Search Results (56)

This study addresses volatile fatty acid (VFA) accumulation, a key issue limiting methane fermentation under high organic loading rate (OLR) conditions. Batch experiments were conducted with GAC (0–10%) under various OLRs (1:0.5–1:10) to investigate its effect on biogas yield, methane purity, and microbial interactions. Higher GAC levels (7.5% and 10%) significantly enhanced biogas production (750–800 mL/g VS) and methane concentration (–70%) while shortening stabilization time. A continuous system with 10% GAC showed suppressed VFA accumulation, stable pH (7.0–8.1), and improved organic matter degradation. This work quantitatively evaluates the link between GAC dosage, DIET induction, and microbial community shifts under high OLR. These findings highlight GAC as an operationally simple and potentially cost-beneficial strategy for stabilizing methane fermentation, particularly in decentralized or small-scale applications. 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

This study investigated the effects of applied voltage on methane fermentation using separate reactors for the anode and cathode, with activated carbon felt as electrodes and a constant voltage of 0.7 V. Compared to the control, the cathode reactor exhibited approximately 1.2 times higher methane production and 1.3 times higher methane concentration, whereas the anode reactor showed a reduction to about 0.5 times and 0.8 times, respectively. Microbial analysis revealed that the anode reactor created an electron-accepting environment, promoting the growth of Clostridium sensu stricto 1 and Fastidiosipila, both contributing to organic acid (electron) production. Conversely, the cathode reactor established an electron-donating environment, enhancing methane production by hydrogenotrophic methanogens such as Methanoculleus and Methanobacterium. Although similar methanogen levels were found in the anode reactor, methane production was higher in the cathode reactor. These findings indicate that the anode facilitates organic acid production via electron acceptance, while the cathode acts as an electron donor that promotes hydrogenotrophic methanogenesis. This study provides a clear evaluation of the effects of microbial electrochemical technologies on methane fermentation, demonstrating their potential to stimulate microbial activities and enhance methane production. Full article

The global increase in municipal and industrial wastewater generation has intensified the need for ecologically resilient and technologically advanced treatment systems. Although traditional biological treatment technologies are effective for organic load reduction, they often fail to remove recalcitrant xenobiotics such as pharmaceuticals, synthetic dyes, endocrine disruptors (EDCs), and microplastics (MPs). Engineered microbial consortia offer a promising and sustainable alternative owing to their metabolic flexibility, ecological resilience, and capacity for syntrophic degradation of complex pollutants. This review critically examines emerging strategies for enhancing microbial bioremediation in wastewater treatment systems (WWTS), focusing on co-digestion, biofilm engineering, targeted bioaugmentation, and incorporation of conductive materials to stimulate direct interspecies electron transfer (DIET). This review highlights how multi-omics platforms, including metagenomics, transcriptomics, and metabolomics, enable high-resolution community profiling and pathway reconstructions. The integration of artificial intelligence (AI) and machine learning (ML) algorithms into bioprocess diagnostics facilitates real-time system optimization, predictive modeling of antibiotic resistance gene (ARG) dynamics, and intelligent bioreactor control. Persistent challenges, such as microbial instability, ARG dissemination, reactor fouling, and the absence of region-specific microbial reference databases, are critically analyzed. This review concludes with a translational pathway for the development of next-generation WWTS that integrate synthetic microbial consortia, AI-mediated biosensors, and modular bioreactors within the One Health and Circular Economy framework. Full article

Anaerobic digestion is a widely adopted technique for biologically converting organic biomass to biogas under oxygen-limited conditions. However, several factors, including the properties of biomass and its complex structure, make it challenging to degrade biomass effectively, thereby reducing the overall efficiency of anaerobic digestion. This review examines the recent advancements in commonly used pretreatment techniques, including physical, chemical, and biological methods, and their impact on the biodegradability of organic waste for anaerobic digestion. Furthermore, this review explores integrated approaches that utilize two or more pretreatments to achieve synergistic effects on biomass degradation. This article highlights various additives and their physicochemical characteristics, which play a vital role in stimulating direct interspecies electron transfer to enhance biomethanation reaction rates. Direct electron interspecies transfer is a crucial aspect that accelerates electron transfer among syntrophic microbial communities during anaerobic digestion, thereby enhancing biomethane formation. Finally, this article reviews potential approaches, identifies research gaps, and outlines future directions to strengthen and develop advanced pretreatment strategies and novel additives to improve anaerobic digestion processes for generating high-value biogas. Full article

