Methane production from formate, acetate and H2/CO2; focusing on kinetics and microbial characterization

Syntrophic methane production from volatile fatty acids: Focus on interspecies electron transfer

Methane is a renewable biomass energy source produced via anaerobic digestion (AD). Interspecies electron transfer (IET) between methanogens and syntrophic bacteria is crucial for mitigating energy barriers in this process. Understanding IET is essential for enhancing the efficiency of syntrophic methanogenesis in anaerobic digestion. Interspecies electron transfer mechanisms include interspecies H2/formate transfer, direct interspecies electron transfer (DIET), and electron-shuttle-mediated transfer. This review summarizes the mechanisms, developments, and research gaps in IET pathways. Interspecies H2/formate transfer requires strict control of low H2 partial pressure and involves complex enzymatic reactions. In contrast, DIET enhances the electron transfer efficiency and process stability. Conductive materials and key microorganisms can be modulated to stimulate the DIET. Electron shuttles (ES) allow microorganisms to interact with extracellular electron acceptors without direct contact; however, their efficiency depends on various factors. Future studies should elucidate the key functional groups, metabolic pathways, and regulatory mechanisms of IET to guide the optimization of AD processes for efficient renewable energy production.

Utilization strategy for algal bloom waste through co-digestion with kitchen waste: Comprehensive kinetic and metagenomic analysis

Anaerobic co-digestion (AcoD) with kitchen waste (KW) is an alternative utilization strategy for algal bloom waste (AW). However, the kinetic characteristic and metabolic pathway during this process need to be explored further. This study conducted a comprehensive kinetic and metagenomic analysis for AcoD of AW and KW. A maximum co-digestion performance index (CPI) of 1.13 was achieved under the 12% AW addition. Co-digestion improved the total volatile fatty acids generation and the organic matter transformation efficiency. Kinetic analysis showed that the Superimposed model fit optimally (R2Adj = 0.9988-0.9995). The improvement of the kinetic process by co-digestion was mainly reflected in the increase of the methane production from slowly biodegradable components. Co-digestion enriched the cellulolytic bacterium Clostridium and the hydrogenotrophic methanogenic archaea Methanobacterium. Furthermore, for metagenome analysis, the abundance of key genes concerned in cellulose and lipid hydrolysis, pyruvate and methane metabolism were both increased in co-digestion process. This study provided a feasible process for the utilization of AW produced seasonally and a deeper understanding of the AcoD synergistic mechanism from kinetic and metagenomic perspectives.

Cultivation of earthworms and analysis of associated bacterial communities during earthworms' growth using two types of agricultural wastes

Earthworm cultivation can effectively promote the resource utilization of agricultural waste. The efficient utilization of agricultural waste by earthworms mainly depends on the microbial communities in the guts. This study used silkworm excrement and cow manure as substrates for earthworm cultivation and investigated the associated bacterial communities during earthworms' growth. The survival rate of earthworms remained above 89% after 21 days of feeding with the two substrates. Proteobacteria, Actinobacteria, and Firmicutes constituted the predominant bacterial communities in earthworm growth, accounting for over 81% of the relative abundance in both guts and vermicompost. The bacteria richness and diversity in the foregut and midgut of earthworm were lower than those in the hindgut. The prediction function of intestinal bacterial communities of earthworms cultured with two substrates mainly involved biosynthesis, decomposition and energy production.

Assessment of the in situ biomethanation potential of a deep aquifer used for natural gas storage

The dihydrogen (H2) sector is undergoing development and will require massive storage solutions. To minimize costs, the conversion of underground geological storage sites, such as deep aquifers, used for natural gas storage into future underground hydrogen storage sites is the favored scenario. However, these sites contain microorganisms capable of consuming H2, mainly sulfate reducers and methanogens. Methanogenesis is, therefore expected but its intensity must be evaluated. Here, in a deep aquifer used for underground geological storage, 17 sites were sampled, with low sulfate concentrations ranging from 21.9 to 197.8 µM and a slow renewal of formation water. H2-selected communities mainly were composed of the families Methanobacteriaceae and Methanothermobacteriaceae and the genera Desulfovibrio, Thermodesulfovibrio, and Desulforamulus. Experiments were done under different conditions, and sulfate reduction, as well as methanogenesis, were demonstrated in the presence of a H2 or H2/CO2 (80/20) gas phase, with or without calcite/site rock. These metabolisms led to an increase in pH up to 10.2 under certain conditions (without CO2). The results suggest competition for CO2 between lithoautotrophs and carbonate mineral precipitation, which could limit microbial H2 consumption.

High-throughput DNA extraction and cost-effective miniaturized metagenome and amplicon library preparation of soil samples for DNA sequencing

Reductions in sequencing costs have enabled widespread use of shotgun metagenomics and amplicon sequencing, which have drastically improved our understanding of the microbial world. However, large sequencing projects are now hampered by the cost of library preparation and low sample throughput, comparatively to the actual sequencing costs. Here, we benchmarked three high-throughput DNA extraction methods: ZymoBIOMICS™ 96 MagBead DNA Kit, MP BiomedicalsTM FastDNATM-96 Soil Microbe DNA Kit, and DNeasy® 96 PowerSoil® Pro QIAcube® HT Kit. The DNA extractions were evaluated based on length, quality, quantity, and the observed microbial community across five diverse soil types. DNA extraction of all soil types was successful for all kits, however DNeasy® 96 PowerSoil® Pro QIAcube® HT Kit excelled across all performance parameters. We further used the nanoliter dispensing system I.DOT One to miniaturize Illumina amplicon and metagenomic library preparation volumes by a factor of 5 and 10, respectively, with no significant impact on the observed microbial communities. With these protocols, DNA extraction, metagenomic, or amplicon library preparation for one 96-well plate are approx. 3, 5, and 6 hours, respectively. Furthermore, the miniaturization of amplicon and metagenome library preparation reduces the chemical and plastic costs from 5.0 to 3.6 and 59 to 7.3 USD pr. sample. This enhanced efficiency and cost-effectiveness will enable researchers to undertake studies with greater sample sizes and diversity, thereby providing a richer, more detailed view of microbial communities and their dynamics.

Rumen microbes, enzymes, metabolisms, and application in lignocellulosic waste conversion - A comprehensive review

The rumen of ruminants is a natural anaerobic fermentation system that efficiently degrades lignocellulosic biomass and mainly depends on synergistic interactions between multiple microbes and their secreted enzymes. Ruminal microbes have been employed as biomass waste converters and are receiving increasing attention because of their degradation performance. To explore the application of ruminal microbes and their secreted enzymes in biomass waste, a comprehensive understanding of these processes is required. Based on the degradation capacity and mechanism of ruminal microbes and their secreted lignocellulose enzymes, this review concentrates on elucidating the main enzymatic strategies that ruminal microbes use for lignocellulose degradation, focusing mainly on polysaccharide metabolism-related gene loci and cellulosomes. Hydrolysis, acidification, methanogenesis, interspecific H2 transfer, and urea cycling in ruminal metabolism are also discussed. Finally, we review the research progress on the conversion of biomass waste into biofuels (bioethanol, biohydrogen, and biomethane) and value-added chemicals (organic acids) by ruminal microbes. This review aims to provide new ideas and methods for ruminal microbe and enzyme applications, biomass waste conversion, and global energy shortage alleviation.

Effect of zero-valent iron nanoparticles on taxonomic composition and hydrogen production from kitchen waste

This study investigated the effects of varying zero-valent iron (ZVI) (0 to 5,000 mg/L) on fermentative hydrogen (H2) production, metabolic pattern, and taxonomic profile by using kitchen waste as substrate. The study demonstrated that the supplementation of 500 mg ZVI/L resulted in the highest H2 yield (219.68 ± 11.19 mL H2/g-volatile solids (VS)added), which was 19% higher than the control. The metabolic pattern analysis showed that acetic and butyric acid production primarily drove the H2 production. The taxonomic analysis further revealed that Firmicutes (relative abundance (RA): 80-96%) and Clostridium sensu stricto 1 (RA: 68-88%) were the dominant phyla and genera, respectively, during the exponential gas production phase, supporting the observation of accumulation of acetic and butyric acids. These findings suggest that supplementation of ZVI can enhance H2 production from organic waste and significantly influence the metabolic pattern and taxonomic profile, including the metalloenzymes.

