. The results indicated that hydrolysis of the organic fractions in MSFF was the most important factor for improving organic extraction from the MSFF to press water for fermentation. Optimal hydrolysis efficiency was achieved with a digestate and process water-to-MSFF of approximately 1000 L/ton, with a feeding rate between 8.8 and 14 tons per hour. Increasing pressure from 2.5 to 4.0 bar had minimal impact on press water properties or biogas production, regardless of the press basket size. The highest volatile solids (29%) and biogas (50%) recovery occurred at 4.0 bar pressure with a 1000 L/ton liquid-to-MSFF. Further improvements could be achieved with longer mixing times before pressing. These findings demonstrate the technical feasibility of the pressing system for preparing an appropriate substrate for the fermentation process, underscoring the potential for optimizing the system. However, further research is required to assess the cost–benefit balance. Full article

The Mediterranean region is increasingly confronted with intersecting environmental, agricultural, and socio-economic challenges, including biowaste accumulation, soil degradation, and high dependency on imported fossil fuels. Biomethane, a renewable substitute for natural gas, offers a strategic solution that aligns with the region’s need for sustainable energy transition and circular resource management. This review examines the current state of biomethane production in the Mediterranean area, with a focus on anaerobic digestion (AD) technologies, feedstock availability, policy drivers, and integration into the circular bioeconomy (CBE) framework. Emphasis is placed on the valorisation of regionally abundant feedstocks such as olive pomace, citrus peel, grape marc, cactus pear (Opuntia ficus-indica) residues, livestock manure, and the Organic Fraction of Municipal Solid Waste (OFMSW). The multifunctionality of AD—producing renewable energy and nutrient-rich digestate—is highlighted for its dual role in reducing greenhouse gas (GHG) emissions and restoring soil health, especially in areas threatened by desertification such as Sicily (Italy), Spain, Malta, and Greece. The review also explores emerging innovations in biogas upgrading, nutrient recovery, and digital monitoring, along with the role of Renewable Energy Directive III (RED III) and national biomethane strategies in scaling up deployment. Case studies and decentralised implementation models underscore the socio-technical feasibility of biomethane systems across rural and insular territories. Despite significant potential, barriers such as feedstock variability, infrastructural gaps, and policy fragmentation remain. The paper concludes with a roadmap for research and policy to advance biomethane as a pillar of Mediterranean climate resilience, energy autonomy and sustainable agriculture within a circular bioeconomy paradigm. Full article

An increasing number of soils, including those in EU countries, are affected by organic matter deficiency and the deterioration of nutrients, and using mineral fertilizers is often associated with negative environmental impacts. One of the basic recommendations for sustainable agriculture is to increase the proportion of organic fertilizers in crop production and preserve soil biodiversity. An increasingly common organic fertilizer is biogas plant digestate, the physical and chemical properties of which depend primarily on the waste material used in biogas production. However, the fertilizer value of this additive and its effects on the soil environment, including beneficial organisms, remain insufficiently studied. Soil macrofauna, particularly earthworms, play a crucial role in soil ecosystems, because they significantly impact the presence of plant nutrients, actively participate in forming soil structures, and strongly influence organic matter dynamics. The present study was undertaken to determine the effects of fertilizing a silt loam soil with the solid fraction of digestate in monoculture crop production on earthworm community characteristics and the resulting changes in selected soil physicochemical properties. The research was conducted at a single site, so the original soil characteristics across the experimental plots were identical. Plots were treated annually (for 3 years; 2021–2023) with different levels of digestate: DG100 (100% of the recommended rate; 30 t ha⁻¹), DG75 (75% of the recommended rate; 22.5 t ha⁻¹), DG50 (15 t ha⁻¹), DG25 (7.5 t ha⁻¹), and CL (a control plot without fertilizer). An electrical method was used to extract earthworms. Those found at the study site belonged to seven species representing three ecological groups: Dendrodrilus rubidus (Sav.), Lumbricus rubellus (Hoff.), and Dendrobaena octaedra (Sav.) (epigeics); Aporrectodea caliginosa (Sav.), Aporrectodea rosea (Sav.), and Octolasion lacteum (Örley) (endogeics); and Lumbricus terrestris (L.) (anecics). Significant differences in the abundance and biomass of earthworms were found between the higher level treatments (DG100, DG75, and DG50), and the lowest level of fertilization and the control plot (DG25 and CL). The DG25 and CL plots showed an average of 24.7% lower earthworm abundance and 22.8% lower biomass than the other plots. There were no significant differences in the earthworm metrics between the plots within each of the two groups (DG100, DG75, and DG50; and DG25 and CL). The most significant influence on the average abundance and average biomass of Lumbricidae was probably exerted by soil moisture and the annual dosage of digestate. A significant increase in the abundance and biomass of Lumbricidae was shown at plots DG100, DG75, and DG50 in the three successive years of the experiment. The different fertilizer treatments were found to have different effects on selected soil parameters. No significant differences were found among the values of the analyzed soil traits within each plot in the successive years of the study. Full article

