Highest Methane Yield Research Articles

Biological saccharification coupled with anaerobic digestion using corn straw for sustainable methane production

In this study, accumulated fermentable sugars from biosaccharified corn straw were used to generate methane through anaerobic digestion (AD). The results showed that reducing sugars from biosaccharification expanded corn straw (BECS) treated with Clostridium thermocellum XF811 accumulated with yields of 94.9 mg/g. The BECS used for AD was converted into a high methane yield (7436 mL), which was 49.3 % higher than that of expanded corn straw (ECS). High-throughput microbial analysis suggested that Methanoculleus and Methanobacterium greatly contributed to the high methane yield. Industrial experiments demonstrated that the methane production from BECS by AD was 72,955 m3, which increased by 13.2 % compared to that from ECS. Biosaccharification pretreatment accelerated ECS destruction and accumulated sugars, thereby increasing methane yields. This study provides a strategy for producing clean energy from lignocellulose biomass.

An integrated leachate bed reactor – anaerobic membrane bioreactor system (LBR-AnMBR) for food waste stabilization and biogas recovery

This study developed an integrated LBR – AnMBR system for efficient stabilization and biogas recovery from food waste (FW) at room temperatures (21–22 °C). First, the leachate recirculation rate (4.4–13.2 L/h) was optimized to maximize hydrolysis and acidification yields. The maximum hydrolysis yield of 551 gSCOD/kg VSadded was achieved at recirculation rate of 13.2 L/h. The VFA concentrations in the FW leachate was as high as 12.5–16.0 g/L, resulting in a high acidification of 468 g CODVFA/kg VS. The solubilized FW was further stabilized by feeding the leachate to AnMBR. Different hydraulic (HRT) and solids retention times (SRT) were tested to achieve high COD removal and methane yields. High COD removal of 86 ± 3% was obtained in the AnMBR at HRT of 13 and SRT of 75 days. High biogas recovery of about 850 kWh per ton FWtreated was achieved along with high quality of AnMBR permeates containing low COD concentration but advantageously high concentration of nutrients (NH4+-N 317–403 mg/L, total phosphate 23–213 mg/L) without any particulates, which can be reused for landscape or liquid fertilizer.

How dose calcium hypochlorite promote the methane production from sludge anaerobic digestion: A mechanism study from enhanced biodegradability of recalcitrant substances

The application of calcium hypochlorite (CH) in waste activated sludge (WAS) treatment for enhancing the anaerobic digestion performance has been widely investigated in recent years. However, the underlying mechanisms for CH affecting the biomethane yield during sludge anaerobic digestion are still largely unknown. This work explored the impacts of CH with different contents on anaerobic methane generation from sludge, as well as the relevant mechanisms. Experimental results showed that the biomethane production firstly increased from 164.8 ± 4.2 to 220.5 ± 6.2 mL/g VSS (volatile suspended solids) with CH content raised from 0 to 1.0 g/L, which then started to decrease with further increased CH content. Mechanism analysis demonstrated that CH efficiently disintegrated sludge flocs, producing more biodegradable organics for anaerobic digestion. Beyond that, the biodegradability of recalcitrant humus and lignocellulose substances was enhanced by CH treatment, as higher methane yields were attained from CH treated model substrates when compared with the control. Microbial community analysis indicated that most of the functional microbes were enriched after CH treatment, with the total abundance detected to be 17.72 % and 25.20 % in control and CH pretreated reactors, respectively. Economic calculations demonstrated that the benefit of the CH-based method efficaciously covered the cost, indicating that the balance between economy and technology was achieved.

Enhanced Methane Production from Sludge Anaerobic Digestion with the Addition of Potassium Permanganate.