A comparative study on polyaluminum chloride (PACl) and Moringa oleifera (MO) chemically enhanced primary treatment (CEPT) in enhanced biogas production: anaerobic digestion performance and the Gompertz model

A comparative study was performed to estimate biogas production from sludge produced by organic and inorganic chemically enhanced primary treatments (CEPTs). To this end, the effects of two coagulants, polyaluminum chloride (PACl) and Moringa oleifera (MO), on CEPT and biogas production in anaerobic digestion were surveyed within an incubation period of 24 days. The optimal dosage and pH of PACl and MO were optimized in terms of sCOD, TSS and VS parameters in the CEPT process. Next, the digestion performance of anaerobic digestion reactors fed with sludge obtained from PACl and MO coagulants at a batch mesophilic reactor (37 ± 1 °C) was surveyed from the biogas production, volatile solid reduction (VSR) and Gompertz model. At the optimal conditions (pH = 7 and dosage = 5 mg L^-1), the removal efficiency of COD, TSS and VS in CEPT assisted with PACL was 63, 81 and 56%, respectively. Moreover, CEPT assisted with MO led to the removal efficiency of COD, TSS and VS until 55, 68 and 25%, respectively. The highest methane yield (0.598 L g_{VS removed}^-1) was obtained in an anaerobic digestion reactor with sludge from the MO coagulant. The anaerobic digestion of CEPT sludge instead of primary sludge resulted in higher sCOD removal efficiency, and 43-50% of sCOD was observed compared with the removal of 32% for the primary sludge. Furthermore, the high coefficient of determination (R²) demonstrated the trustworthy predictive precision of the modified Gompertz model with actual data. The combination of CEPT and anaerobic digestion, especially using natural coagulants, provides a cost-effective and practical way to increase BMP from primary sludge.

Synergetic effects of anaerobic co-digestion of food waste and algae on biogas production

Anaerobic co-digestion of food waste and algae was assessed to offset the drawbacks of anaerobic mono-digestion of each substrate. Batch test results indicated that a food waste and algae mixture ratio of 8:2 facilitated the highest CH₄ yield (334 mL CH₄/g COD_Input). This ratio was applied to the anaerobic co-digestion reactor, resulting in a CH₄ yield that was twice that of the anaerobic mono-digestion reactors, thereby facilitating high operational stability. In contrast to the anaerobic mono-digestion, anaerobic co-digestion resulted in stable CH₄ production by overcoming volatile fatty acid accumulation and a decreased pH, even under a high organic loading rate (3 kg COD/m³∙d). Furthermore, a comparative metagenomic analysis revealed that the abundance of volatile fatty acid-oxidizing bacteria and hydrogenotrophic and methylotrophic methanogens was significantly increased in the anaerobic co-digestion reactor. These findings indicate that the anaerobic co-digestion of food waste and algae significantly improves CH₄ production and process stability.

Salinity impact on the metabolic and taxonomic profiles of acid and alkali treated inoculum for hydrogen production from food waste

This study evaluated the combined impact of salinity (2.5, 13, and 19.3 g NaCl/L) and inoculum pretreatment (acid/alkali) on the genomic and metabolic profiles of mesophilic fermentative bacteria for hydrogen (H₂) production from food waste. Experimental results revealed that acid-treated inoculum showed the highest H₂ yield (201.12 ± 13.84 mL H₂/g of volatile solids added) under medium salinity levels compared to other experimental conditions. A 7-56% increase in H₂ yield was observed for pretreated inoculum than untreated inoculum. Genomic analysis and metabolic pattern revealed that the H₂ production was mainly through Clostridial-type fermentation under medium to high salinity levels, whereas Enterococcus-type fermentation under low salinity levels. Further, the genomic analysis uncovered that phyla Firmicutes (69.71-96.81%) and genus Clostridium sensu stricto 1 (33.28-94.04%) dominated during the exponential gas production phase. Overall, this study showed the significance of inoculum pretreatment for the bioconversion of food waste at different salinity levels.

Biological selenate and selenite reduction by waste activated sludge using hydrogen as electron donor

Biological reduction of selenium oxyanions is widely used for selenium removal from wastewater. The process is, however, limited by the availability of a suitable, efficient and low cost electron donor. In this study, selenite and selenate reduction by waste activated sludge using hydrogen as the electron donor was investigated. Both selenite and selenate (80 mg/L) were completely removed using H₂ within 8 days of incubation. In the presence of sulfate in the medium, the Se removal efficiency decreased to 77.8-95.4% (for selenite) and 88.2-99.4% (for selenate) at different temperatures and initial sulfate concentrations. Thermophilic conditions (50 °C) were better suited for both selenite and selenate reduction using H₂ as electron donor with a 0.8-13.5% increase in overall Se removal. Similarly, sulfate reduction also increased from 69.1- 88% at 30 °C to 72-94.6% at 50 °C. Most of the H₂ utilized was diverted towards Se and sulfate reduction with minimal production of byproducts such as methane (<0.32 mM) or volatile fatty acids (<0.92 mg/L). The elemental Se produced from selenite and selenate reduction ranged between 33.9 and 52.1 mg/L. The elemental selenium nanoparticles produced as a result of selenite and selenate reduction were characterized using transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and dynamic light scattering (DLS) spectroscopy. Furthermore, characterization of the biomass using Fourier-transform infrared spectroscopy (FTIR) and excitation emission matrix (EEM) spectra of the extracellular polymeric substances (EPS) produced by the waste activated sludge were performed to elucidate the mechanism of selenium oxyanion reduction to elemental selenium nanoparticles.

Semi-continuous mesophilic-thermophilic two-phase anaerobic co-digestion of food waste and spent mushroom substance: Methanogenic performance, microbial, and metagenomic analysis

The methanogenic efficiency and system stability of anaerobic co-digestion of food waste (FW) and spent mushroom substance (SMS) are important for its application. A 90-day semi-continuous study was conducted to compare the co-digestion performance of an ethanologenic-methanogenic two-phase system and an acidogenic-methanogenic system using FW and SMS as substrates. The results showed that the ethanologenic-methanogenic system increased the contents of ethanol and acetate in the hydrolytic acidification phase. Microbial-community analysis showed that ethanologenic-methanogenic system enriched hydrolytic acidifying bacteria and methanogens such as Methanoculleus, resulting in an increase in the average methane yield of methanogenic phase by 1.91-2.43 times at the same organic loading rate (OLR = 3.0-4.0 g-VS·L^-1·d^-1). Metagenomic analysis indicated that the ethanologenic-methanogenic system increased the abundance of enzyme-encoding genes and promoted the degradation of acetate and CO₂/H₂, thereby enhancing methanogenic metabolic pathways, compared to the acidogenic-methanogenic system.

Semi-solid anaerobic co-digestion of source-separated fecal slag and food waste: focusing on methane production, ecological risk assessment, and quality evaluation as fertilizer

Toilet revolution is driven by the urgent need for solutions to improve sanitation and access to high-quality organic fertilizer for rural areas, which is tagged "resource recovery from human waste." This study provides a possible solution via semi-solid anaerobic co-digestion (Aco-D) of source-separated fecal slag (SFS) and food waste (FW) (3:1). A comprehensive investigation of Aco-D at different inoculum/substrate ratios (ISR) was conducted. Results revealed that the reactor with ISR of 1:4 reached the highest methane yield (255.05 mL/gVS), which enhanced Methanosaetaceae, Methanomicrobiales, and Syntrophomonas. Additionally, the reactor with low feedstock (ISR of 1:2) showed higher removal efficiency of antibiotics (74.75%). The ecological risk of digestate decreased to an insignificant hazard quotient level, and the contents of nutrients and heavy metals were in line with the standard requirement for fertilizer. This study could serve as an alternative technology to support further research in SFS management and digestate utilization as fertilizer.