Adequate treatment of the organic fraction of municipal solid waste (OFMSW) in co-digestion with sewage sludge (SS) through dark fermentation (DF) technologies has been widely studied and recognized. However, there is little experience with a high-solids approach, where practical and scalable conditions are established to lay the groundwork for further development of feasible industrial-scale projects. In this study, the biochemical hydrogen potential of OFMSW using a 7 L batch reactor at mesophilic conditions was evaluated. Parameters such as pH, redox potential, temperature, alkalinity, total solids, and substrate/inoculum ratio were adjusted and monitored. Biogas composition was analyzed by gas chromatography. The microbial characterization of SS and post-reaction percolate liquids was determined through metagenomics analyses. Results show a biohydrogen yield of 38.4 NmLH₂/gVS OFMSW, which forms ~60% of the produced biogas. Aeration was proven to be an efficient inoculum pretreatment method, mainly to decrease the levels of methanogenic archaea and metabolic competition, and at the same time maintain the required total solid (TS) contents for high-solids conditions. The microbial community analysis reveals that biohydrogen production was carried out by specific anaerobic and aerobic bacteria, predominantly dominated by the phylum Firmicutes, including the genus Bacillus (44.63% of the total microbial community), Clostridium, Romboutsia, and the phylum Proteobacteria, with the genus Proteus. Full article

Mechanical–biological treatment plants face challenges in effectively separating organic fractions from residual municipal solid waste for biological treatment. This study investigates the optimization measures carried out at the Erbenschwang MBT facility, which transitioned from solely aerobic treatment to integrated anaerobic digestion using a screw press. This study focused on evaluating the efficiency of each mechanical pretreatment step by investigating the composition of the residual waste, organic fraction recovery rate, and screw press performance in recovering organic material and biogas to press water. The results showed that 92% of the organic material from the residual waste was recovered into fine fractions after shredding and trommel screening. The pressing experiments produced high-quality press water with less than 3% inert material (0.063–4 mm size). Mass balance analysis revealed that 47% of the input fresh mass was separated into press water, corresponding to 24% of the volatile solids recovered. Biogas yield tests showed that the press water had a biogas potential of 416 m³/ton VS, recovering 38% of the total biogas potential. In simple terms, the screw press produced 32 m³ of biogas per ton of mechanically separated fine fractions and 20 m³ per ton of input residual waste. This low-pressure, single-step screw press efficiently and cost-effectively prepares anaerobic digestion feedstock, making it a promising optimization for both existing and new facilities. The operational configuration of the screw press remains an underexplored area in current research. Therefore, further studies are needed to systematically evaluate key parameters such as screw press pressure (bar), liquid-to-waste (L/ton), and feed rate (ton/h). Full article

Volatile fatty acids (VFA) are valuable intermediates with growing demand in chemical, pharmaceutical, and environmental applications. Their sustainable production from organic waste is increasingly explored in the context of circular economy and biorefinery models. This study investigates the co-fermentation of waste-activated sludge (WAS) and the organic fraction of municipal solid waste (OFMSW) as a strategy for integrated VFA and biogas production. Semi-continuous experiments were carried out to assess the effect of the substrates ratio (WAS:OFMSW = 90:10 and 30:70), hydraulic retention time (HRT), and pH control (5, 9, no control) on VFA yield and composition. Results showed that higher OFMSW content and alkaline conditions favoured VFA production, with a maximum yield of 144.9 mgHAc·gVS⁻¹ at pH 9 and 70:30 ratio. Acetate dominated, while butyrate production peaked at 114.1 mgHBu·gVS⁻¹ under high sludge conditions. However, the addition of alkali required for pH control may lead to excessive accumulation of alkaline-earth metal ions, which can disrupt biological processes due to their potential toxicity. Anaerobic digestion of fermentation residues enhanced biomethane yields significantly (0.27 NL·gVS⁻¹ vs. 0.05 NL·gVS⁻¹ from raw sludge). The proposed process demonstrates potential for converting wastewater treatment plants into biorefineries, maximising resource recovery while reducing environmental impact. Full article