Anaerobic digestion (AD) converts organic waste into methane-rich biogas but often faces performance issues due to organic acid and ammonium nitrogen accumulation. This hinders methanogen growth and reduces methane production. Recent studies show that incorporating conductive materials (CMs) into the AD matrix can mitigate these issues by facilitating electron transfer between microorganisms. This process accelerates the oxidation of organic acids and ammonium ions, enhancing methane recovery. The effectiveness of CMs depends on their type, porosity, surface morphology, and conductivity, which foster a symbiotic microbial community. This comprehensive review paper aimed to (i) describe the influence of CMs on the growth and enrichment of the AD microbial community, (ii) quantify the enhancement of biodegradation and methane generation, and (iii) observe syntrophic interactions and interspecies electron transfer. The review also summarized the impact of different conductive materials on methane generation and the effect of operational parameters, e.g., dose, size, and external voltage application, on the conductive electrodes. The study summarized that the different conductive materials have different influences, and their application in the AD matrix has to be realistic based on availability and economic benefits. Full article

Direct interspecies electron transfer (DIET) is a syntrophic metabolism wherein free electrons are directly transferred between microorganisms without the mediation of intermediates such as molecular hydrogen or formate. Previous research has demonstrated that Methanothrix harundinacea 6Ac is capable of reducing carbon dioxide through DIET. However, the mechanisms underlying electron uptake in M. harundinacea 6Ac during DIET remain poorly understood. This study aims to elucidate the electron and proton flux in M. harundinacea 6Ac during DIET and to propose a model for electron uptake in this organism, primarily based on the analysis of gene transcript levels, genomic characteristics of M. harundinacea 6Ac, and the pathways generating fully reduced ferridoxin (Fd_red²⁻), reduced coenzyme F₄₂₀ (F₄₂₀H₂), coenzyme M (CoM-SH), and coenzyme B (CoB-SH) during DIET. The findings suggest that membrane-bound heterodisulfide reductase (HdrED), F₄₂₀H₂-dehydrogenase lacking subunit F (Fpo⁻), and cytoplasmic heterodisulfide reductase (HdrABC)-subunit B of F₄₂₀-reducing hydrogenase (FrhB) complex play critical roles in electron uptake in M. harundinacea 6Ac during DIET. Specifically, Fpo⁻ is responsible for generating Fd_red²⁻ with reduced methanophenazine (MPH₂), driven by a proton motive force, while HdrED facilitates the reduction of heterodisulfide of coenzyme M and coenzyme B (CoM-S-S-CoB) to CoM-SH and CoB-SH using MPH₂. Additionally, cytoplasmic heterodisulfide reductase HdrABC and subunit B of coenzyme F₄₂₀-hydrogenase complex (HdrABC-FrhB complex) catalyzes the reduction of oxidized coenzyme F₄₂₀ (F₄₂₀) to F₄₂₀H₂, utilizing CoM-SH, CoB-SH, and Fd_red²⁻. This study represents the first genetics-based functional characterization of electron and proton flux in M. harundinacea 6Ac during DIET, providing a model for further investigation of electron uptake in Methanosaeta species. Furthermore, it deepens our understanding of the mechanisms underlying electron uptake in methanogens during DIET. Full article

This study aims to investigate the effect of different applied voltages on the biomethanation performance and microbial community characteristics of corn stover (CS) in a microbial electrolysis cell (MEC)-assisted anaerobic digestion (AD) system (MEC-AD). The results showed that the MEC-AD system operating at 0.8 V achieved the highest methane yield of 192.40 mL CH₄/g VS (volatile solids), an increase of 14.98% compared to the conventional AD. The system obtained methane yields of 187.74 to 191.18 mL CH₄/g VS at lower voltages (0.4 V and 0.6 V), and 156.11–182.75 mL CH₄/g VS at higher voltages (1.0 V and 1.2 V), respectively, suggesting that lower or higher voltages would have adversely impacted the methane yield. Correspondingly, the MEC-AD system operating at 0.4–0.8 V achieved over 71.47% conversion rates of total solids (TS), VS, and cellulose. The microbial community analysis revealed that 0.8 V optimally enriched fermentative acidogenic bacteria (FABs, 24.55%) and electroactive bacteria (13.50%), enhancing both hydrolysis acidification efficiency and direct interspecies electron transfer (DIET). Both Methanosarcina and Methanoculleus demonstrated significant positive correlations with FABs, SOBs, and electroactive bacteria. This study reveals that 0.8 V represents the optimal operating voltage for biomethane production in MEC-AD systems, providing critical insights for agricultural waste valorization. Full article

