Search Results (54)

Polyethylene microplastics (PE-MPs) are ubiquitous constituents of waste activated sludge (WAS), acting as a major land-based source threatening coastal environmental integrity. However, how particle size and dose govern the methanogenic outcome during WAS digestion remains poorly defined. This study evaluated two particle sizes (50 vs. 300 µm) and doses (100 vs. 200 particles/gTS) to elucidate the differential effects of PE-MPs on methane yield and the underlying biological mechanisms. The results show that, while low-dose treatments either slightly inhibited methane yield (RS1) or had no significant effect (RL1), high-dose treatments (RS2 and RL2) achieved a net positive effect, with significant increases of 10.2% (p < 0.05) and 9.0% (p < 0.05) relative to the control, respectively. Nevertheless, RS2 and RL2 achieved methanogenic enhancement via distinctly different biological pathways. RS2 harnessed the stress of reactive oxygen species (ROS) (110.5% of the control) to drive community restructuring and biomass accrual (positive correlation between ROS intensity and total VS, Pearson’s r = 0.99). Key syntrophic and electrogenic taxa (e.g., Syntrophales, Bacteroidetes vadinHA17) exhibited a fully interconnected, decentralized network, thereby achieving tight coupling between hydrolysis and methanogenesis. RL2 leveraged the physical carrier effect to promote granulation and biomass growth, enriching Syntrophobacter to enhance propionate degradation. This culminated in a highly modular, sparse network characterized by localized competitive interactions. Together, dosage governs the net methanogenic effect of PE MPs, whereas particle size dictates the mechanistic routes of action. This work offers a mechanistic framework to optimize energy recovery from PE-MP-contaminated sludge while mitigating secondary environmental risks, providing a science-based strategy for the sustainable management of plastic-laden sludge that reconciles renewable energy recovery with pollution control. Full article

Anaerobic digestion (AD) is an important method for sewage sludge (SS) stabilization and methane recovery. Fe compounds are widely present in SS because they are commonly used for phosphorus removal and organic matter (OM) capture in wastewater treatment plants. Endogenous Fe occurs in different forms, but the roles of these forms in SS AD remain unclear. This study systematically compared the effects of FeCl₃, Poly-FeCl₃, Fe₃O₄, FeOOH, and Fe₅HO₈·4H₂O on AD. The results showed that FeCl₃ and Poly-FeCl₃ decreased methane yield by 9.90% and 11.92%, respectively, whereas Fe₃O₄, FeOOH, and Fe₅HO₈·4H₂O increased it by 18.54%, 15.23%, and 15.09%. The analysis suggested that flocculating salts FeCl₃ and Poly-FeCl₃ groups increased sludge particle size, decreased SCOD concentrations by 10.21% and 12.41%, as well as F₄₂₀ by 16.88% and 28.63%, respectively, thereby inhibited the methanogenesis process. In contrast, Fe₃O₄, FeOOH, and Fe₅HO₈·4H₂O enhanced methane production by promoting OM hydrolysis, with SCOD concentrations increased by 12.71%, 8.99%, and 7.47%, respectively. XRD, CV, and EIS results showed that Fe₃O₄ likely promoted methanogenesis through a stable Fe(III)/Fe(II) cycle and electron transfer. Although FeOOH and Fe₅HO₈·4H₂O also underwent Fe(III)/Fe(II) conversion, their promoting effects were weaker than that of Fe₃O₄, possibly because the lack of a bulk mixed-valence structure reduced the efficiency of continuous electron transfer. This study highlights that the chemical form of Fe in SS fundamentally determines its effects on AD performance. Full article

