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The overuse of mineral fertilizers has brought about numerous matters such as deteriorating soil health, crop safety concerns, and environmental pollution. The global requirements for effective waste handling and sustainable agricultural production have been growing continuously. Therefore, integrated nutrient management method might be a key way to achieve circular agriculture, such as replacing chemical fertilizers with organic fertilizers. In modern agriculture, digestate that is a byproduct of anaerobic digestion as a fertilizer is becoming increasingly favored as a viable method for improving crop yield and quality. However, the application of digestate in agriculture have not yet been fully explored. This review addresses a knowledge gap by synthesizing current research on digestate as a fertilizer. Firstly, the physical–chemical and biological properties of digestate are discussed. Following that, this review focuses on its specific impact on crop growth and quality. Lastly, it outlines the challenges faced in the application of digestate and looks ahead to future trends. With appropriate policy support and technological innovation, digestate holds promise for advancing environmental sustainability. This review aims to provide direction and reference for future research on the application of digestate. Full article

Wastewater treatment plants (WWTPs) are significant contributors to anthropogenic greenhouse gas (GHG) emissions through both direct biological processes generating methane (CH₄), nitrous oxide (N₂O), and biogenic carbon dioxide (CO₂) and indirect energy consumption. This comprehensive research paper synthesizes findings from 30 peer-reviewed studies to present a holistic analysis of carbon footprints in wastewater treatment, with a specific quantitative assessment of a sequencing batch reactor (SBR) facility processing 5000 m³/day. The SBR operates with anoxic–aerobic cycles (fill–anoxic react–aerobic react–settle–decant–idle). The analysis reveals that N₂O emissions can constitute up to 75% of a plant’s carbon footprint, while aeration accounts for 40–75% of total energy consumption. For the SBR facility, the baseline carbon footprint is 1648 tCO₂e/year [95% CI: 1420–1910] (0.90 kg CO₂e/m³) under conservative assumptions, with CH₄ yield of 0.03 kg CH₄/kg COD removed and N₂O yield of 0.008 kg N₂O-N/kg TN removed. A realistic baseline using median literature values gives 0.52 kg CO₂e/m³. The carbon footprint of WWTPs varies by treatment technology, scale, and operational conditions, ranging from 61 to 161 kg CO₂e per population equivalent (PE) annually. Through anaerobic digestion with biogas recovery and anoxic phase optimization, emissions can be reduced by 38% to 1018 tCO₂e/year [95% CI: 860–1190]. The findings underscore that achieving carbon neutrality requires extending accounting beyond plant boundaries to include effluent exports, sludge management, and urban infrastructure integration. This paper provides a transparent, practitioner-ready framework for understanding, quantifying, and mitigating carbon emissions from wastewater treatment, with particular emphasis on SBR technology. Full article

Three natural celluloses (softwood pulp, straw grass pulp, and degreased cotton) were used for anaerobic digestion tests to research methane yield, substrate conversion and microbial community structure, and further supplemented and clarified the metabolic pathway mechanisms of anaerobic digestion of cellulosic biomass. The results showed that natural cellulose could be significantly degraded and converted into methane by anaerobic microorganisms. The cumulative specific methane yields of wood pulp fiber (F1), straw pulp fiber (F2), and degreased cotton fiber (F3) were 373.57 ± 10.70 mL/g VS, 349.15 ± 13.20 mL/g VS and 346.16 ± 1.60 mL/g VS, respectively. The corresponding biodegradability values were 93.97%, 85.95% and 84.32%. Although the fermentation cycles in F1, F2, and F3 were identical (T95 was 12 days), the three groups exhibited distinct biogas production patterns. Metagenomic analysis indicated that F1 and F2 were dominated by the acetoclastic methanogenesis pathway, while the proportion of the hydrogenotrophic methanogenesis pathway increased in F3. Meanwhile, the cell motility pathway category was significantly enriched in F3. These results supplement the existing research on the anaerobic digestion of natural cellulose and provide theoretical support for the efficient anaerobic bioconversion of natural cellulosic biomass. 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