This study evaluates the effects of solid-state fermentation inoculated with Aspergillus spp. on the nutritional profile of selected agro-industrial by-products (AIBPs: cowpea shell, groundnut shell, soybean hull, and maize shaft). These AIBPs were assessed as potential feedstuffs in weaner rabbit diets, which often exhibit digestive disorders when introduced to highly lignified feed ingredients. The AIBPs were milled to a particle size of 2 mm, sterilized, and subjected to fermentation with Aspergillus spp. under microaerophilic conditions at 28 ± 2 °C for 10 days. Samples (four replicates per treatment) were analyzed for chemical constituents (mineral and proximate composition, anti-nutritional factors, and fibre fractions) before and after fermentation. Digestible energy and digestibility coefficient of gross energy were calculated. Data were subjected to two-way analysis of variance (ANOVA). There was an increase (p < 0.05) in mineral profile, proximate composition, digestible energy, digestibility coefficient of gross energy, and dry matter, with a reduction (p < 0.05) in crude fibre, fibre fractions, and anti-nutritional factors. It was concluded that fermentation with Aspergillus spp. improved the nutritional value of the selected agro-industrial by-products. Therefore, fermented materials possess a better nutritional profile to be used in feeding programs for weaner rabbits. This will ensure sustainable animal production and add value to agricultural waste, which would otherwise constitute an environmental nuisance. Full article

Active and abandoned mining sites are significant sources of heavy metals and metalloid pollution, leading to serious environmental issues. This study assessed the environmental risks posed by potentially toxic elements (PTEs), specifically arsenic (As) and antimony (Sb), in the Technosols (mining residues) of the former Pejão coal mine complex in Northern Portugal, a site impacted by forest wildfires in October 2017 that triggered underground combustion within the waste heaps. Our methodology involved determining the “pseudo-total” concentrations of As and Sb in the collected heap samples using microwave digestion with aqua regia (ISO 12914), followed by analysis using hydride generation-atomic absorption spectroscopy (HG-AAS). The concentrations of As an Sb ranging from 31.0 to 68.6 mg kg⁻¹ and 4.8 to 8.3 mg kg⁻¹, respectively, were found to be above the European background values reported in project FOREGS (11.6 mg kg⁻¹ for As and 1.04 mg kg⁻¹ for Sb) and Portuguese Environment Agency (APA) reference values for agricultural soils (11 mg kg⁻¹ for As and 7.5 mg kg⁻¹ for Sb), indicating significant enrichment of these PTEs. Based on average I_geo values, As contamination overall was classified as “unpolluted to moderately polluted” while Sb contamination was classified as “moderately polluted” in the waste pile samples and “unpolluted to moderately polluted” in the downhill soil samples. However, total PTE content alone is insufficient for a comprehensive environmental risk assessment. Therefore, further studies on As and Sb fractionation and speciation were conducted using the Shiowatana sequential extraction procedure (SEP). The results showed that As and Sb levels in the more mobile fractions were not significant. This suggests that the enrichment in the burned (BCW) and unburned (UCW) coal waste areas of the mine is likely due to the stockpiling of lithic fragments, primarily coals hosting arsenian pyrites and stibnite which largely traps these elements within its crystalline structure. The observed enrichment in downhill soils (DS) is attributed to mechanical weathering, rock fragment erosion, and transport processes. Given the strong association of these elements with solid phases, the risk of leaching into surface waters and aquifers is considered low. This work underscores the importance of a holistic approach to environmental risk assessment at former mining sites, contributing to the development of sustainable remediation strategies for long-term environmental protection. Full article