Dairy cattle are a substantial contributor to global agricultural greenhouse gas emissions, primarily producing enteric methane through the ruminal anaerobic fermentation of dietary fiber. As China formally pledges to achieve carbon neutrality before 2060, accurately quantifying these emissions and developing localized mitigation strategies within the livestock sector has become a critical priority. Enteric methane emissions in dairy cattle are not a static physiological baseline; rather, they represent a highly dynamic phenotype profoundly influenced by an intricate network of physiological and environmental parameters. These include the animal’s age, anatomical and ruminal development, parity, lactation stage, and the precise stoichiometric balance of dietary carbohydrates. This review synthesizes extensive experimental data to construct a robust, scientifically logical framework elucidating the profound physiological mechanisms that govern apparent methane emission parameters. Accordingly, this paper reviews our recent research on methane emissions from Holstein dairy cattle across various ages and lactation stages, including heifers, lactating cows, and dry cows. Furthermore, it extensively evaluates the modulation of methanogenesis under diets with varying neutral detergent fiber to non-fibrous carbohydrate (NDF/NFC) ratios, demonstrating that an increased NDF/NFC ratio is positively correlated with higher enteric methane production, yield, and intensity due to the promotion of acetate-type ruminal fermentation. Ultimately, this review aims to provide robust theoretical support for the accurate quantification of enteric methane emissions and the formulation of precision mitigation strategies tailored to specific physiological states. Full article

In this study, the methane yield, substance conversion rate and microbial community structure of individual components of lignocellulose, synthetic mixtures, and corn straw subjected to different pretreatments (thermal hydrolysis, chemical, biological, and combined pretreatment) during anaerobic digestion were comparatively investigated. The synthetic mixture of cellulose and hemicellulose (MCXY) exerted a positive promoting effect on biomethane production, with a synergistic effect index of 101.51%. The methane yield per volatile solids (VS) of microcrystalline cellulose (MC), xylan (XY), and MCXY reached 320.81 ± 1.85 mL/g VS, 352.70 ± 6.58 mL/g VS, and 340.60 ± 10.94 mL/g VS, respectively. Lignin did not produce biogas in anaerobic digestion (AD) system, and its presence had an inhibitory effect on the methanogenesis of cellulose and hemicellulose, especially that of hemicellulose. Notably, pretreatment significantly improved the methane production potential of corn stover. Deep eutectic solvent-pretreated corn stover (DES_CS) achieved the highest methane yield of 356.57 ± 8.50 mL/g VS, which was 55.46% higher than that of the untreated group. DES pretreatment deconstructed lignocellulosic matrix and distinctly increased DOM molecular diversity, thus providing superior substrate conditions for improving anaerobic digestion performance. Microbial community analysis revealed that DES pretreatment significantly reshaped the bacterial structure, enriching syntrophic taxa over the carbohydrate-degrading Bacteroides found in raw corn stover, thereby fostering a more robust metabolic network for methane production. While acetoclastic Methanothrix dominated the pretreated groups, its synergistic coexistence with hydrogenotrophic Methanobacterium across all digesters facilitated stable dual-pathway methanogenesis. This work can provide a theoretical basis and technical reference for the optimization and application of pretreatment strategies for efficient anaerobic digestion of corn stover. Full article

The efficient resource utilization of duck manure and agricultural/forestry wastes (AFW) plays a significant role in environmental protection and promoting the sustainable development of the economy and society. This study examined the effects of hydrochar derived from AFW in the anaerobic digestion (AD) process, determining the optimal addition ratio. This research systematically investigated the impact of hydrochar on methane yield, as well as changes of short-chain fatty acids, microbial community dynamics, and metabolic pathways during AD of duck manure. The underlying mechanisms were clarified by metagenomic and metabolomic analyses. This experiment used duck manure as substrate and added hydrochar of four different dosage levels. Laboratory batch tests ran for 32 days at 37 ± 0.5 °C, with three parallel samples for each group. The results indicated that hydrochar additive significantly improved methane yield (p < 0.05), with a maximum increase of 27.13% at an optimal dosage of 10.91 g·L⁻¹. This amendment enhanced the abundance of Firmicutes, Bacteroidota, Chloroflexota, Halobacteriota, and Methanosarcina significantly. Compared to the control group, the abundances of functional genes involved in hydrolysis, acidogenesis, and acetogenesis pathways increased by 28–254% in the optimal treatment group, with methanogenesis-related genes showing a 16–155% enhancement (p < 0.05). Full article