Tracking the diversity and interaction of methanogens in the energy recovery process of a full-scale wastewater treatment plant

Methanogens have been significant for the achievement of carbon neutrality in wastewater treatment plants due to their crucial roles in the anaerobic digestion of sludge. Nevertheless, the phylogenetic diversity of methanogens and their versatile metabolism have been continuously investigated, the current scientific knowledge regarding these microbes appears inadequate and requires more evaluations. This study is considered an endeavor in which functional genes sequencing was used to reveal the diversity of methanogens in the sludge process of the wastewater treatment plant. The information obtained was substantially more than that employing 16s sequencing. The methanogenic microbial resources were appropriate to sustain a self-inoculated energy recovery with a potential ability to boost methane production. A constancy was observed in 16 S rRNA gene and mcrA gene sequencing results, where the bacterial or Methanosaeta concilii dominated community of DS (digest sludge) was distinct from the inoculum sources TS (total sludge), CTS (concentrated total sludge), and HTS (hydrolysis total sludge), indicating the independent development of DS. A quantitative cross-network was constructed by coupling the absolute quantify of 16 S rRNA and mcrA sequences. The Methanobacterium petrolearium actively interacted with bacteria in the DS community rather than the dominant species (Methanosaeta concilii). Moreover, the unclassified methanogens were identified to be significantly prevalent in all communities, suggesting that unknown methanogenic taxa might be imperative in accomplishing community functions. Collectively, the findings of this research study will shed light on the comprehensive knowledge of microbial communities, especially the methanogenic microbiota. This will further enhance the exploration of the phylogenetic diversity of methanogens and their corresponding impacts in energy recovery from wastewater treatment plants.

Quorum quenching of autoinducer 2 increases methane production in anaerobic digestion of waste activated sludge

The ubiquitous signaling molecule autoinducer 2 (AI-2) is involved in intra- and interspecies communication, most notably between Gram-negative and Gram-positive bacteria. AI-2 accumulates during the exponential phase of the Escherichia coli (E. coli) monoculture and then rapidly decreases upon entry into the stationary phase. However, deleting both the genes encoding AI-2 synthase (LuxS) and the lsr operon regulator (LsrR) in the E. coli genome causes impaired AI-2 production and continuous AI-2 scavenging from the environment. This genetically-engineered E. coli mutant capable of quenching AI-2 quorum sensing (QS) system was utilized to evaluate the effect of AI-2 quenching on the anaerobic digestion of waste activated sludge (WAS) because the role of QS system via AI-2 in the process remains obscure. In this study, E. coli ∆luxS lsrR mutant cells were microencapsulated in sodium alginate beads and incubated with WAS anaerobically. After 15 days of anaerobic fermentation, the WAS containing double mutant cells produced significantly more methane than that of the parent E. coli cells. AI-2 quenching occurred concurrently with a shift of microbial communities that contribute to increasing acetate consumption by the Methanosarcina spp. resulting in an increase in methane production. KEY POINTS: • Impact of autoinducer 2 quenching in complex bacterial populations were determined. • Key microorganisms contributing to the increase of methane in WAS anaerobic digestion were found. • The AI-2 quenching is a potential regulatory in wastewater treatment and bioenergy research.

Effect of formic acid inflow on microbial properties of the anaerobic granular sludge in a UASB reactor

In the production of natural rubber, formate or acetate is added to the latex solution to coagulate the rubber; therefore, the wastewater contains high concentrations of organic acids, requiring the application of anaerobic treatment technology. In this study, a two-phase continuous flow experiment using a laboratory-scale upflow anaerobic sludge blanket (UASB) was conducted to investigate the influence of formate inflow on the microbial and physical characteristics of the retained granular sludge. In phase 1, acetate-based wastewater was used as feed, while in phase 2, formate-based wastewater was used as feed. In phase 1, the UASB exhibited high COD removal efficiency (97.2%); in addition, the retained sludge showed increased methane production from acetate and proliferation of acetate-utilizing Methanosaeta species. In phase 2, the UASB performed as well as phase 1, with 98.2% COD removal efficiency. Microbial community structure analysis confirmed that relatives of Methanobacterium formicicum present in the retained sludge were responsible for the degradation of formate in phase 2. However, decreased diameter and slight deterioration of granular sludge settleability were observed. In conclusion, formate inflow has low risk of interference with the process performance of the UASB, but it has negative effects on the physical properties of the granular sludge.

A pilot-scale study of granule-based anaerobic reactors for biogas recovery from municipal wastewater under sub-mesophilic conditions

The influence of hydraulic retention time (HRT of 3-5 h) and temperature (20-25 °C) on performance and microbial dynamics of two pilot-scale upflow anaerobic sludge blanket (UASB) reactors with different granule size distribution (UASB1 = 3-4 mm and UASB2 = 1-2 mm) were investigated for 217 days. Increasing the HRT to 5 h even at a lower temperature of 20 °C enhanced COD removal and biogas production with average of 59 ± 16% (up to 85%) and 73 ± 9 L/(m³·d) (up to 102 L/(m³·d)) for UASB1; 63 ± 16% (up to 85%) and 75 ± 9 L/(m³·d) (up to 90 L/(m³·d)) for UASB2, respectively. This is explained by sufficient contact time between microorganisms and substrate. Acetoclastic methanogenic activity was higher in UASB1 because Methanosaetaceae (produces methane from acetate) dominated (64 ± 4%). However, Methanoregulaceae (29 ± 3%) and Methanomicrobiales_unassigned (20 ± 6%) which produce methane from H₂/CO₂ and formate were significant in UASB2. The extent of change in the microbial dynamics with HRT and temperature was more obvious in the smaller granule reactor.

Optimization of bioprocesses with Brewers’ spent grain and Cellulomonas uda

Brewers’ spent grain (BSG) is a low‐value by‐product of the brewing process, which is produced in large quantities every year. In this study, the lignocellulosic feedstock (solid BSG) was used to optimize fermentations with Cellulomonas uda. Under aerobic conditions, maximum cellulase activities of 0.98 nkat∙mL⁻¹, maximum xylanase activities of 5.00 nkat∙mL⁻¹ and cell yields of 0.22 g_Cells∙g_BSG⁻¹ were achieved. Under anaerobic conditions, enzyme activities and cell yields were lower, but valuable liquid products (organic acids, ethanol) were produced with a yield of 0.41 g_Prod∙g_BSG⁻¹. The growth phase of the organisms was monitored by measuring extracellular concentrations of two fluorophores pyridoxin (aerobic) and tryptophan (anaerobic) and by cell count. By combining reductive with anaerobic conditions, the ratio of ethanol to acetate was increased from 1.08 to 1.59 mol_EtOH∙mol_Ac⁻¹. This ratio was further improved to 9.2 mol_EtOH∙mol_Ac⁻¹ by lowering the pH from 7.4 to 5.0 without decreasing the final ethanol concentration. A fermentation in a bioreactor with 15 w% BSG instead of 5 w% BSG quadrupled the acetate concentration, whilst ethanol was removed by gas stripping. This study provides various ideas for optimizing and monitoring fermentations with solid substrates, which can support feasibility and incorporation into holistic biorefining approaches in the future.