The widespread implementation of anaerobic digestion (AD) systems for organic waste treatment is increasingly challenged by emerging contaminants, including microplastics (MPs), antibiotics, and heavy metals (HMs), which exhibit environmental persistence and pose risks to ecological and human health. This review critically examines the sources, transformation pathways, and advanced mitigation strategies for these contaminants within AD systems. MPs, primarily derived from fragmented plastics and personal care products, accumulate in digestates and act as vectors for adsorbing toxic additives and pathogens. Antibiotics, introduced via livestock manure and wastewater, exert selective pressures that propagate antibiotic resistance genes (ARGs) while disrupting methanogenic consortia. HMs, originating from industrial and agricultural activities, impair microbial activity through bioaccumulation and enzymatic interference, with their bioavailability modulated by speciation shifts during digestion. To combat these challenges, promising mitigation approaches include the following: (1) bioaugmentation with specialized microbial consortia to enhance contaminant degradation and stabilize HMs; (2) thermal hydrolysis pretreatment to break down MPs and antibiotic residues; (3) chemical passivation using biochar or sulfides to immobilize HMs. Co-digestion practices inadvertently concentrate these contaminants, with MPs and HMs predominantly partitioning into solid phases, while antibiotics persist in both liquid and solid fractions. These findings highlight the urgency of optimizing mitigation strategies to minimize contaminant mobility and toxicity. However, critical knowledge gaps persist regarding the long-term impacts of biodegradable MPs, antibiotic transformation byproducts, and standardized regulatory thresholds for contaminant residues in digestate. This synthesis underscores the necessity for integrated engineering solutions and policy frameworks to ensure the safe resource recovery from AD systems, balancing energy production with environmental sustainability. Full article

This study presents an integrated approach for the remediation of zinc- and ammonium-contaminated water using duckweed, followed by the valorization of the harvested biomass through anaerobic digestion for biogas production. Duckweed was cultured with various initial concentrations of zinc (Zn, 0 mg/L, 2.5 mg/L, and 5 mg/L) and ammonium (NH₄⁺-N, 0 mg/L, 20 mg/L, and 40 mg/L). Subsequently, duckweed was subjected to chemical pretreatment with sulfuric acid and the obtained residual solid and liquid fractions were evaluated as substrates for methane production. The liquid fraction consistently yielded higher methane production compared to the solid fraction. However, when duckweed was grown in zinc- and ammonium-rich conditions (2.5 or 5.0 mg/L Zn and 20 mg/L NH₄⁺-N), methane production from the liquid hydrolysate was significantly reduced (120.90 ± 12.03 mL/g COD and 129.82 ± 10.65 mL/g COD, respectively) compared to the control duckweed (201.67 ± 5.72 mL/g COD). The lowest methane yields were observed for duckweed grown solely in zinc (111.32 ± 5.72 and 99.88 ± 10.49 mL/g COD for 2.5 and 5.0 mg/L Zn, respectively), attributed to the inhibitory effect of high dissolved zinc concentrations in the liquid fraction. The applicability of this integrated system is particularly relevant for the agricultural and industrial sectors, where wastewater streams are often co-contaminated with nutrients and trace metals. By demonstrating that acid-pretreated, zinc-rich duckweed biomass can be used for biogas production—provided that process conditions are optimized to mitigate metal inhibition and acidification—this study provides actionable strategies for developing circular, sustainable wastewater treatment systems. The approach not only maximizes pollutant removal and resource recovery, but also addresses environmental safety concerns associated with residual metals in the digestate. Full article

The valorization of plant biomass is one of the main strategies for sustainable development. However, its use as energy, biofuels, fertilizers, value-added products, or even food is severely affected by the complexity of the plant cell wall. Therefore, the evaluation of fungi with high production of lignocellulolytic enzymes capable of efficiently degrading these substrates constitutes a viable, clean, and eco-friendly solution, allowing, for example, an increase in the digestibility and nutritional quality of alternative animal feed sources. For these reasons, the present study evaluated the ability of the mutant strain Trichodema viride M5-2 to improve the nutritional composition of the forage legumes Lablab purpureus and Mucuna pruriens through solid-state fermentation. Endo- and exoglucanase cellulolytic activity was assessed, as well as the effect of fermentation on the fiber’s physical properties and chemical composition. Molecular changes in the structure of plant fiber were analyzed using infrared spectroscopy. Increased production of the cellulolytic complex of the enzymes endoglucanase (3.29 IU/mL) and exoglucanase (0.64 IU/mL) was achieved in M. pruriens. The chemical composition showed an increase in true protein and a decrease in neutral fiber, hemicellulose, and cellulose, with a consequent improvement in nutritional quality. Fiber degradation was evident in the infrared spectrum with a significant decrease in the signals associated with cellulose and, to a lesser extent, with lignin. It can be concluded that the mutant strain T. viride M5-2 produced chemical, physical, and molecular changes in the fibrous and protein fractions of L. purpureus and M. pruriens through SSF, which improved their nutritional value as an alternative feed for animal nutrition. By promoting the use of this fungus, the nutritional quality of this source is increased through an effective and eco-friendly process, which contributes to mitigating the environmental impact of food production, in accordance with sustainability objectives and the need for more responsible agricultural practices. Full article