In the process of anaerobic digestion for manure treatment, adding conductive materials is one of the most used methods to enhance methane yield. Biochar, a stable conductive material, shows significant potential in facilitating direct interspecies electron transfer in anaerobic digestion systems. However, biochar’s structure and properties are influenced by its preparation method, and the mechanisms by which structural characteristics affect methane yield and microbial community structure in fermentation systems require further investigation. This study investigates the effects of pyrolysis duration (1 h for A3O and 2 h for A3T) at 550 °C using corn straw as raw material. Through characterization analyses including SEM, FTIR, conductivity, and elemental composition, we explore the impacts on gas production efficiency and key parameters in anaerobic digestion systems. By analyzing microbial community structure and changes in methanogenic functional bacteria, we elucidate the mechanisms by which biochar materials with different pyrolysis times influence anaerobic digestion processes and microbial community composition. These findings provide theoretical foundations and support for optimizing biochar preparation techniques and their targeted applications in anaerobic digestion fields. It was found that the biochar-treated group exhibited higher methane production. Compared with the CK group without biochar, the methane production of A3O and A3T increased by 8.53% and 5.16%, respectively. While methane yield differed little between A3O and A3T, longer pyrolysis time increased the biochar’s specific surface area, promoting the system’s reaction rate and enabling faster methanogenesis. High-throughput analysis showed that biochar enriched methanogenic archaea like Methanosarcina and Methanobrevibacter while upregulating methanogenesis metabolic pathways and enhancing system metabolic potential. This study elucidates the influence of pyrolysis conditions on biochar performance and its regulatory role in anaerobic digestion, providing a basis for energy recovery from organic waste and biochar application in anaerobic fermentation. Full article

The bioconversion of propionate, a well-known intermediate of anaerobic digestion (AD), to methane is energetically unfavorable under standard conditions, which typically occurs in the syntrophy of bacteria and methanogens via methylmalonyl-CoA (MMC) and the dismutation pathway. Since the latter, which is reported only in Smithella, possessed a thermodynamic advantage over the former, enriching Smithella and promoting the syntrophic interaction and metabolism of the microbiota are important for improving AD efficiency. In this study, lysine-modified Fe₃O₄ (Lys@Fe₃O₄) significantly enhanced the bioconversion of propionate to methane. The methane yield and the maximum methane production rate (R_max) in a Lys@Fe₃O₄ reactor were 278.7% and 271.7% of Blank, and the corresponding values were 201.9% and 201.6% of bare Fe₃O₄, respectively. The metaproteomic results indicated that Lys@Fe₃O₄ increased not only the abundance of Smithella but also the expression of cell surface and adhesion proteins, thereby promoting syntrophic interaction between Smithella and methanogens and facilitating electron and acetate transfer from Smithella to methanogens. Moreover, the expression of quorum-sensing proteins was enhanced, benefiting the cooperation of Smithella and its associated bacterium (Syntrophomonas). Furthermore, the expressions of key enzymes related to metabolism and electron transfer in propionate oxidation, butyrate oxidation, CO₂-reductive methanogenesis and acetoclastic methanogenesis were all significantly upregulated. The results are of great significance for maintaining low propionate concentration and stability of AD. Full article