This work aims to reveal the effect of potassium permanganate (KMnO4) on the sludge anaerobic digestion process, as well as the relevant mechanisms. Experimental data showed that the biomethane production was gradually increased from 159.3 ± 3.0 to 211.5 ± 5.1 mL/g VSS (volatile suspended solids) when the KMnO4 content was increased from 0 to 0.08 g/g VSS, with an increasing rate of 32.8%. A further increase in the KMnO4 dosage however resulted in the decline of the methane yield. First-order kinetic model analysis indicated that higher methane production potentials and hydrolysis rates were achieved in KMnO4-added reactors than in the control. Mechanism analysis demonstrated that KMnO4 not only efficiently disintegrated the sludge flocs, which resulted in the increased contents of dissolved organics, but also enhanced the proportion of biodegradable substances in the sludge liquor. Meanwhile, the biodegradabilities of recalcitrant humus and lignocellulose substances were found to be promoted by KMnO4 treatment as higher methane yields were attained from KMnO4-treated model substrates. 16S rRNA analysis illustrated that the functional microbes participated in anaerobic digestion were largely enriched in the KMnO4-pretreated digestor. Furthermore, efficient inactivation of the fecal coliform was achieved by KMnO4 pretreatment.

Short sludge age denitrification as alternative process for energy and nutrient recovery

High rate activated sludge (HRAS) systems redirect organics into highly biodegradable sludge and nutrients into microbial proteins. This study evaluates anoxic HRAS for nitrogen and carbon recovery. The reactor treated synthetic wastewater at solids retention times (SRTs) of 5, 3 and 1 days. Denitrification rates varied between 0.15 and 0.19 g-NO3-N g-TSS-1 d-1 (total suspended solids per day) and all conditions showed favourable settling. The highest sludge yield, obtained at SRT 1 d, was 0.75 g-TSS g-CODremoved-1, double that observed for aerobic HRAS. The highest methane yield (322 mL-CH4 g-VSsludge-1) was obtained from sludge wasted at 3 d SRT. Both 1 d and 3 d SRTs showed favourable energy recovery, with 14 % of the organics recovered as methane. All conditions yielded sludge with protein content ranging between 24 and 27 % of dry weight and similar amino acid profile, comparable to traditional proteins. Thus, denitrifying HRAS recovers resources as its aerobic counterpart, allowing for nitrogen removal via denitrification, more stable compared to mainstream partial nitritation anammox typically combined with aerobic HRAS.

High-solids anaerobic co-digestion performances and microbial community dynamics in co-digestion of different mixing ratios with food waste and highland barley straw

High-solids anaerobic co-digestion (HSACoD) performances and microbial community dynamics were investigated. Food waste (FW) and highland barley straw (HBS) were used as co-digestion substrates with mixing ratios of 1:1, 1:3, 1:5, 5:1, and 3:1 (FW: HBS) in an anaerobic digestion (AD) system with 20% total solids. The results showed that the highest methane yield was 509.5 mLN/gVS for FH51, which was 129.8% higher than that of the single HBS. Synergistic effects contributed 20.4%–30.9% to improving methane yields in the first 25 days. The HSACoD system displayed excellent buffer capacity. Fastidiosipila, Gallicola, Proteiniphilum, Aminobacterium, Syntrophaceticus, and W5053 were dominant genera. Methanobacterium, Methanosphaera, Methanoculleus, and Methanosarcina were the key methanogens, displaying high relative abundance. Functional profiling indicated a relative abundance of metabolic pathways of 36.9%–39.6%, with the next most prevalent pathways being cellular processes and signaling, and information storage and processing. Therefore, HSACoD of FW and HBS can enhance AD speed in the early phase of AD and improve system stability.

Anaerobic Co-Digestion to Enhance Waste Management Sustainability at Yosemite National Park

This study evaluated the co-digestion of domestic wastewater solids (WWS) and food waste (FW) at the bench-scale for Yosemite National Park, California, which operates a 1900 m3/d wastewater treatment plant in El Portal, California. A 35-day biochemical methane potential test was performed on varying amounts of FW as a percentage of total waste (WWS plus FW) on a volatile solids basis (%FW). Specific methane yield and volumetric methane yield increased substantially with increasing %FW. A higher %FW was also associated with slower degradation kinetics but higher methane content in biogas. The 75 %FW treatment had relatively rapid kinetics, a high cumulative specific methane yield (453 mL CH4/g VS), and an elevated methane content in biogas, and is suggested as an upper limit %FW mixture for full-scale co-digestion. This, coincidently, is near the estimated ratio of WWS and FW production at the Park (70 %FW). Co-digesting the Park’s feedstock of FW with WWS in existing anaerobic digestion facilities could increase methane production five-fold. Combusting this methane in a combined heat and power system would produce about twice the energy needed to heat anaerobic digestors and power the treatment plant.