Anaerobic digestion (AD) is a preferred method for food waste (FW) treatment due to its sustainability and potential for production of renewable bioenergy. However, the accumulation of volatile fatty acids (VFAs) and ammonia often destabilizes the AD process, and managing the digestate byproduct poses additional challenges. This study investigates the use of co-pyrolysis biochar synthesized from digestate and rice straw (DRB) to enhance methane production and AD efficiency. DRB addition increased cumulative methane yield by 37.1%, improved VFA conversion efficiency, and achieved a 42.3% higher NH₃-N-removal rate compared to the control group. The COD-removal rate was 68.7% throughout the process. Microbial analysis revealed that DRB selectively enriched Fastidiosipila and Methanosarcina, promoting direct interspecies electron transfer (DIET) and methane yield. These findings highlight DRB’s potential to enhance AD efficiency and support closed-loop resource utilization. Full article

Anaerobic digestion is an important technology for energy recovery from organic waste. However, methanogenesis is restricted by some barriers, such as the low-speed bottleneck of interspecies electron transfer (IET), the low hydrogen partial pressure limitation, trace element deficiency, etc., resulting in poor system stability and low methane production. Recently, multiple iron accelerants have been employed to overcome the above challenges and have been proven effective in enhancing methanogenesis. This study reviews the effects of iron accelerants (Fe⁰, Fe₃O₄ and magnetite, Fe₂O₃ and hematite, iron salts and other iron accelerants) on anaerobic digestion in terms of methane production, process stability and the microbial community and elaborates the mechanisms of iron accelerants in mediating the direct interspecies electron transfer (DIET) of the syntrophic methanogenic community, strong reducibility promoting methanogenesis, provision of nutrient elements for microorganisms, etc. The potential engineering application of iron accelerants in anaerobic digestion and the current research advances regarding the environmental impacts and the recovery of iron accelerants are also summarized. Although iron accelerants exhibit positive effects on anaerobic digestion, most of the current research focuses on laboratory and small-scale investigations, and its large-scale engineering application should be further verified. Future research should focus on elucidating the mechanisms of iron accelerants for enhancing anaerobic digestion, developing diverse application methods for different types of anaerobic systems, optimizing large-scale engineering applications, and exploring the environmental impacts and high-efficiency recovery strategies of iron accelerants. Full article

This study developed a system (MEC-AD) by integrating a single-chamber microbial electrolysis cell (MEC) with anaerobic digestion (AD), aiming to enhance the conversion efficiency of kitchen waste (KW) into biomethane and optimize metabolic pathways. The performance and microbial metabolic mechanisms of MEC-AD were investigated and compared with those of conventional AD, through inoculation with original inoculum (UAD) and electrically domesticated inoculum (EAD), respectively. The results show that the MEC-AD system achieved a CH₄ yield of 223.12 mL/g VS, which was 31.27% and 25.24% higher than that of conventional UAD and EAD, respectively. The system also obtained total solid (TS) and volatile solid (VS) conversion rates of 82.32% and 83.39%, respectively. Furthermore, the MEC-AD system enhanced the degradation of soluble chemical oxygen demand (SCOD) and mitigated biogas production stagnation by reducing the accumulation of volatile fatty acids (VFAs) as intermediate products. Microbial metagenomics analysis revealed that the MEC-AD system enhanced microbial diversity and enriched functional genera abundance, facilitating substrate degradation and syntrophic relationships. At the molecular level, the system upregulated the expression of key enzyme-encoding genes, thereby simultaneously strengthening both direct interspecies electron transfer (DIET) and mediated interspecies electron transfer (MIET) pathways for methanogenesis. These findings demonstrate that MEC-AD significantly improves methane production through multi-pathway synergies, representing an innovative solution for efficient KW-to-biomethane conversion. Full article

The reverse polarity biocathode culture (RPBC) is a technology for the rapid preparation of biocathodes, which quickly enrich electroactive bacteria (EAB) in the microbial fuel cell (MFC) anode and then transform the electrode function from bioanode to biocathode by reversing bioelectrode polarity. However, the mechanism of RPBC is still unclear, and methods to regulate performance and ensure the long-term stability of cultured biocathodes have not been established. This study investigated the correlation between electrogenic bacteria and the target reducing EAB, from two aspects: energy supply and the formation of a composite biofilm. The results showed that electrogenic bacteria provided energy for the reducing EAB through interspecies electron transfer. This process could be regulated by changing the electrode potential and substrate concentration to obtain an optimized biocathode. In addition, the RPBC forms a composite biofilm of electrogenic bacteria and reducing EAB, which significantly improves the enrichment efficiency and the amount of reducing EAB (compared with a direct biocathode culture, respectively, shortening the enrichment time by 80%, increasing the electroactivity by 12.4 times, and increasing the nitrate degradation rate by 4.85 times). This study provides insights into regulating the performance and maintaining the long-term stability of RPBC-cultured biocathodes. Full article