Anaerobic co-digestion of substrates offers synergistic benefits, enhancing methane production and improving the operational stability of wastewater treatment. The present study, for the first time, evaluated the biochemical methane potential and kinetics modeling performance of two regional wastewater streams—swine wastewater (SW) and nejayote wastewater (NW)—under mesophilic batch conditions. Five substrate ratios (SW/NW: 100/0 to 0/100) were tested, and interaction effects were measured using the co-digestion performance index (CPI). All mixtures demonstrated synergistic effects, with CPI values ranging from 1.12 to 1.26. NW exhibited the highest methane yield (438 ± 25 NL-CH₄/kgCOD_T-removed), nearly twice that obtained for SW (227 ± 18 NL-CH₄/kgCOD_T-removed). In addition, co-digestion improved the methane yield of SW as mono-digestion, with production increasing from 281.8 ± 12.4 to 304.7 ± 27.8 NL-CH₄/kgCOD_T-removed in all mixtures. The methane production kinetics were analyzed using six mathematical models. The multi-phase Gompertz model provided the best fit (R² > 0.99), while the two-phase model offered the best balance of accuracy and simplicity according to Akaike’s criterion. The present model effectively described the diauxic patterns of methane production resulting from substrate heterogeneity with an error of <8% for all experimental assays. Full article

Augmenting aquaculture feeds with black soldier fly (Hermetia illucens L.) larvae is an emerging solution to the industry’s fishmeal and fish oil dependence. However, the larva’s nutritional plasticity often results in bioaccumulation of metals from the rearing substrates. Larvae can be nutritionally enriched with microalgae, but research investigating growth impacts and metals uptake are lacking. In this study, a Stichococcaceae algae strain that is used to phycoremediate effluent from commercial anaerobic digesters was investigated as a rearing substrate. Larvae were reared on chicken feed enriched with stepped ratios of algae and spent coffee grounds (a reference waste feed). Growth, survival and metals content (ICP-OES) were recorded when 10% of larvae were prepupal. Survival was >98.5% across all treatments with a trend of increased growth with microalgal inclusion, and no significant impact of metals on growth. Metals uptake as determined by a bioaccumulation factor was significantly lower in the highest algae treatment compared to the coffee-only treatment. Larvae consistently accumulated cadmium and lead whereas arsenic bioaccumulation was only observed in three treatments. Cadmium had the highest bioaccumulation factor (up to 4.06) and arsenic the lowest (down to 0.41). Larvae did not exceed current European Union maximum metal ions levels for inclusion into aquafeeds. These findings highlight the potential of using Stichococcaceae to enrich black soldier fly larvae, offering a dual sustainable solution for wastewater remediation and aquaculture feed provision. Full article

Lignocellulose is degraded in the rumen by diverse microorganisms. This study aimed to select the top ruminal microbes associated with an anaerobic fungus (AF) capable of forming consortia that facilitate biogas production from wheat straw. The workflow included the following steps: (1) batch reactors, divided into three compartments with porous membrane bags containing wheat straw, were assembled. The outermost compartment was inoculated with freshly collected rumen content. The first microbes colonizing the wheat straw in the innermost compartment within 72 h were identified. (2) Synthetic consortia were assembled comprising the following identified microbes: an anaerobic fungus (AF) (Neocallimastix lanati); methanogenic archaea (M) (Methanobrevibacter ruminantium or Methanobrevibacter gottschalkii); bacteria (B) (Butyrivibrio hungatei or Succinoclasticum ruminis). (3) Wheat straw was subjected to 7-day pretreatments with these synthetic consortia. (4) The pretreated straw served as substrate in biochemical methane potential (BMP) tests that used a biogas reactor digestate as the inoculum. The pretreated straw produced elevated biomethane yields; nonetheless, this process needs further optimization. The cross-kingdom AF + M + B consortia increased methane production by 35–70%, and superior volatile fatty acid production was confirmed via HPLC. The results suggest novel strategies for advanced practical biogas/biomethane technologies. Full article

Olive pomace is an abundant agro-industrial residue whose recalcitrant composition limits its efficient valorisation, highlighting the need for sustainable recovery strategies. This study investigated the use of Hermetia illucens larvae for the bioconversion of olive pomace and its integration with anaerobic digestion to evaluate the simultaneous recovery of biomass and energy. Larvae were initially reared on olive pomace, goat manure, and their mixture to assess growth performance, survival, and substrate suitability. Subsequently, olive pomace was subjected to thermal pre-treatment and anaerobic digestion with goat manure, and the resulting digestates were reused as substrates for larval rearing. Their phytotoxicity was also evaluated through germination tests on alfalfa. Larval performance was higher on non-digested substrates, confirming the suitability of raw mixtures for insect rearing. Thermal pre-treatment did not result in a significant increase in methane production. In contrast, digestates, particularly those obtained under more severe thermal treatments, resulted in reduced larval growth and survival under the tested conditions and showed variable phytotoxic effects. Overall, although anaerobic digestion enabled energy recovery, its integration with insect-based bioconversion resulted in reduced larval performance under the tested conditions. These findings highlight the importance of optimising substrate treatment to ensure effective integration of insect rearing within circular bioeconomy systems. Full article