Methane production from acetate, formate and H2/CO2 under high ammonia level: Modified ADM1 simulation and microbial characterization

This study evaluated the methanogenic performance of typical substrates (acetate, formate, H₂/CO₂, and glucose) under low and high ammonia levels and the Anaerobic Digestion Model No.1 (ADM1) was extended and modified for better simulation and understanding of the process. Formate-utilizing and hydrogen-utilizing methanogenesis showed stronger ammonia resistance than acetate-utilizing methanogenesis (13-23% vs. 34% decrease in methane production (MP)). Model extension, based on foundational experiments fed with three typical precursors (R² > 0.92), was then validated with glucose degradation experiments, and satisfactory predictions of MP and total volatile fatty acids were obtained (R² > 0.91). Based on the modified ADM1, the carbon fluxes of glucose degradation were determined, and formate-utilizing methanogenesis showed its importance with a 28-34% contribution of the total methanation, becoming the dominant pathway under high ammonia level. Formate-utilizing methanogenesis also had a thermodynamic advantage among the three pathways. 16S rRNA sequencing suggested a homology between the hydrogen-utilizing and formate-utilizing methanogens. Methanobacterium and Methanobrevibacter were found to be key methanogens, and their enrichment under high ammonia level confirmed the stronger ammonia tolerance of formate-utilizing and hydrogen-utilizing methanogenesis. The microbial characterization and modified ADM1 simulations supported each other.

Deeper investigation on methane generation from synthetic wastewater containing oxytetracycline in a scale up acidic anaerobic baffled reactor

Acidic anaerobic digestion attracted much attention and interest due to its significant advantage in wastewater treatment. In the present study, methanogenic fermentation was successfully operated under acidic condition during treating wastewater containing oxytetracycline (OTC) in a scale up anaerobic baffled reactor (ABR). After start-up process, the pH value in the first compartment was 4.60 with high activity of methanogenesis. After stabilization, different OTC loading of 1.0, 3.3 and 5.0 g/m3/d was added in the influent for OTC removal. The resulted showed that OTC addition had little impact on the methane generation with whole COD and OTC removal rate of 95% and 60%, respectively. The microbial analysis, OTC addition could significantly influence the bacteria and archaea communities. To be more specific, Methanosaeta showed the highest relative abundance and tolerance to OTC under acidic condition. The present work supplied deeper insights into methane generation from acidic condition during wastewater containing OTC treatment.

Enhanced in-situ biomethanation of food waste by sequential inoculum acclimation: Energy efficiency and carbon savings analysis

The increasing rate of food waste (FW) generation globally, makes it an attractive resource for renewable energy through anaerobic digestion (AD). The biogas recovered from AD can be upgraded by the methanation of internally produced carbon dioxide, CO₂ with externally sourced hydrogen gas, H₂ (biomethanation). In this work, H₂ was added to AD reactors processing FW in three successive phases, with digestate from preceding phases recycled in succession with the addition of fresh inoculum to enhance acclimation. The concentration of H₂ was increased for succeeding phases: 5%, 10% and 15% of the reactor headspace in Phase 1 (EH1), Phase 2 (EH2) and Phase 3 (EH3), respectively. The H₂ utilisation rate and biomethane yields increased as acclimation progressed from EH1 through EH3. Biomethane yield from the controls: EH1_Control, EH2_Control and EH3_Control were 417.6, 435.4 and 453.3 NmL-CH₄/gVS_added accounting for 64.8, 73.9 and 77.8% of the biogas respectively. And the biomethane yield from the test reactors EH1_Test, EH2_Test and EH3_Test were 468.3, 483.6, and 499.0 NmL-CH₄/gVS_added, accounting for 77.2, 78.1 and 81.0% of the biogas respectively. A progressive in-situ biomethanation could lead to biomethane production that meets higher fuel standards for gas-to-grid (GtG) injections and vehicle fuel - i.e. >95% CH₄. This would increase the energy yield and carbon savings compared to conventional biogas upgrade methods. For example, biogas upgrade for GtG by in-situ biomethanation could yield 7.3 MWh/t_FW energy and 1343 kg-CO₂e carbon savings, which is better than physicochemical upgrade options (i.e., 4.6-4.8 MWh/t_FW energy yield and 846-883 kg-CO₂e carbon savings).

The application status, development and future trend of nano-iron materials in anaerobic digestion system

Growing environment problem and emphasis of environmental protection motivate intense research efforts in exploring technology to improve treatment efficiency on refractory organic pollutants. Hence, finding a method to make up for the deficiency of anaerobic digestion (AD) is very attractive and challenging tasks. The recent spark in the interest for the usage of some nanomaterials as an additive to strengthen AD system. The adoption of iron compounds can influence the performance and stability in AD system. However, different iron species and compounds can influence AD system in significantly different ways, both positive and negative. Therefore, strengthening mechanism, treatment efficiency, microbial community changes in Nanoscale Zero Valent Iron (nZVI) and Fe₃O₄ nanoparticles (Fe₃O₄ NPs) added AD systems were summarized by this review. The strengthening effects of nZVI and Fe₃O₄ NPs in different pollutants treatment system were analyzed. Previous study on the effects of nZVI and Fe₃O₄ NPs addition on AD have reported the concentration of nZVI and Fe₃O₄ NPs, and the types and biodegradability of pollutants might be the key factors that determine the direction and extent of effect in AD system. This review provides a summary on the nZVI and Fe₃O₄ NPs added AD system to establish experiment systems and conduct follow-up experiments in future study.

Novel strategy for enhancing acetic and formic acids generation in acidogenesis of anaerobic digestion via targeted adjusting environmental niches

Optimization of acetic acid and formic acid production efficient methanogenesis is always the research hot spot in anaerobic digestion. It is a promising approach to adjust the operation parameters to influence the functional microorganisms for better acetic acid and formic acid production in acidogenesis. Herein, the effects of pH, oxidation-reduction potential (ORP) and carbon-nitrogen (C/N) ratio were determined in batch experiments to probe acetic and formic acids production, and were further verified in continuous stirred tank reactor (CSTR). The results revealed that the content of volatile fatty acids (VFAs) reached to maximum at pH 6.0 or ORP -350 mV, while the production of acetic and formic acids was the highest at pH 7.0 or ORP -450 mV in 9 h fermentation. Also, fermentation products dominated by acetic and formic acids were adjusted in the CSTR under the operating conditions of pH 7.0 and ORP -450 mV. Microbiological analysis from batch test showed that fermentation at pH value of 7.0 enriched the diversity of microorganism, and provided a niche for microbes (Petrimonas, norank_f__Synergistaceae, vadinBC27_wastewater-sludge_group, and Trichococcus) to produce acetic and formic acids. Correspondingly, 78.70% of the carbon was converted to acetic and formic acids in pH 7.0. This study provides a promising strategy for the targeted regulation of acetic and formic acids production in acidogenesis of anaerobic digestion.

The Anaerobic Fungi: Challenges and Opportunities for Industrial Lignocellulosic Biofuel Production

Lignocellulose is a promising feedstock for biofuel production as a renewable, carbohydrate-rich and globally abundant source of biomass. However, challenges faced include environmental and/or financial costs associated with typical lignocellulose pretreatments needed to overcome the natural recalcitrance of the material before conversion to biofuel. Anaerobic fungi are a group of underexplored microorganisms belonging to the early diverging phylum Neocallimastigomycota and are native to the intricately evolved digestive system of mammalian herbivores. Anaerobic fungi have promising potential for application in biofuel production processes due to the combination of their highly effective ability to hydrolyse lignocellulose and capability to convert this substrate to H2 and ethanol. Furthermore, they can produce volatile fatty acid precursors for subsequent biological conversion to H2 or CH4 by other microorganisms. The complex biological characteristics of their natural habitat are described, and these features are contextualised towards the development of suitable industrial systems for in vitro growth. Moreover, progress towards achieving that goal is reviewed in terms of process and genetic engineering. In addition, emerging opportunities are presented for the use of anaerobic fungi for lignocellulose pretreatment; dark fermentation; bioethanol production; and the potential for integration with methanogenesis, microbial electrolysis cells and photofermentation.