Immobilization of hydrolytic/fermentative bacteria to achieve ultra-low fouling in anaerobic membrane bioreactor

Membrane fouling remains the key challenge that limits the application of anaerobic membrane bioreactors (AnMBRs). Free-living bacteria, particularly hydrolytic/fermentative bacteria (HFB) in sludge supernatant, have recently been identified as the predominant foulants in AnMBRs. Here, a pretreatment tank filled with porous polyurethane sponges as carriers for biomass attachment and/or entrapment was developed for influent organic hydrolysis/fermentation, named HFB-immobilization tank. Then, two parallel AnMBRs (i.e., with and without an HFB-immobilization tank) treating synthetic domestic wastewater were operated to determine the roles of HFB capture in fouling mitigation as well as reactor performance. Higher methane yields were observed in the experimental than in the control AnMBR. Interestingly, the HFB-immobilization helped to achieve a non-fouling state of the experimental AnMBR for more than 100 days, while the average fouling cycle of the control AnMBR was 20 days. Dead-end filtration tests corroborated better filterability of sludge supernatant in the experimental than the control AnMBR. The 16S rRNA gene sequencing revealed substantially lower abundances of HFB (e.g., Aminicenantales, SJA-15) but higher abundances of methanogens (e.g., Methanosaeta, Methanolinea, Methanospirillum) in the mixed liquor of the experimental than the control AnMBR, indicating efficient niche separation by the HFB immobilization. Particularly, the sequencing data, together with fluorescent staining, showed that the amounts and abundances of HFB in sludge supernatant were largely lower in the experimental than the control AnMBR. This study provides a promising alternative for fouling control in AnMBRs.

Application fuzzy DEMATEL methodology to investigate some technical parameters of biochemical methane potential (BMP) test produced vegetable waste anaerobic biogas.

As the transition to renewable energy systems is accelerating, anaerobic digestion, which is one of the methods of energy recovery from organic substrates, continues to be studied with great interest by scientists. Anaerobic digestion research and applications are mostly carried out with biochemical methane potential (BMP) tests to decide the methane potency of sewage sludge, energy crops, and organic wastes. Unlike long and costly continually reactor experiments, actually, BMP tests are cumulative and can be performed with a relatively low investment of materials, technical labor, and also time. For the BMP to give accurate results, the effect of all the tools and technical parameters used in the implementation of the BMP should be well understood. In such situations, it is very useful to apply fuzzy logic methods in multi-criteria decision-making stages when more than one parameter changes at the same time. Therefore, in this study, fifteen parameters were determined and analyzed with the fuzzy DEMATEL (decision-making trial and evaluation laboratory) method to understand the cause-effect mechanism of the technical parameters of BMP. As a result of these analyses, it was seen that the material of the reactor (ri-cj value of 0.55), the particle size (ri-cj value of 0.43), the effect of mixing (ri-cj value of 0.32), and the amount of the total solids (TSA) (ri-cj value of 0.25) had a high effect in the causal sense. It was observed that the first-order parameter (material of reactor) was 27% stronger than the second-order (the particle size) parameter in terms of causality. Likewise, the second-order parameter is 34% stronger than the third-order parameter (the effect of mixing) in terms of cause effect. In addition, it was understood that the most effective parameters in the mechanism of effect were pH (ri + cj value of 3.41), C/N ratio (ri + cj value of 3.26), and temperature (ri + cj value of 3.07), respectively. Besides, high methane yield is seen in mesophilic conditions. The average cumulative biogas yield of the reactor is 282.1 NmL/g VS. The highest percentage of methane formed in the biogas occurred on the 21st day. Briefly, this study is important to provide a facilitating way for researchers working on BMP to understand the cause-effect mechanism of system technical requirements.