The European Urban Wastewater Treatment Directive revision introduced the energy neutrality concept, accelerating the transition of wastewater treatment plants (WWTPs) towards a 100% renewable energy share. Energy audits must be initially conducted to assess current energy consumption levels, identifying deviations from benchmarking values, and energy efficiency measures must be implemented. Strategies should be then diversified according to WWTP size: anaerobic digestion (AD) is a core technology for large-scale plants. The refurbishment of conventional digesters into “enhanced” AD, including sludge pretreatment, co-digestion, or two-stage AD, significantly increases energy yields, providing most of the required electricity/heat. Enhanced AD can be complemented by photovoltaic (PV) panels and thermal energy recovery from effluents. For medium-scale plants, instead, PV implementation is a key solution for electricity production, coupled with hydroenergy recovery and, eventually, wind turbines, while heat can be provided by solar thermal panels or thermal energy recovery from effluents. Hybrid systems, which integrate multiple renewable sources, are often the best solution to reach energy neutrality, improving the system’s resiliency; however, dedicated mathematical models are needed to size and operate the different components, considering local factors. Future research must connect theoretical and in-field studies to allow a wider implementation of hybrid systems. Full article

This study designed an integration of anaerobic digestion, partial nitrification/Anammox (PN/A), and Fenton process to efficiently treat high-concentration organic liquor wastewater (HCLW). Results indicated that when the diluted ten-fold mixture of boiler bottom water and cellar bottom water with the ratio of 5:1 was used as influent, the average concentrations of COD, TN, NH₄⁺-N, NO₂⁻-N, and NO₃⁻-N in effluent of biological treatment for this process were 180.00, 12.64, 1.74, 0.13, and 2.45 mg/L, respectively. To meet the requirement for direct discharge of HCLW, Fenton oxidation with 600 mg H₂O₂/L and 300 mg Fe²⁺/L was used to further reduce the COD concentration. Three-dimensional fluorescence spectra analysis revealed that the process effectively altered the organic molecular structure and degraded some large molecular proteins. Microbial community analysis showed that Methanobacterium (20.98% → 31.52%) and Methanosaeta (9.70% → 19.34%) in AD, Azoarcus (no detected → 10.49%) and Nitrosomonas (1.68% → 6.16%) in PN, and Candidatus_Brocadia (18.80% → 20.31%) and Ignavibacterium (no detected → 5.11%) in Anammox were dominant in this system. This study provided a pioneering industrial solution for the efficient and stable treatment of HCLW. 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 plants utilizing organic waste play an important role in the transition toward a circular economy and renewable energy systems. However, evaluating their actual performance is challenging due to the diversity of anaerobic digestion technologies, the wide range of feedstocks, and the use of various pretreatment methods. Consequently, assessing operational efficiency requires a comprehensive approach that goes beyond installed capacity alone. This paper synthesizes and systematizes existing approaches for evaluating the efficiency of biogas plants based on key operational indicators reported in the literature. The analysis considers a broad spectrum of feedstocks, highlighting the variability of input materials and their influence on plant performance. Particular attention is given to the internal energy consumption of electricity and heat, which directly affects net energy output and overall efficiency. The relationship between annual energy production (MWh) and installed capacity (MW) is analyzed as a core performance indicator enabling comparison between plants using different technologies, substrates, and scales. The proposed framework supports transparent performance assessment, operational optimization, and evidence-based decision-making in the development and management of waste-based biogas systems. Full article