Amid the pressing challenge of global climate change, biogas (marsh gas) has garnered recognition as a clean and renewable energy source with significant potential to reduce greenhouse gas emissions and support sustainable energy production. Composed primarily of methane (CH₄) and carbon dioxide (CO₂), enhancing the CH₄ content in biogas is essential for improving its quality and expanding its high-value applications. This review examines the mechanisms underlying CH₄ and CO₂ production in anaerobic digestion (AD) processes; investigates the effects of raw material types, process routes, and fermentation conditions on biogas production and CH₄ content; and proposes feasible technical pathways for producing CH₄-rich biogas. Research indicates that CH₄-rich biogas can be produced through various strategies. Raw material pretreatment technologies and co-digestion strategies can enhance substrate performance, stabilize the AD process, and boost CH₄ production. Process optimizations, such as multiphase AD and CH₄ co-production techniques, significantly improve carbon utilization efficiency. Introducing exogenous reinforcement materials, including biochar and zero-valent iron nanoparticles, fosters microbial interactions and facilitates direct interspecies electron transfer (DIET). Furthermore, microbial regulation through genetic engineering and microbial community design presents promising prospects. By reviewing the mechanisms of gas production, influencing factors, and feasible pathways, this work aims to provide valuable insights for the technical research of AD to produce CH₄-rich biogas. Full article

When vegetable waste (VW) is used as a sole substrate for anaerobic digestion (AD), the rapid accumulation of volatile fatty acids (VFAs) can impede interspecies electron transfer (IET), resulting in a relatively low biogas production rate. In this study, Chinese cabbage and cabbage were selected as the VW substrates, and four continuous stirred tank reactors (CSTRs) were employed. Different concentrations of biochar-loaded nano-Fe₃O₄(Fe₃O₄@BC) (100 mg/L, 200 mg/L, 300 mg/L) were added, and the organic loading rate (OLR) was gradually increased during the AD process. The changes in biogas production rate, VFAs, and microbial community structure in the fermentation tanks were analyzed to identify the optimal dosage of Fe₃O₄@BC and the maximum OLR. The results indicated that at the maximum OLR of 3.715 g (VS)/L·d, the addition of 200 mg/L of Fe₃O₄@BC most effectively promoted an increase in the biogas production rate and reduced the accumulation of VFAs compared to the other treatments. Under these conditions, the biogas production rate reached 0.658 L/g (VS). Furthermore, the addition of Fe₃O₄@BC enhanced both the diversity and abundance of bacteria and archaea. At the genus level, the abundance of Christensenellaceae_R-7_group, Sphaerochaeta, and the archaeal genus Thermovirga was notably increased. Full article

Propionate, a critical intermediate in anaerobic digestion, and its syntrophic removal, is sensitive to stress. To our knowledge, this study investigates for the first time the response of a metabolic gene panel to organic loading rate (OLR) stress in propionate-degrading methanogenic consortia in lab-scale upflow anaerobic sludge blanket (UASB) reactors. The experimental phases included stabilisation (1.4–2.8 g COD/L/day), electroactive enrichment, OLR shock (6 g COD/L/day), and early recovery. Quantitative PCR was used to assess the abundance of key functional genes (16SrRNA, mcrA, pilA, and hgtR). During stabilisation, ~200 mLCH₄/h was produced, the mcrA/16SrRNA ratio was 0.78–2.64, and pilA and hgtR abundances were 1.29–2.27 × 10⁵ and 2.12–4.37 × 10⁴ copies/gVS. Following the OLR shock, methane production ceased entirely, accompanied by a sharp decline in the mcrA/16S ratio (0.08–0.24) and significant reductions in pilA (1.43-log) and hgtR (1.34-log) abundance. Partial recovery of pilA and hgtR abundance (1.19 × 10⁵ and 8.57 × 10⁴) was observed in the control reactor after the early recovery phase. The results highlight the utility of mcrA, 16SrRNA, pilA, and associated ratios, as reliable indicators of OLR stress in lab-scale UASB reactors. This study advances the understanding of molecular stress responses in propionate-degrading methanogenic consortia, focusing on direct interspecies electron transfer in process stability and recovery. Full article