Deeper insights into effect of activated carbon and nano-zero-valent iron addition on acidogenesis and whole anaerobic digestion

Conductive materials presented promising advantages for enhancing anaerobic digestion (AD) performance. This study evaluated the effects of activated carbon (AC) and nano-zero-valent iron (nZVI) on the acidogenesis and whole AD to explore their potential mechanisms. AC increased the content of lactic and propionic acids in acidogenesis. nZVI increased the production of formic acid, acetic acid and H₂ in acidogenesis, thus significantly promoted the methane yield in the whole AD. Mechanism exploration proved that AC enriched Trichococcus, and norank_f__Bacteroidetes_vadinHA17, and then improved the activity of enzymes involved in the production of lactic and propionic acids. nZVI buffered the pH to increase the activity of pyruvate formate-lyase (PFL) in formic acid production. Furthermore, nZVI enriched the Methanobacterium which use H₂ and formic acid as substrate. The research paves pathway for the efficient enhancement of conductive materials added novel AD process.

Integrating microbial electrolysis cell based on electrochemical carbon dioxide reduction into anaerobic osmosis membrane reactor for biogas upgrading

It has been reported that anaerobic osmosis membrane bioreactors have the potential for energy recovery since dissolved methane was almost rejected by commercial forward osmosis membranes. Notwithstanding, upgraded biogas has to be achieved by removing as much carbon dioxide as possible. In this study, a novel anaerobic osmotic membrane bioreactor-microbial electrolysis cell (AnOMBR-MEC) system was developed for simultaneous biogas upgrading and wastewater treatment. The AnOMBR-MEC elicited an excellent and stable soluble chemical oxygen demand and phosphorus removal. As the experiment progressed, unwanted carbon dioxide produced from biogas was reduced to formate using a SnO₂ nanoparticles electrocatalytic cathode in an electrocatalytic-assisted MEC, with the highest faradic efficiency of formate being 85% at 1.2V. Compared to AnOMBR, the methane content increased from 55% to 90% at the end of operation and methane yield experienced a1.6-fold increment in the AnOMBR-MEC. Microbial community analysis revealed that hydrogenotrophic methanogens (e.g. Methanobacterium and Methanobrevibacter) converted the produced H₂ and formate to methane at saline conditions. These results have demonstrated an efficient strategy based on the integration of an electrocatalytic-assisted MEC into AnOMBR for upgrading biogas, enhancing methane yield and wastewater treatment.

Zero-valent iron enhanced in-situ advanced anaerobic digestion for the removal of antibiotics and antibiotic resistance genes in sewage sludge

The in-situ advanced anaerobic digestion (AAD) enhanced with zero-valent iron powder (ZVI) under mesophilic condition was investigated to remove 5 antibiotics (sulfamerazine (SMR), sulfamethoxazole (SMZ), ofloxacin (OFL), tetracycline (TC), and roxithromycin (ROX)) and 11 antibiotic resistance genes (ARGs) (AAC (6')-IB-CR, qnrS, ermF, ermT, ermX, sul1, sul2, sul3, tetA, tetB, and tetG) in sewage sludge. The effects of different ZVI dosages, antibiotic concentrations, and solid retention time (SRTs) on the removal were explored. Also, the correlation coefficient of antibiotics and ARGs, microbial community structure, biogas production and methane yield were analyzed. All conducted treatments operated stably, and the modified Gompertz model described the cumulative methane yield well. The antibiotics, with the exception of OFL, were effectively removed in the sewage sludge at a dosage of 1000 mg/L ZVI, SRT 20 d, and an antibiotic concentration of 20 μg/L during AAD. The removal rates of SMZ, SMR, TC, and ROX reached 97.39%, 74.54%, 78.61%, and 56.58%, respectively. AAC (6')-IB-CR and tetB could be effectively reduced during the in-situ AAD. Through the redundancy analysis, AAC (6')-IB-CR, ermT, ermX, sul2, tetB, and tetG had strong positive correlations with the antibiotics in the reactor. The principle component analysis revealed that the community structure was similar when the SRT was 10 d and 20 d at the same amount of ZVI and antibiotic concentrations in the sludge. Under the operating parameters of 1000 mg/L ZVI dosage, SRT 20 d, and an antibiotic concentration of 20 μg/L, Erysipelotrichia, Verrucomicrobia, Clostridia, Caldiserica, and Alphaproteobacteria of the class were dominated microorganisms in the anaerobic digestion.

Synergetic effect of nano zero-valent iron and activated carbon on high-level ciprofloxacin removal in hydrolysis-acidogenesis of anaerobic digestion

Ciprofloxacin is the most commonly prescribed antibiotic, and its widespread use poses threat to environmental safety. The removal of ciprofloxacin from contaminated water has remained a major challenge. The present study investigated adding nanoscale zero-valent iron (NZVI) and activated carbon (AC) on high-level ciprofloxacin removal in hydrolysis-acidogenesis stage of anaerobic digestion. The results showed that the degradation rate of ciprofloxacin increased from 22.61% (Blank group) to 72.41% after adding NZVI/AC with concentration of ciprofloxacin in effluent decreasing from 8.25 mg L^-1 to 3.48 mg L^-1. The volatile fatty acids (VFAs) yield increased by 173.7% compared with the Blank group. In addition, the NZVI/AC group achieved the highest chemical oxygen demand (COD) removal rate and acidogenesis rate. The microbial community analysis presented that hydrolytic and acidogenic bacteria and microorganisms related to degrading ciprofloxacin were obviously improved in the NZVI/AC group. Moreover, eleven transformation products and the main degradation pathways were proposed based on mass spectrometry information. In summary, the NZVI/AC addition supplied promising approach for ciprofloxacin wastewater treatment.

Towards the practical application of bioelectrochemical anaerobic digestion (BEAD): Insights into electrode materials, reactor configurations, and process designs

Anaerobic digestion (AD) is one of the most widely adopted bioenergy recovery technologies globally. Despite the wide adoption, AD has been challenged by the unstable performances caused by imbalanced substrate and/or electron availability among different reaction steps. Bioelectrochemical anaerobic digestion (BEAD) is a promising concept that has demonstrated potential for balancing the electron transfer rates and enhancing the methane yield in AD during shocks. While great progress has been made, a wide range of, and sometimes inconsistent engineering and technical strategies were attempted to improve BEAD. To consolidate past efforts and guide future development, a comprehensive review of the fundamental bioprocesses in BEAD is provided herein, followed by a critical evaluation of the engineering and technical optimizations attempted thus far. Further, a few novel directions and strategies that can enhance the performance and practicality of BEAD are proposed for future research to consider. This review and outlook aim to provide a fundamental understanding of BEAD and inspire new research ideas in AD and BEAD in a mechanism-informed fashion.

Novel strategy for relieving acid accumulation by enriching syntrophic associations of syntrophic fatty acid-oxidation bacteria and H2/formate-scavenging methanogens in anaerobic digestion

Aiming at relieving acid accumulation in anaerobic digestion (AD), syntrophic associations of syntrophic fatty acid-oxidation bacteria and H₂/formate-scavenging methanogens were enriched by feeding propionate, butyrate and formate in an up-flow anaerobic sludge blanket (UASB) reactor. Results showed that methane yield increased by 50% with increasing formate concentration (0-2000 mg COD/L). In addition, the abundance and quantity of SFOB (Syntrophobacter, Smithella and Syntrophomonas) and H₂/formate-scavenging methanogens (Methanobacteriales and Methanomicrobiales) were increased after microbial acclimation. The enriched syntrophic associations showed higher propionate and butyrate removal efficiencies of 98.48 ± 1.14% and 99.71 ± 0.71%, respectively. Furthermore, encoding genes of formate dehydrogenase and hydrogenases presented higher abundances after microbial enrichment, which suggested that the enhancements of interspecies formate transfer and interspecies hydrogen transfer between syntrophic associations benefited volatile fatty acids (VFAs) conversion. This research provided an effective strategy to relieve acid accumulation.