In this study, the impact of zero-valent iron (ZVI) on methane production during sludge thermophilic anaerobic fermentation was investigated. The results showed that ZVI addition significantly enhanced cumulative methane production, with an optimum concentration of 5 g/L increasing the biochemical methane potential by 51.4% compared to the control. ZVI primarily promoted the acidogenesis and methanogenesis stages rather than hydrolysis, as indicated by the enhanced production of short-chain fatty acids and increased activities of key enzymes. Specifically, the activity of the methanogenic enzyme F₄₂₀ increased by 28.09%, which contributed to a higher methane yield. Moreover, the synergistic effect of ZVI and its decomposition products (Fe²⁺ > Fe³⁺) facilitated a more reduced environment. Furthermore, ZVI addition enriched acetate-utilizing methanogens, i.e., Methanosarcina, which helps rapidly degrade organic acids, thereby stabilizing the fermentation process. These findings demonstrated the potential of ZVI to improve methane recovery and process stability in thermophilic anaerobic fermentation systems. Full article

Natural gas represents a pivotal transitional clean energy resource, and biogenic coalbed methane (CBM) is ubiquitously distributed in coal reservoirs worldwide. In the context of carbon neutrality targets and the growing demand for large-scale commercial CBM exploitation, innovative technological solutions are urgently required. CBM bioengineering aims to substantially enhance CBM production by stimulating biomethane generation, promoting gas desorption, and improving reservoir permeability, while simultaneously enabling effective CO₂ sequestration. The potential for biomethane generation is largely governed by the intrinsic physicochemical characteristics of coal, including aromatic structures, maceral composition, and pore–fracture architecture. In addition, hydrogeological conditions—such as geothermal gradients, pH variability, and redox potential—play critical roles in regulating microbial functional gene expression and metabolic enzyme synthesis. Core pretreatment strategies in coalbed gas bioengineering can be broadly classified into approaches that enhance coal bioconversion potential and those that optimize functional microbial consortia. Electric fields and conductive materials can influence microbial community structure by enriching electroactive microorganisms and facilitating interspecies electron transfer. In addition to engineered conductive interventions, reservoir environmental conditions also play an important role in shaping methanogenic community structure. Experimental observations under reservoir-relevant CO₂ pressure and temperature conditions indicate that deep coalbed environments are associated with shifts in methanogenic community composition, including an increased relative abundance of hydrogenotrophic methanogens. These observations suggest that physicochemical conditions in deep coal seams may favor hydrogen-dependent CO₂ reduction pathways, thereby supporting hydrogenotrophic methanogenesis and contributing to biomethane generation. The integration of supercritical CO₂ with microbially acclimated stimulation fluids as an innovative reservoir fracturing strategy offers multiple advantages, including effective reservoir stimulation, permanent carbon sequestration, and sustainable biomethane generation. Future research should focus on modulating coal matrix bioavailability, optimizing microbial consortia, enhancing interspecies metabolic synergies, and advancing carbon fixation bioprocesses to facilitate the large-scale implementation of coalbed gas bioengineering systems. This review synthesizes recent advances in microbially mediated CBM enhancement and CO₂ sequestration, with a particular focus on field-scale evidence and the key challenges that must be addressed for large-scale implementation. Full article

Anaerobic digestion (AD) is a proven and promising technology for recovering energy from biowastes, such as pig slurry (PS) from the fattening/finishing phase. The mechanisms of AD are widely studied, and nowadays, it is of the utmost importance to investigate strategies that give end-users the confidence to choose this technology and to adapt it to their reality, promoting the energy transition and circular economy. This study investigated how collection and storage period affect PS samples, and how hydraulic retention time (HRT) (15 versus 20 days) influences AD performance and stability. Seasonality was the primary factor influencing feedstock characteristics. Samples presented no significant differences during the storage period. A 20-day HRT led to higher digestate pH, total ammonia nitrogen (TAN), and free ammonia nitrogen (FAN) concentrations, which can cause process instability and methanogenesis inhibition. However, 20-day HRT led to a specific methane production that was 7% higher and to a methane quality (expressed in % v/v CH₄) that was 6% higher than 15-day HRT. Overall, methane quality, digestate pH, TAN, and FAN values may be considered key points that need to be monitored to prevent the AD system from being compromised. Nevertheless, these results provide the operational freedom to choose either HRT, allowing reduced reactor volume and investment. Full article