Effect of substrates and voltages on the performance of bio-electrochemical anaerobic digestion

This research study investigates the effect of feedstock chemical composition and voltage supply on methane production from bio-electrochemical anaerobic digestion (BEAD). The experimental work was conducted on batch biomethane potential (BMP) bottles equipped with electrodes and connected to external power. Three substrates with distinct elemental compositions, waste activated sludge (WAS), and cow manure (CM), organic fraction of municipal solid waste (OFMSW), were tested under different voltages (0.7–1.5V). The BMP tests showed that OFMSW had the highest methane yield in the AD control reactors, followed by WAS and CM, which concurred with the estimated theoretical yields. Supplying voltage enhanced the performance of all BEAD reactors, but with different levels of improvement; methane yield, organics removal, and net energy efficiency increased by 7–46%, 16–54%, and 5–37%, respectively, compared to the equivalent controls. The strong relationship between the voltage and substrates with methane production was confirmed statistically through analysis of variance and t-test. Moreover, a regression model was established between the feedstock elemental fractions, power supply, and methane yield, with an adjusted correlation coefficient of 0.9 and a maximum prediction error of 5.4%. The overall findings of this research provide valuable operation and performance insights on the potential bio-electrochemical upgrades to AD systems.

Thermophilic-mesophilic temperature phase anaerobic co-digestion of sewage sludge, wine vinasse and poultry manure: Effect of hydraulic retention time on mesophilic-methanogenic stage

The present study investigated the influence of the hydraulic retention time (HRT) in temperature-phase anaerobic co-digestion (TPAcD) for methane production. The reactors were started-up with a mixing ratio of 49.5:49.5:1 of sewage sludge, wine vinasse and poultry manure. The TPAcD was operated at thermophilic temperatures in the first stage and mesophilic temperatures in the second stage. The thermophilic stage operated with a constant HRT of 5 days, while the methanogenic stage was optimized under the HRT of 15, 12, 10, 8, 5, 4, and 3 days. The best results were obtained for an HRT of 12 days in the methanogenic stage, 56.35 % of volatile solids (VS) biodegradation was achieved, with a methane yield of 391 mL CH4/gVSadded. Regarding the whole TPAcD process (acidogenic following by methanogenic), the vS and total volatile fatty acids reached, respectively, 93.13 % and 97.43 % of removal efficiency. The microbial population revealed that Eubacteria was higher than the Archaea at the HRT with the highest methane yield, and the microbial activity increased proportionally to the organic loading rate, which in turn was related to methane production. Due to the strong pathogen reduction in the TPAcD, the digestate obtained can be classified as class A biosolids in all HRT evaluated, being a promising alternative for its application as agricultural fertilizer. Finally, the presented TPAcD process can be an environmeltally friendly alternative for the management of sewage sludge, wine vinasse, and poultry manure in an integrated biorefinery for the recovery of bioenergy and fertilizer, advocating a sustainable approach for the circular economy transition.

Heavy metals migration and antibiotics removal in anaerobic digestion of swine manure with biochar addition

Anaerobic digestion (AD) is a key technology for the utilization of swine manure, but the complex pollution of heavy metals and antibiotics in manure is catching more attentions. In this study, AD of swine manure loading with four types of biochar was performed in a 1 L-frosted jar with the total fresh weight of 840 g, in which the effects of biochar on the AD stability and methane yields were focused, and the removal of antibiotics and migration of heavy metals were investigated as well. The results showed that the removal efficiencies of sulfadimidine and tylosin were remarkable, which were up to around 70% and 100%, respectively. The maximum methane yield of 146.98 mL/ (g VS) was noted in the digester added with the biochar derived from paper mill sludge (BPMS) which contained 87.35% of ash, having a high pH PZC of 8.95 and more mesopores and macropores. In this reactor, 12.40% of Cu was moved to the solid phase, and 70.26% of Zn and 82.80% of As were migrated to the solid digestate. Compared to the control, the content of the acid soluble Cu in the digester loaded with BPMS was decreased to 7.99%, and no acid soluble Zn was found along with a drop of 29.20% for the reducible Zn, and the residual As content was increased by 16.60%. It revealed that BPMS had a great potentiality to remove As and Zn in wastewater and synchronously mitigate the bioavailability of Cu/Zn/As in the digestate. Herein, applying BPMS in the AD of swine manure could reduce the complex pollution of heavy metals and antibiotics, and obtain a higher methane yield simultaneously. • Paper mill sludge biochar had high pH PZC and was rich of ash, macropores and mesopores. • Around 70% of SDM and 100% of TYL were degraded in AD. • Highest methane yield was noted in the digester loaded with paper mill sludge biochar. • 70.26% Zn and 82.80% As migrated to solid digestate via paper mill sludge biochar adsorption. • Paper mill sludge biochar could reduce the eco-toxicity of Cu, Zn, As in the digestate.