Reversibility of propionic acid inhibition to anaerobic digestion: Inhibition kinetics and microbial mechanism

Anaerobic digestion is a technology that simultaneously treats waste and generates energy in the form of biogas. Unfortunately, when a high organic loading rate is applied, anaerobic digestion can suffer from volatile fatty acid accumulation that results in pH drop and decreased biogas production. In particular, propionic acid has shown to inhibit biogas production even at a very low concentration. Therefore, the kinetics of biogas production in relation to propionic acid concentration needs to be investigated. In batch experiments on anaerobic co-digestion of food waste and dairy manure in the present study, cumulative biogas production showed little inhibition by propionic acid in the concentration range of 6.5-14.6 mM, but a lag phase of 9.4, 16.3 and 60.8 d was detected in the digesters with initial propionic acid concentrations of 22.7, 36.2, and 56.4 mM, respectively. After the lag phase, these digesters accelerated to specific biogas yields of 0.59-0.70 L g-VS^-1. The similar specific biogas yields across all of the digesters at initial propionic acid concentrations of 6.5-56.4 mM indicated reversibility of the inhibition. The reversibility was made possible by microbial acclimation and the shift to hydrogenotrophic methanogenesis in syntrophy with acetogenic bacteria. Evidently, an increase of hydrogenotrophic Methanobacterium and Methanoculleus abundances was found at 36.2 and 56.4 mM. Batch digestion experiments must be extended beyond the lag phase in order to fully reveal the inhibition kinetics. This paper highlights the need for a standard protocol that experimentally evaluates inhibition in anaerobic digestion.

Effect of biogas recirculation strategy on biogas upgrading and process stability of anaerobic digestion of sewage sludge under slightly alkaline condition

In this study, in-situ biogas upgrading was examined and compared in lab-scale semi-continuous anaerobic digestion (AD) systems coupled with biogas recirculation under neutral and slightly alkaline conditions. Results showed that neutral pH (~7.5) condition yielded 6-7% higher CH₄ content and 52-53% less H₂S production, while slightly alkaline condition (pH ~8.0) achieved 14-15% higher CH₄ content and 76-77% less H₂S production under biogas recirculation in comparison to the control (no biogas recirculation). Little effect was noticed on daily CH₄ production rate at slightly alkaline condition under biogas recirculation. Endogenous free ammonia nitrogen-dissolved inorganic carbon-volatile fatty acids (FAN-DIC-VFAs) buffer system was found to regulate the pH and alkalinity in the bulk liquor. Therefore, biogas recirculation under slightly alkaline condition is proposed as an efficient and sustainable strategy for biogas upgrading during AD of sewage sludge.

Bioelectrosynthetic Conversion of CO2 Using Different Redox Mediators: Electron and Carbon Balances in a Bioelectrochemical System

Microbial electrosynthesis (MES) systems can convert CO2 to acetate and other value-added chemicals using electricity as the reducing power. Several electrochemically active redox mediators can enhance interfacial electron transport between bacteria and the electrode in MES systems. In this study, different redox mediators, such as neutral red (NR), 2-hydroxy-1,4-naphthoquinone (HNQ), and hydroquinone (HQ), were compared to facilitate an MES-based CO2 reduction reaction on the cathode. The mediators, NR and HNQ, improved acetate production from CO2 (165 mM and 161 mM, respectively) compared to the control (without a mediator = 149 mM), whereas HQ showed lower acetate production (115 mM). On the other hand, when mediators were used, the electron and carbon recovery efficiency decreased because of the presence of bioelectrochemical reduction pathways other than acetate production. Cyclic voltammetry of an MES with such mediators revealed CO2 reduction to acetate on the cathode surface. These results suggest that the addition of mediators to MES can improve CO2 conversion to acetate with further optimization in an operating strategy of electrosynthesis processes.

A comparative long-term operation using up-flow anaerobic sludge blanket (UASB) and anaerobic membrane bioreactor (AnMBR) for the upgrading of anaerobic treatment of N, N-dimethylformamide-containing wastewater

Synthetic industrial wastewater containing approximately 2000 mg/L N, N-dimethylformamide (DMF) was treated using a lab-scale anaerobic sludge blanket (UASB) and an anaerobic membrane bioreactor (AnMBR) in this study. The inoculum consisted of two sources of sludge: Co-culture of anaerobic digested sludge (ADS) with DMF-hydrolyzing activated sludge (DAS) for the AnMBR, and co-culture of anaerobic granular sludge (AGS) with DAS for the UASB. Effective DMF methanogenic degradation of nearly 100% removal was achieved in both reactors on the first day. Both reactors obtained excellent DMF removal efficiency and high methane production under a low organic loading rate (OLR) of around 3-4 g COD/L/d. However, excessive elevation of OLR significantly limited DMF hydrolysis. When OLR exceeded 6 g COD/L/d, the removal efficiency and methane production in both reactors dramatically dropped. Despite their different forms and shapes, the ADS and AGS both provide methanogens which are responsible for methanogenesis. The UASB tolerated a higher OLR while the AnMBR was limited by membrane fouling due to the increased sludge concentration. However, the AnMBR obtained high-quality effluent without suspended solid. Whether DMF can be effectively degraded depends on DAS, in which abundant DMF-hydrolyzing bacteria (DHB) provide sufficient quantities of the hydrolytic enzyme for effective hydrolysis of DMF. However, these DHB were facultative and were also identified as denitrifying bacteria which require nitrate as the electron acceptor or otherwise survive under the aerobic condition. They gradually decayed rather than proliferated and were outcompeted by methanogens. Therefore, it is conceivable that a slight dosage of nitrate would enrich the abundance of DHB in both the UASB and the AnMBR, and provide a sufficient quantity of enzymes for the DMF hydrolysis. The cultivation of the anaerobic DMF-degrading granular sludge using the UASB is considered an upgraded solution to the effective treatment of DMF-containing wastewater.

Impact of physical structure of granular sludge on methanogenesis and methanogenic community structure

Physical structures of sludge are critical factors determining the performance of the anaerobic digestion process, especially for the rate-limiting step, methanogenesis. Thus, to evaluate the effect of granular physical structure on methanogenesis and methanogenic community variation, intact and disintegrated granules were applied as inocula with formate, hydrogen and acetate as sole substrates in batch reactors. Kinetics results revealed that the physical structure of sludge had little impact on methane yield potential from three substrates, while a significantly different impact on methanogenesis rates of formate, hydrogen and acetate. The methanogenesis rate of formate in disintegrated granules was higher than that in the intact granular system, the methanogenesis rate of H2/CO2 in the intact granular system was higher than that in the disintegrated granules and the methanogenesis rate of acetate was similar with the in intact and disintegrated granular systems. Besides, in both intact and disintegrated granular systems, methanogenesis rates of formate were the highest, then followed the H2/CO2 and acetate was the lowest, indicating formate consumption has an advantage over hydrogen in the studied system. A microbial assay indicated that Methanobacteriales, Methanosarcinales and Methanomicrobiales are dominant methanogens on the order level, and the physical structure of granular sludge has little influence on methanogenic communities on the order level but showed significant influence on the species level. It enlightens us that the physical structure of sludge could be considered for regulating the anaerobic digestion via influencing the methanogenesis rates.