Enteric methane (CH₄) emissions from ruminants contribute significantly to agricultural greenhouse gases. Anti-methanogenic feed additives (AMFA), such as Asparagopsis spp. and 3-nitrooxypropanol (3-NOP), reduce CH₄ emissions by inhibiting methanogenic enzymes. However, CH₄ inhibition often leads to dihydrogen (H₂) accumulation, which can impact rumen fermentation and decrease dry matter intake (DMI). Recent studies suggest that co-supplementation of CH₄ inhibitors with alternative electron acceptors, such as phloroglucinol, fumaric acid, or acrylic acid, can redirect excess H₂ during methanogenesis inhibition into fermentation products nutritionally beneficial for the host. This review summarizes findings from rumen simulation experiments and in vivo trials that have investigated the effects of combining a CH₄ inhibitor with an alternative H₂ acceptor to achieve effective methanogenesis inhibition. These trials demonstrate variable outcomes depending on additive combinations, inclusion rates, and adaptation periods. The use of phloroglucinol in vivo consistently decreased H₂ emissions and altered fermentation patterns, promoting acetate production, compared with fumaric acid or acrylic acid as alternative electron acceptors. As a proof-of-concept, phloroglucinol shows promise as a co-supplement for reducing CH₄ and H₂ emissions while enhancing volatile fatty acid profiles in vivo. Optimizing microbial pathways for H₂ utilization through targeted co-supplementation and microbial adaptation could enhance the sustainability of CH₄ mitigation strategies using feed additive inhibitors in ruminants. Further research using multi-omics approaches is needed to elucidate the microbial mechanisms underlying the redirection of H₂ toward beneficial fermentation products during enteric methanogenesis inhibition. This knowledge will help guide the formulation of novel co-supplements designed to reduce CH₄ emissions and improve energy efficiency for sustainable livestock production. Full article

Poly(butylene adipate-co-terephthalate) (PBAT) wastewater, characterized by high chemical oxygen demand (COD) and acidity, poses significant challenges to anaerobic digestion (AD) due to toxicity and volatile fatty acids (VFAs) accumulation. This study coupled granular activated carbon (GAC) and waste iron scraps (WISs) to synergistically enhance AD performance. Batch experiments demonstrated that, compared with the control, the GAC/WISs group achieved a COD removal efficiency of 53.18% and a methane production of 207.53 ± 5.80 mL/g COD, which were 5.48- and 12.14-fold increases, respectively, while reducing the accumulation of total VFAs by 98.48% (to 15.09 mg/L). Mechanistic analysis revealed that GAC adsorbed inhibitors and enriched methanogens, while WISs buffered pH and promoted direct interspecies electron transfer (DIET) through hydrogenotrophic methanogenesis. Metagenomic sequencing showed shifts in microbial communities, with enrichment of syntrophic bacteria (Syntrophobacter) and functional genes (pta, bcd, and pccA), indicating metabolic reprogramming. This study provided a theoretical foundation and engineering strategy for the anaerobic treatment of PBAT wastewater. Full article