Thermophilic anaerobic co-digestion of glycerol and cheese whey – Effect of increasing organic loading rate

The organic loading rate (OLR) is one of the variables that directly affects anaerobic digestion. An increase in OLR can improve methane production, but it can also lead to process failures. Therefore, determining the proper OLR for the process is of great importance. This study analyzed the effect of increasing the OLR on cheese whey and glycerol co-digestion in a thermophilic anaerobic fluidized bed reactor (AFBR) by varying both the influent concentration and the hydraulic retention time (HRT). The increase in OLR from 5 to 20 gCOD.L−1.d−1 was initially performed by increasing the influent concentration from 5 to 7.5, 10, 15, and 20 gCOD.L−1, at HRT of 24 h. Organic matter removal (87.7%), methane yield (MY) (253.0 mL CH4.gCOD−1rem), and methane production rate (MPR) (3.2 L CH4. L−1.d−1) showed the best performance at 10 gCOD.L−1.d−1. Conversely, raising the OLR to 20 gCOD.L−1 enhanced the buildup of volatile fatty acids, resulting in a 38.5% decrease in MY. Based on these findings, the HRT was reduced from 24 h to 20 h and 16 h at influent concentration of 10 gCOD.L−1 resulting in OLR of 10, 12, and 15 gCOD.L−1.d−1, respectively. The HRT at 20 h (12 gCOD.L−1.d−1) provided the highest MY values (around 292.5 mL CH4.gCOD−1rem), while the maximum MPR was observed at 16 h (5.1 L CH4. L−1.d−1). The main metabolites observed in all conditions were acetic acid (31.1–66.8%) and propionic acid (10.9–61.5%). The OLR also affected the microbial community, with Proteobacteria accounting for 53.1% of the relative abundance at 5 gCOD.L−1.d−1 while Firmicutes accounted for 76.6% at 10 gCOD.L−1.d−1. At 10 gCOD.L−1.d−1, with maximum MY, the bacteria observed with the highest relative abundance were an unidentified genus of Lentimicrobiaceae (8.9%), “Blvii28 wastewater sludge group” (2.9%), and Tolumonas (1.4%). The archaea identified with the highest relative abundance at both OLRs (5 and 10 gCOD.L−1.d−1) was hydrogenotrophic Methanobacterium (27.3% and 2.6%, respectively).

Anaerobic codigestion of excess sludge with chicken manure with a focus on methane yield and digestate dewaterability

The effects of the mixing ratio of excess sludge (ES) with chicken manure (CM) on methane yield and digestate dewaterability were investigated at thermophilic and mesophilic temperatures. Batch experiments were performed at five different volatile solid ratios (ES/CM). Compared with anaerobic digestion of ES alone, codigestion of ES with CM increased methane yield 82.4–123.1 mL g−1-VSadded (mesophilic) and 33.9–171.3 mL g−1-VSadded (thermophilic), respectively. The normalized capillary suction time (NCST) and TS of thickened digestate (3.09–8.29 s g−1-TS, 17.51 %–18.83 %) at mesophilic temperatures were superior to these (6.81–10.14 s g−1-TS, 16.66 %–18.30 %) at thermophilic temperatures. But the gap decreased with increasing CM proportion in the substrate. The results indicated that anaerobic codigestion of ES and CM at a proper mixing ratio (ES/CM = 1:1.5–2) could obtain a high methane yield and an adequate digestate dewaterability.