Advanced nanomaterials are becoming increasingly critical for improving the efficiency, durability, and sustainability of bioenergy systems, with applications spanning biomass conversion, catalysis, and bioelectrochemical energy generation. This systematic bibliometric and thematic review analyses Scopus-indexed literature from 2020 to 2025 to elucidate global research trends in nanomaterial characterisation and performance evaluation for bioenergy applications. Bibliometric mapping using VOSviewer version 1.6.18 reveals a rapidly growing research landscape structured around three dominant themes: nanocatalysts for biodiesel and bioethanol production, nanostructured enhancements in bioelectrochemical and anaerobic digestion systems, and surface-engineered materials for energy conversion and storage. The review highlights the pivotal role of structural and morphological characterisation techniques including SEM, TEM, AFM, and XRD in establishing structure–property–performance relationships that underpin catalytic activity, electron transfer efficiency, and system stability. Beyond short-term catalytic and electrochemical metrics, increasing attention is given to mechanical stability, durability, and long-term operational reliability, which are shown to be critical determinants of scalability. Emerging strategies such as additive manufacturing and hybrid material systems enable the integration of nanomaterials into architected, mechanically robust structures, mitigating degradation and enhancing sustained performance. A concise conceptual framework is presented to link nanomaterial classes, characterisation challenges, targeted bioenergy applications, and scalability constraints. Despite significant progress, gaps remain in standardised characterisation protocols, durability-focused testing, and life-cycle assessment. Addressing these challenges is essential for translating laboratory-scale advances into scalable, sustainable bioenergy technologies. Full article

Biogas production through anaerobic digestion is increasingly recognised as a strategic renewable energy pathway capable of addressing South Africa’s energy insecurity, organic waste management challenges, and climate mitigation goals. However, the water-intensive nature of anaerobic digestion raises critical sustainability concerns in water-scarce regions. This systematic review critically examines the water footprint of biogas-based bioenergy systems, with a specific focus on South Africa’s water-stressed context, to understand how water availability, feedstock selection, digester configuration, and governance frameworks influence system viability and scalability. This study adopts a systematic literature review (SLR) approach guided by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) methodology; peer-reviewed literature published between 2010 and 2025 was retrieved from Scopus and Web of Science and synthesised through descriptive analysis and qualitative meta-synthesis. The review integrates blue, green, and greywater footprint concepts to assess water use across diverse biogas pathways, including livestock manure, agricultural residues, food waste, wastewater sludge, and aquatic biomass. Findings indicate that wet digestion systems, dominant in South Africa, are highly sensitive to freshwater availability, particularly where slurry dilution relies on blue water. In contrast, wastewater-integrated, semi-wet, and co-digestion systems substantially reduce freshwater demand while enhancing methane yields and process stability. The reuse of greywater, industrial effluents, and digestate emerges as a key strategy for lowering water footprints and strengthening circular water–energy linkages. Despite strong technical potential, the adoption of water-efficient anaerobic digestion systems remains constrained by fragmented governance, infrastructure deficits, and limited empirical data on dry and low-water digestion technologies. The review concludes that embedding water footprint considerations into bioenergy planning, policy, and system design is essential for the sustainable expansion of biogas in South Africa. Integrated water–energy–waste governance, coupled with targeted technological innovation, is critical to ensuring that biogas development enhances both energy security and water sustainability in water-scarce regions. Full article

Anaerobic digestion is a key technology for chicken manure valorization, but ammonia accumulation often causes system instability. In this study, a 100-day continuous stirred tank reactor (CSTR) experiment was conducted under mesophilic conditions to investigate the mechanisms of ammonia inhibition in chicken manure at total solids (TS) contents of 8% (T1), 12% (T2), and 16% (T3). Compared to T1, the peak TAN concentrations in T2 and T3 were 64.28% and 73.82% higher. After 100 days, pH in T2 and T3 dropped by 5.19% and 7.65% relative to T1. Volatile fatty acid (VFA) accumulation increased by 4.6- and 6.5-fold, while the TS-based methane yield decreased by 52.94% and 73.11%, respectively. Metagenomic analysis revealed the mechanisms of ammonia inhibition: high-ammonia conditions not only directly suppressed the gene abundance of methanogenic pathways but also systematically reduced the abundance of hydrolytic bacteria and acidogenic fermentative bacteria, leading to a disruption in the supply chain of methanogenic precursors, while ammonia-tolerant microbiota became competitively enriched. This study elucidates the multi-level mechanism of ammonia inhibition in high-TS chicken manure digestion at the functional gene level, providing a theoretical basis for the precise regulation of ammonia stress and improvement of system stability. Full article