Anaerobic digestion (AD) technology is universally acknowledged as the most economically viable and efficient approach for energy recovery from livestock manure. To validate the efficacy of riboflavin-functionalized carbon-based conductive materials (CCM-RF) in enhancing methane production at pilot scale, three pilot-scale upflow anaerobic sludge blanket (UASB) reactors were constructed and separately supplemented with carbon cloth (CC), granular activated carbon (GAC), and a combination of CC and GAC. During reactor initialization, riboflavin and a concentrated inoculum of Methanosarcina barkeri (M. barkeri) were introduced to investigate the mechanistic role of CCM-RF in promoting direct interspecies electron transfer (DIET) and optimizing treatment efficiency during anaerobic digestion of cattle manure wastewater. The results showed that all reactors improved AD performance and maintained stable operation at the OLR of 15.66 ± 1.95 kg COD/(m³·d), with a maximum OLR of 20 kg COD/(m³·d) and the HRT as short as 5 days. Among the configurations, the CC reactor outperformed the others, achieving a methane volumetric yield of 6.42 m³/(m³·d), which represents an eight-fold increase compared to conventional AD systems. Microbial community analysis revealed that, although M. barkeri was initially inoculated in large quantities, Methanothrix—a methanogen with DIET capability—eventually became the dominant species. The enrichment of Methanothrix and the simultaneous enhancement in sludge conductivity collectively verified the mechanistic role of CCM-RF in promoting CO₂-reductive methanogenesis through strengthened DIET pathways. Notably, M. barkeri showed progressive proliferation under conditions of high organic loading rates (OLR) and short hydraulic retention time (HRT). This phenomenon provides a critical theoretical basis for the development of future strategies aimed at the targeted enrichment of Methanosarcina-dominant microbial consortia. Full article

Improving the anaerobic digestion (AD) of swine manure is crucial for sustainable waste-to-energy systems, given its high organic load and process instability risks. This study examined the combined effects of substrate-to-inoculum ratio (SIR, 0.1–3.2) and magnetite-mediated direct interspecies electron transfer on biogas production, effluent quality, and microbial community dynamics. The highest methane yield (262 ± 10 mL CH₄/g COD) was obtained at SIR 0.1, while efficiency declined at higher SIRs due to acid and ammonia accumulation. Magnetite supplementation significantly improved methane yield (up to a 54.1% increase at SIR 0.2) and reduced the lag phase, particularly under moderate SIRs. Effluent characterization revealed that low SIRs induced elevated soluble COD (SCOD) levels, attributed to microbial autolysis and extracellular polymeric substance release. Furthermore, magnetite addition mitigated SCOD accumulation and shifted molecular weight distributions toward higher fractions (>15 kDa), indicating enhanced microbial activity and structural polymer formation. Microbial analysis revealed that magnetite-enriched Syntrophobacterium and Methanothrix promoted syntrophic cooperation and acetoclastic methanogenesis. Diversity indices and PCoA further showed that both SIR and magnetite significantly shaped microbial structure and function. Overall, an optimal SIR range of 0.2–0.4 under magnetite addition provided a balanced strategy for enhancing methane recovery, effluent quality, and microbial stability in swine manure AD. Full article

Extremophilic biological process (EBP) pretreatment increases substrate availability in anaerobic digestion, but the effect on downstream microbial community composition in industrial systems is not characterized. Changes in microbial communities were determined at an industrial facility processing dairy manure in a modified split-stream system with three reactor types: (1) EBP tanks at 70–72 °C; (2) mesophilic Continuously Stirred Tank Reactors (CSTRs); (3) mesophilic Induced Bed Reactors (IBRs) receiving combined CSTR and EBP effluent. All reactors had a two-day hydraulic retention time. Samples were collected weekly for 60 days. pH, volatile fatty acid and bicarbonate concentrations, COD, and methane yield were measured to assess tank environmental conditions. Microbial community compositions were obtained via 16S rRNA gene sequencing. EBP pretreatment increased acetate availability but led to a decline in the relative abundance of acetoclastic Methanosarcina species in downstream IBRs. Rather, syntrophic methanogens, e.g., members of Methanobacteriaceae, increased in relative abundance and became central to microbial co-occurrence networks, particularly in association with hydrogen-producing bacteria. Network analysis also demonstrated that these syntrophic relationships were tightly coordinated in pretreated digestate but absent in the untreated CSTRs. By promoting syntrophic methanogenesis while increasing acetate concentrations, EBP pretreatment requires system configurations that enable acetoclast retention to prevent acetate underutilization and maximize methane yields. Full article