Bacterial and archeal dynamics of a labscale HYBRID gas fermentation bioreactor fed with CO2 and H2

16s rDNA-based methods were used in order to identify the dynamics of microbial profiles in a HYBRID gas fermentation bio-methanization reactor. The effects of various H₂ and CO₂ ratios on microbial community were investigated. The HYBRID gas fermentation reactor was composed of granular anaerobic seed and the system fed with only H₂ and CO₂ gases. No additional organic material and trace element was fed during the throughout the experiments; thus, the microbial diversity was directly related to production of methane. The dynamics of the microbial communities were investigated with DGGE and real-time PCR analysis. The results showed that Methanobacteriales members were more dominated than Methanosarcinales and Methanomicrobiales members in the system. DGGE results indicated that Methanosaeta concilii, Methanoculleus sp., Methanosphaerula palustris, Methanofollis formosanus, Methanolinea sp., and Methanobacterium palustre were the most prominent methanogens depending on different H₂/CO₂ ratios. DGGE profiles suggested that hydrogenotrophic and acetoclastic species were responsible for the production of methane. The survival of syntrophic bacteria and acetoclastic methanogens was attributed to their utilization of organic materials provided by lysis. To the best of our knowledge, this is the first microbial profile detection study in a hybrid bioreactor system operated with only pure hydrogen and carbon dioxide.

Three-stage anaerobic co-digestion of food waste and waste activated sludge: Identifying bacterial and methanogenic archaeal communities and their correlations with performance parameters

A three-stage anaerobic digester setup was configured and evaluated for enhanced methane production during co-digestion of food waste and waste activated sludge and the corresponding bacterial and methanogen communities were characterized. Results showed that the average methane yield (0.496 L/gVS) in the three-stage digester was 13-52% higher than that of one- and two-stage digesters. Compared to controls, an increase of 12-47% in volatile solids reduction was achieved in the three-stage digester (69.3 ± 6.7%). Bacterial phyla Proteobacteria, Firmicutes and Bacteroidetes dominated in one-, two- and three-stage digester while genera Pseudomonas, Tissierella, and Petrimonas were selectively enriched in the three-stage digester due to functional segregation. Taxonomic analysis identified 8 dominant methanogen genera, of which Methanosarcina, Methanosaeta, Methanobacterium and Methanolinea collectively accounted for 80%. With increasing OLR and digester stage number, the dominant methanogenic pathway shifted from hydrogenotrophic pattern to acetoclastic pattern and reached a final synergy of these two.

Dissection and characterization of the prokaryotic community during the long-term operation of a submerged anaerobic membrane bioreactor for the anaerobic treatment of N, N-dimethylformamide-containing wastewater with a co-cultured inoculum

A submerged anaerobic membrane bioreactor (SAnMBR) was operated for the anaerobic treatment of wastewater containing approximately 2000 mg L^-1N, N-dimethylformamide (DMF). Inoculated with a co-cultured inoculum, the SAnMBR obtained an excellent DMF removal under a low organic loading rate (OLR) of 3.14-4.16 g COD L^-1 d^-1. However, the elevation of OLR limited hydrolysis. While the co-cultured inoculum initially contains abundant DMF-hydrolyzing bacteria with potential to hydrolyze DMF into intermediates, such as Paracoccus, Hyphomicrobium, Burkholderia, Catellibacterium, Bacillus and Bradyrhizobium, since these bacteria are facultative anaerobes which survive anaerobically, they kept decaying rather than proliferating, resulting in the weakening of the DMF-hydrolyzing ability. Each re-inoculation of new sludge only temporarily revitalized hydrolysis activity for a short period. Due to the lack of nitrate, these bacteria were unable to proliferate. This suggests that a small dose of nitrate would help to enrich these bacteria and establish a stable DMF-degrading consortium.

Anaerobic treatment of N,N-dimethylformamide-containing high-strength wastewater by submerged anaerobic membrane bioreactor with a co-cultured inoculum

The anaerobic treatment of wastewater containing approximately 2000 mg L^-1N,N-dimethylformamide (DMF) was conducted by a lab-scale submerged anaerobic membrane bioreactor (SAnMBR). The inoculum consisted of aerobic DMF-hydrolyzing activated sludge (DAS) and anaerobic digested sludge (ADS). A rapid start-up was achieved with thorough DMF methanogenic degradation on the first day. The results of a 250-day long-term experiment demonstrated that under a low organic loading rate (OLR) of 3.14-4.16 g COD L^-1 d^-1, SAnMBR maintained excellent DMF removal efficiency along with high methane conversion. However, the elevation of OLR significantly limited DMF hydrolysis. When OLR exceeded 6.54 g COD L^-1 d^-1, both removal efficiency and methane production dramatically dropped. The DMF-hydrolyzing bacteria originating from the DAS gradually decayed under the anaerobic condition, resulting in the weak hydrolysis of DMF. The shortening of hydraulic retention time (HRT) is not recommended for the SAnMBR because severe membrane fouling occurred when HRT was shortened to 8 h. To handle high OLRs, an appropriate solution is to maintain a low F/M ratio by increasing both the influent DMF concentration and sludge concentration. The high CH₄ content in the biogas, exceeding 85%, was shown to be the reason for the suitability of anaerobic treatment to DMF. Some improvements which would help to maintain the effective hydrolysis are proposed: a side-stream system to replenish DAS to the SAnMBR is helpful; slight dosage of nitrate could also help to enrich the DMF-hydrolyzing bacteria; and the co-digestion of DMF and other organics might be convenient to establish a stable DMF-degrading consortium.

Reversibility of hydrolysis inhibition at high hydrogen partial pressure in dry anaerobic digestion processes fed with wheat straw and inoculated with anaerobic granular sludge

In dry anaerobic digestion (AD), methanogenic performances are lowered by high solid contents. Low performances are often caused by a decrease of the gas-liquid transfer kinetics leading to local accumulation of inhibitory by-products. Hydrogen was previously identified as an inhibitor of hydrolytic and acetogenic microbial activities in dry AD. CO₂ is also generated but its impact on the microbial activity remains unknown. In this study, the reversibility of dry AD inhibition at high H₂ partial pressure (P_H2 of 1 bar) was investigated by adding CO₂ (400 mbars) after 11 and 18 days of methanogenesis inhibition, in an AD process operated at 25% TS, using wheat straw as substrate and inoculated with anaerobic granular sludge. As soon as CO₂ was added, the methanogenic activity rapidly recovered within 3 days, from 0.41 ± 0.1 to 3.77 ± 0.8 and then 2.25 ± 0.3, likely through the hydrogenotrophic pathway followed by the acetoclastic pathway, respectively. This result was confirmed by the high abundance of Methanomicrobiales (83%) and the emergence of Methanosarcinales sp (up to 17%) within the methanogenic community. Furthermore, the recovery kinetics were impacted by the duration of the inhibition period suggesting a different impact of the high P_H2 on hydrogenotrophic and acetoclastic methanogens.

A comparative study of biogasification of wheat straw, sugarcane bagasse and pressmud

A study to compare biogas production potentials of wheat straw, sugarcane bagasse and pressmud was conducted at pH 8.0, temperature 40 °C and substrate concentration 20 g/L. Raw substrates were thermogravimetrically and Fourier-transform infrared spectroscopically characterised. TGA showed the weight loss of samples attributable to moisture, hemicellulose, cellulose and lignin losses. FTIR analysis indicated functional groups characteristics of hemicellulose, cellulose and lignin. Biogas production was the maximum between 10th and 25th day for all the tests. WS with 10% inoculum showed the highest cumulative biogas production of 370 mL/g followed by the SB (316 mL/g) and PM (211 mL/g) counterparts. The corresponding values with 5% inoculum were 303 mL/g (WS), 244 mL/g (SB) and 152 mL/g (PM). The inoculum volume also positively affected the cumulative biogas production (22.1, 29.5 and 38.8% respectively). The higher volatile fatty acids as observed in case of WS which further facilitated higher biogas production could be due to its maximum volatile solids content (88.9%) and water swelling capacity (7.37). A consistently increasing trend in the methane content (varying between 54 and 61%) in all the tests was observed till the 20th day. The biogas (7.7-21.7 mL/g) and the methane (35-42%) contents showed a decreasing trend thereafter, the lowest being observed during the 35-40-day period.