Novel anaerobic digestion of waste activated sludge via isoelectric-point pretreatment: Ultra-short solids retention time and high methane yield

The prolonged reaction period and low methane yield have become a pivotal bottleneck in the anaerobic digestion (AD) of waste activated sludge (WAS), severely limiting its use in bioenergy. This study evaluated the long-term semi-continuous AD of WAS with isoelectric-point (pI) pretreatment in terms of the reaction period, methane yield, material resource recovery, and economic and environmental benefits. The experimental results show that after pI pretreatment, at the same solids retention time (SRT), the average methane yield and methane content in the biogas of WAS increased by 120.9% and 15.5%, respectively. The average methane yield from AD with pI pretreatment at a 5-day SRT was 1.6 times that of the AD without pI pretreatment at a 15-day SRT, indicating that pI pretreatment significantly (P < 0.001) increased the methane yield from the AD of WAS at different SRTs, even at an ultra-short SRT. Statistical analyses of the changes in the concentrations of soluble organic matter in the AD process with and without pI pretreatment confirmed that pI pretreatment enhanced the biochemical reactions related to the transformation of sludge organic matter during the whole AD process, but not the initial organic solubilisation and hydrolysis of the sludge. Furthermore, pI pretreatment recovered the NaHCO3, Na2S, and multivalent metals from the sludge, indicating that AD with pI pretreatment efficiently recovered both bioenergy and material resources from the sludge. Economic and environmental analyses of AD with and without pI pretreatment further indicated that with pI pretreatment, the annual economic benefit of AD increases by 162% and carbon emissions decreases by approximately 2.63 × 107 kg CO2/year compared with conventional AD. These findings serve as an important reference for the development of a novel AD technology for sludge that incorporates second-generation pretreatment and operates at an ultra-short SRT.

Serial anaerobic digestion improves protein degradation and biogas production from mixed food waste

Optimization of the biogas generation process is important to achieve efficient degradation and high methane yield, and to reduce methane emissions from the digestate. In this study, serial digester systems with two or three biogas reactors were compared with a single reactor, with the aim of improving degree of degradation and methane yield from food waste and assessing adaptation of microbial communities to different reactor steps. All systems had the same total organic load (2.4 g VS/(L d)) and hydraulic retention time (55 days). Serial systems increased methane yield by >5% compared with the single reactor, with the majority of the methane being obtained from the first-step reactors. Improved protein degradation was also obtained in serial systems, with >20% lower outgoing protein concentration compared with the single reactor and increasing NH4+-N concentration with every reactor step. This resulted in separation of high ammonia (>384 mg NH3-N/L) levels from the main methane production, reducing the risk of methanogen inhibition. Methanosarcina dominated the methanogenic community in all reactors, but increases in the hydrogenotrophic genera Methanoculleus and Methanobacterium were observed at higher ammonia levels. Potential syntrophic acetate-oxidizing bacteria, such as MBA03 and Dethiobacteraceae, followed the same trend as the hydrogenotrophic methanogens. Phylum Bacteroidota family Paludibacteraceae was highly abundant in the first steps and then decreased abruptly, potentially linked to an observed decrease in degradation in the last-step reactors. Nevertheless, the results indicated a trend of increasing relative abundance of the potentially proteolytic genera Proteiniphilum and Fastidiosipila with successive reactor steps.

Carbon cloth self-forming dynamic membrane enhances anaerobic removal of organic matter from incineration leachate via direct interspecies electron transfer

Anaerobic dynamic membrane bioreactors (AnDMBRs) can convert organic matter in wastewater to methane with excellent filtration performance, and can effectively intercept suspended solids and macromolecules in wastewater. The objectives of this study were to explore the feasibility of enhancing AnDMBRs by using conductive carbon cloth as the supporting material for self-forming dynamic membrane to treat waste incineration leachate, determine the strengthening efficiency and reactor operation mechanism, and further study the enhancement mechanism. Incineration leachate was treated using a laboratory-scale AnDMBR with either conductive carbon cloth or polyester cloth as the supporting material. Compared with the polyester cloth, the carbon cloth reactor resulted in higher COD removal (96.5% vs 85.5%) and higher methane yield (217 vs 169 mL(STP)/h) when the reactors were fed diluted leachate with COD of 15,000 mg/L. In addition, the carbon cloth r withstood a higher organic load of 5.129 kg COD/(m3d) compared with only 4.014 kg COD/(m3d) for the polyester cloth reactor. A suitable filtering performance was also observed in both reactors, which could effectively intercept > 4000 Da macromolecules, and remove > 97% of dissolved organic matter in the leachate. Electroactive microorganisms including Syntrophomonas and Methanothrix species were enriched on the conductive carbon cloth surface, and genes associated with reduction of CO2 to methane were more highly expressed by Methanothrix cells attached to the carbon cloth than that of the polyester cloth, which possibly accelerated the methanogenesis by direct interspecies electron transfer.