This study evaluates the potential of spent coffee grounds (SCGs) as a co-substrate to improve anaerobic co-digestion (AcD) performance, with a focus on biogas yield, methane (CH₄) content, and the removal of volatile solids (VS) and total chemical oxygen demand (TCOD). Biochemical methane potential (BMP) tests were conducted in two stages. In Stage I, SCGs were blended with active sludge (AS) and the organic fraction of municipal solid waste (OFMSW) at varying ratios. The addition of 25% SCGs increased biogas production by 24.47% (AS) and 20.95% (OFMSW), while the AS50 mixture yielded the highest methane yield (0.302 Nm³/kg VS, 66.42%). However, SCG concentrations of 75% or higher reduced process stability. In Stage II, we evaluated the impact of mixing. The AS25 configuration maintained stable biogas under varying mixing conditions, showing system resilience, whereas OFMSW25 showed slight improvement. Biogas production kinetics were modeled using modified Gompertz, logistic, and first-order equations, all of which demonstrated high predictive accuracy (R² > 0.97), with the modified Gompertz model offering the best fit. Overall, SCGs show promise as a sustainable co-substrate for the improvement of methane recovery and organic matter degradation in AcD systems when applied at optimized concentrations. Full article

Rearing broiler chickens generates large quantities of waste material in the form of bedding. Anaerobic digestion (AD) is a technology that can be applied to this waste. This study aimed to evaluate the AD of broiler litter, either screened (S) or unscreened (US), from different flocks, collected from each production batch, totaling nine, from a commercial farm. Anaerobic digestion was conducted in batch biodigesters, and fraction separation was performed through screening prior to loading. The S substrate from the second and fifth flocks did not produce biogas. Reductions in total (TS) and volatile solids were highest for S substrates from the third flock (50.5% and 58.3%, respectively). Only the third flock’s S substrates showed greater reductions in solids than the US substrates. Potential biogas and methane production were also highest in the third flock’s bedding for both the S substrate (336.8 and 218.2 L/kg of TS, respectively) and the US substrate (296.8 and 213.4 L/kg of TS, respectively). The methane concentration in the S substrate was highest in the third flock (64.8%), while in the US substrate, it was highest in the third and fourth flocks (70.3%). Screening the litter reduced the process efficiency. We conclude that fraction separation is inadvisable for broiler litter. Full article

Nutrient recovery from anaerobic digestate has gained increasing importance in recent years due to its potential to reduce resource dependency and to close nutrient cycles. The aim of this work is to evaluate the influence of a previous solid–liquid separation phase on nutrient recovery efficiency using two innovative membrane technologies, namely, gas-permeable membranes (GPM) and electrodialytic (ED) processes, applied individually or in combination. The obtained results were compared with those obtained through the centrifugation of the raw digestate and direct chemical precipitation followed by centrifugation in terms of the efficiency in the recovery of N (nitrogen) and P (phosphorous). A total of nine scenarios of digestate processing were compared. GPM technology allowed for the recovery of 65% of the N content in the raw digestate (41.5 g total solids (TS) kg⁻¹) and 67% of N in the liquid fraction (28.0 g TS kg⁻¹), without any significant difference between the two scenarios. However, the results revealed significant differences in the P recovery with ED from the raw digestate (15%) and the liquid fraction (34%), suggesting that phosphorous extraction can be improved by the application of a prior solid–liquid phase. The recovery of N with the GPM technology also enhanced the further recovery of total P with the ED processes. Furthermore, the combination of these technologies allowed for the recovery of N- and P-rich solutions, which were used to precipitate secondary struvite with an efficiency of up to 85%. This research provides a practical framework for sustainable nutrient management, advancing solutions for resource efficiency and environmental stewardship. Full article