Insights into the methanogenic degradation of N, N-dimethylformamide: The functional microorganisms and their ecological relationships

The methanogenic degradation of N, N-dimethylformamide (DMF) was investigated using anaerobic digested sludge (ADS), aerobic activated sludge (AAS) and co-cultured sludge (CCS), respectively. Both the metabolic pathway and the corresponding microorganisms which function in the methanogenic degradation of DMF were elucidated. DMF was unable to be degraded anaerobically by ADS due to the lack of DMF-hydrolyzing bacteria. DMF can be effectively degraded by AAS, however, no methane was recovered under the aerobic condition. The co-culture of DMF-hydrolyzing bacteria and methanogens in the CCS allowed for both hydrolysis of DMF and methane production to proceed successfully under the anaerobic condition, realizing the complete conversion from DMF to methane. However, a niche overlap due to the competition for the intermediates lowered the abundance of DMF-hydrolyzing bacteria. The introduction of nitrate, timely replenishment of AAS, micro-aeration and co-digestion were likely to maintain a high abundance of DMF-hydrolyzing bacteria to ensure an effective hydrolysis.

Formation and characteristics of a ternary pH buffer system for in-situ biogas upgrading in two-phase anaerobic membrane bioreactor treating starch wastewater

Biochemical biogas upgrading retaining more CO₂ from biogas to form biomethane opens new avenues for sustainable biofuel production. For developing this technology, maintaining sustain pH for CO₂·H₂O is fundamental. This study proposes an innovative control strategy for in-situ biogas upgrading retaining and converting the CO₂ in the biogas into CH₄, via hydrogenotrophic methanogenesis without external agent. The Biogas-pH strategy limited pH drop over 7.4 by stop feeding and maintained the methanogenesis activity by biogas flow rate over 98 ml·h^-1. Low pH (7.37-7.80) decrease CO₂·H₂O as a substrate in stage-I, higher pH (7.40-8.41) enhances CO₂ to CO₂·H₂O transfer by 6.29 ± 2.20% in stage-II. Because of that 95% CO₂·H₂O converts to HCO₃^- and CO₃^2- when pH > 7.9, higher pH > 7.9 did not further upgrading the biogas. The carbonate buffer system shown open and close system characteristics in gas and liquid phase. The biogas CH₄ was upgraded from 67.27 ± 5.21% to 73.56 ± 5.01%.

A gradual change between methanogenesis and sulfidogenesis during a long-term UASB treatment of sulfate-rich chemical wastewater

The competition between methane-producing archaea and sulfate-reducing bacteria is an important topic in anaerobic wastewater treatment. In this study, an Up-flow Anaerobic Sludge Blanket Reactor (UASB) was operated for 330 days to evaluate the treatment performance of sulfate-rich wastewater. The effects of competition change between methane production and sulfate reduction on the organic removal efficiency, methane production, and electrons allocation were investigated. Synthetic wastewater was composed of ethanol and acetate with a chemical oxygen demand (COD)/SO₄^2- of 1.0. As a result, the COD removal efficiency achieved in long-term treatment was higher than 90%. During the initial stage, methane production was the dominant reaction. Sulfate-reducing bacteria (SRB) could only partially oxidize ethanol to acetate, and methane-producing archaea (MPA) utilized acetate for methane production. Methane production declined gradually over the long-term operation, whereas the sulfate-reducing efficiency increased. However, UASB performed well throughout the experiment because there was no significant inhibition. After the complete reduction of the sulfate, MPA converted the remaining COD into methane.

Anaerobic treatment of N, N-dimethylformamide-containing wastewater by co-culturing two sources of inoculum

The complete methanogenic degradation of N, N-dimethylformamide (DMF) was achieved in this study. Initially, DMF was found to be feebly degradable by a lab-scale submerged anaerobic membrane bioreactor (SAnMBR) using normal anaerobic digestion sludge (ADS) even after 120-day's culturing. However, aerobic DMF-degrading activated sludge (AS) was rapidly cultivated in a continuous aeration reactor (CAR). A specially designed anaerobic co-cultured sludge (ACS) made by artificially mixing AS with ADS was successfully domesticated by a long term repeated batch experiment. The results demonstrated that ACS could effectively degrade over 5000 mg L^-1 DMF for methane recovery. The metabolic pathway and stoichiometric equation of DMF methanogenic degradation were also revealed and verified in detail. It was confirmed that under the anaerobic condition, with the help of enzyme, DMF converts into dimethylamine and formic acid, and the intermediates are effectively fermented through methylotrophic/hydrogenotrophic methanogenesis. Analysis of the microbial community suggested that some facultatively anaerobic bacteria played the key roles in methanogenic degradation due to their DMF-hydrolyzing ability. By co-culturing two sources of inoculum under the anaerobic condition, the symbiosis of facultatively anaerobic DMF-hydrolyzing bacteria and methylotrophic/hydrogenotrophic methanogens makes methanogenic degradation of DMF available. This study also provides a novel sludge cultivation method for anaerobic treatment of degradation-resistant organics.

Differentiated stimulating effects of activated carbon on methanogenic degradation of acetate, propionate and butyrate

Granular activated carbon (GAC) could promote methane production from organic wastes, but a wide range of dosages has been reported. In present study, different GAC dosages of 0, 0.5, 5 and 25 g/L were supplemented into anaerobic digesters and the methanogenic degradation kinetics of acetate, propionate and butyrate were characterized, respectively. At high organic load of 5 g/L, the degradation rates of propionate and butyrate increased by 1.5-4.7 and 2.5-7.0 times at varied GAC dosages. The methane production rates (R_max) from propionate and butyrate were significantly elevated when increasing GAC dosage up to 5 g/L. However, only a minor increment was found for acetate degradation either at 1 g/L or 5 g/L. The stimulatory mechanism of GAC for accelerated syntrophic degradation of propionate and butyrate can be primarily attributed to the triggering effect on acetogenesis, as evidenced by the enrichment of syntrophic bacteria e.g. Thermovirga, Synergistaceae, and Syntrophomonas etc.

mcrA sequencing reveals the role of basophilic methanogens in a cathodic methanogenic community

Cathodic methanogenesis is a promising method for accelerating and stabilising bioenergy recovery in anaerobic processes. The change in composition of microbial (especially methanogenic) communities in response to an applied potential-and especially the associated pH gradient-is critical for achieving this goal, but is not well understood in cathodic biofilms. We found here that the pH-polarised region in the 2 mm surrounding the cathode ranged from 6.9 to 10.1, as determined using a pH microsensor; this substantially affected methane production rate as well as microbial community structure. Miseq sequencing data of a highly conserved region of the mcrA gene revealed a dramatic variation in alpha diversity of methanogens concentrated in electrode biofilms under the applied potential, and confirmed that the dominant microbes at the cathode were hydrogenotrophic methanogens (mostly basophilic Methanobacterium alcaliphilum). These results indicate that regional pH variation in the microenvironment surrounding the electrode is an ecological niche enriched with Methanobacterium.

A review on the bioenergetics of anaerobic microbial metabolism close to the thermodynamic limits and its implications for digestion applications

The exploration of the energetics of anaerobic digestion systems can reveal how microorganisms cooperate efficiently for cell growth and methane production, especially under low-substrate conditions. The establishment of a thermodynamically interdependent partnership, called anaerobic syntrophy, allows unfavorable reactions to proceed. Interspecies electron transfer and the concentrations of electron carriers are crucial for maintaining this mutualistic activity. This critical review summarizes the functional microorganisms and syntroph partners, particularly in the metabolic pathways and energy conservation of syntrophs. The kinetics and thermodynamics of propionate degradation to methane, reversibility of the acetate oxidation process, and estimation of microbial growth are summarized. The various routes of interspecies electron transfer, reverse electron transfer, and Poly-β-hydroxyalkanoate formation in the syntrophic community are also reviewed. Finally, promising and critical directions of future research are proposed. Fundamental insight in the activities and interactions involved in AD systems could serve as a guidance for engineered systems optimization and upgrade.