Enhanced methane production by bimetallic metal–organic frameworks (MOFs) as cathode in an anaerobic digestion microbial electrolysis cell

Cathode materials play important roles in the optimization of methane upgrading in microbial electrolysis cell-coupled anaerobic digestion (MEC-AD) systems. In this study, metal organic framework (MOF)-derived carbon-based bimetallic hybrids (Ni/Co-NC) was synthesized as cathode material and its performances and microbial activities were evaluated in MEC-AD. A high methane production rate (0.57 m3 CH4/m3·d) and yield (0.34 m3 CH4/kgCOD) were achieved with a Ni/Co-NC cathode at a potential of 0.6 V. The methane content in the biogas was upgraded to ca. 90%. The enhanced methane production was likely related to the positive surface charge and the good conductive capacity and hydrogen evolution of the Ni/Co-NC cathode, as demonstrated by electrochemical assessment. Additionally, roughness and extensive surface areas provided additional room for cathodic biofilm adhesion via hydrogen bonding and electrostatic interactions. A significant enrichment of Methanobacterium (79.6%) in the MEC-AD-Ni/Co-NC system, especially in the biofilm of the cathode surface, indicated that the methane production pathway shifted from acetoclastic to hydrogenotrophic methanogenesis. Moreover, Methanosaeta harundinacea 6Ac species, which can directly accept electrons, were identified in the MEC-AD systems. These findings revealed that Ni/Co-NC cathode probably enhanced bioelectrochemical reduction of CO2 to CH4 by accelerating hydrogenotrophic methanogenesis and direct electron transfer.

Using the Mixed Culture of Fodder Mallow (Malva verticillata L.) and White Sweet Clover (Melilotus albus Medik.) for Methane Production

The ever-growing number of biogas plants also leads to an increasing demand for suitable, alternative plant substrates. A currently dominant plant substrate is maize silage. However, intensive cultivation of maize (Zea mays L.) as an energy crop in monocultures poses risk to the environment (soil erosion, depletion of soil nutrient supplies, increased concentration of pests—Ostrinia nubilalis). In this study, results of experimental methane production from silages of alternative substrates, such as fodder mallow (Malva verticillata var. crispa L.; FM), white sweet clover (Melilotus albus Medik.; WSC) and their mixture, are presented. Based on the dry matter yield of a mixed culture of mallow and sweet clover, the value of the land equivalent ratio parameter (LER) was set as 1.05. The obtained LER (&gt;1) value shows that the cultivation of the two tested crops in the mixed culture is more beneficial than their monocultures. Methane production from all three silage variants was fully comparable with results of methane production from the maize silage. Anaerobic fermentation of the mixture of FM and WSC did not result in higher methane yield than the average result of monosubstrates.

Co-digestion of biofuel by-products: Enhanced biofilm formation maintains high organic matter removal when methanogenesis fails

Ethanol and biodiesel industries generate large volumes of by-products, such as vinasse and glycerol, which are suitable for biogas exploitation. This paper assessed the applicability and process performance of the anaerobic structured-bed reactor (AnSTBR) for the mesophilic (30 °C) continuous (105 days) anaerobic co-digestion of sugarcane vinasse and distilled glycerol under increasing organic loading rates (OLR) (0.5–5.0 kgCOD m−3d−1). The highest methane yield (211 NmL g−1CODrem.) and volumetric production (668 NmL L−1d−1) occurred at an OLR of 3.5 kgCOD m−3d−1. The performance of the AnSTBR showed high removal efficiencies of total COD (77.1%), carbohydrates (81.9%), and glycerol content (99.7%). Biofilm growth enhancement within the reactor offset the impairment of methanogenesis activity at high organic loads. The prompt biodegradability of glycerol reinforced the importance of gradually increase the organic load to prevent the buildup of volatile acids and maintain a stable long-term co-digestion system.