Increased Biomethane Production Research Articles

Field conditions for the synergistic increase of biomethane in the goaf of coal mines filled with corn straw

AbstractSynergistic fermentation of coal and corn straw is an effective tool to increase biomethane production. However, a large gap exists between the biomethane production conditions of corn straw filling coal mine goafs and laboratory experiments. In order to determine the effect of the field environment on synergistic biomethane production, biomethane production experiments with coal and corn straw were carried out under different conditions to find the key factors restricting the potential of biomethane production. The obtained results showed that various bacterial sources had significant influences on the biomethane production of coal and corn straw, and domesticated bacterial sources provided fermentation systems with more efficient biomethane production capacities than mine water sources. Biomethane production of coal and corn straw was relatively high under mixed conditions, but it was also promoted under unmixed conditions. Different inorganic minerals had different effects on synergistic biomethane production, which varied. For example, calcite, montmorillonite, and kaolin are common minerals in coal‐bearing strata that significantly enhance synergistic biomethane production of coal and corn straw. However, pyrite was found to significantly inhibit the synergistic biomethane production effect of coal and corn straw. Highly metamorphosed anthracite coal also presented biomethane production potential when stimulated by corn straw as a carbon source. The obtained results revealed the influences of different field conditions on the biomethane production of coal and corn straw and provided a reference for the field application of corn straw filling in coal mine goafs.

Anaerobic Co-digestion of the Liquid Fraction of Food Waste with Waste Activated Sludge

The present study investigated the feasibility of the anaerobic co-digestion (AcoD) of condensate, resulting from drying food waste, with Waste Activated Sludge in a pilot scale continuous stirred tank reactor. Different parameters were assessed in order to enhance the AcoD performance; the condensate potency (condensate A: 13 gCOD/L and condensate B: 4 gCOD/L), the volumetric ratio of condensate to WAS (0–67% v/v) and the hydraulic retention time (HRT) (20, 15 and 12 days). The results showed that increasing the condensate content in the feed from 0 to 67% v/v, enhances the organic load removal (up to 41% increased total COD removal) and the bioenergy production (up to 35% increased biomethane production). Moreover, in the case of condensate A, the reduction of HRT from 20 to 15 days enhanced the bioenergy production (up to 19% increased biomethane production) while the reduction of HRT from 20 to 12 days in the case of condensate B did not significantly affect the reactor’s performance. Overall it is concluded that condensate can be safely introduced in existing facilities for anaerobic digestion, while maintaining a more stable operation and improved effluent quality and bioenergy production in comparison with conventional anaerobic sludge digestion.Graphical

Enhanced coal biomethanation by microbial electrolysis and graphene in the anaerobic digestion

The combination of microbial electrolytic cells and conductive materials can effectively promote direct interspecies electron transfer (DIET) to increase methane production, which has great potential for enhanced anaerobic degradation of organic matter. A single-chamber microbial electrolytic cell containing graphene was constructed using long-flame coal as a substrate. The results showed that the external electric field and graphene increased the abundance of hydrolytic bacteria (Paraclostridium, Sedimentibacter) and hydrogen-producing acetogenic bacteria (Anaerovorax) in the AD system. The consumption rate of alkanes, volatile fatty acids and alcohols was accelerated, which provided sufficient nutrients for methanogens and increased biomethane production by 53.1 %. The abundance of related genes involved in the carbon dioxide reduction pathway was significantly increased. The abundance of pilA gene involved in electron transport in the AD system increased by 153.7 %, and the abundance of electroactive microorganisms Geobacter and Methanosarcina capable of DIET increased significantly, which further promoted coal biomethanation.

Environmental assessment of anaerobic digestion and biological methanation in power-to-methane systems

Biomethane can play a crucial role in achieving European climate goals and replacing natural gas. One way to increase biomethane production is through biological methanation, converting the remaining CO2 from biogas with hydrogen into methane. The required hydrogen is produced via electrolysis, making this technology also a valuable storage and balance solution for the energy grid. This study determines the ecological development potentials of biological methanation. For this purpose, different feedstock for anaerobic digestion are investigated as carbon source, effects of technological advances in electrolysis and biological methanation are determined and optimal modes of operation are evaluated. Results indicate that the biological methanation with an anaerobic digestion using manure as feedstock has the lowest greenhouse gas emissions. However, the biological methanation achieves the greatest reduction in emissions compared to traditional biomethane production (biogas upgrading with high pressurized water scrubbing) when using energy crops and reducing emissions from feedstock production. In the base scenario which represents the current state of technology, biogas from maize without additional methanation step produces 151.8 kgCO2eq/MWhCH4. In combination with biological methanation, biomethane can be produced in the future with CO2eq emissions of 121.4 kgCO2eq/MWhCH4. Other factors such as agricultural land occupation or water consumption show even more significant improvements. The main drivers of emissions are the electricity demand for the electrolysis and fugitive methane emissions. The study shows that there is great ecological potential in biological methanation, especially in the future with prognosed technological advances in electrolysis and biological methanation.

Effects of trace elements (Fe, Cu, Ni, Co and Mg) on biomethane production from paper mill wastewater

Abstract Trace elements have a significant effect on biochemical reactions and therefore the presence of optimum levels of trace elements is essential for bioreactor performances. In this study, the effects of five trace elements on biomethane production have been investigated. Experimental studies have been carried out with multiple batch reactors at 15 day HRT and mesophilic temperatures. The optimum concentrations for each of the trace elements Fe, Cu, Ni, Co and Mg were found as 5, 0.5, 0.5, 0.5 and 100 mg/L, respectively. Among tested trace elements, Cu was the one which provided the highest biomethane production. Cu addition was resulted in a 46 % increase in biomethane production followed by Co with 24 %. The biomethane production rate for these two trace elements was 191.70 and 110.77 ml CH4/g COD, respectively. Optimum levels for Ni, Fe and Mg increased biomethane production rate by approximately 14.3, 10 and 17 % compared to control groups, respectively. Because the exact amount of trace element requirement for each industry/reactor is different, specific case studies should be performed for each application. These results could be used as initial trace element concentrations for further continuous studies.

Increased biomethane production from herbaceous peat through pretreatment with dilute sulfuric acid

Increased biomethane production from herbaceous peat through pretreatment with dilute sulfuric acid

Simultaneous study of the interaction effect of chemical and hydrothermal pretreatment on the yield of methane produced from municipal waste

Municipal waste has the potential to be a significant source of energy production. This study investigated pretreatment methods such as NaOH, hydrothermal, and ozonation to increase biomethane production from municipal waste. In addition, these pretreatments were further evaluated using ultrasonic pretreatment after achieving optimal conditions by RSM CCD methods. The optimum pretreatment conditions were observed to be 8% NaOH concentration, 132 °C hydrothermal temperature, and O3 equal to 0.19 g/g TS. The maximum biomethane produced and achieved during the tests was 394 mL/kg TS, which increased to 410 mL/kg TS after ultrasonic pretreatment. The best sCOD reduction in the optimal pretreatment conditions and after the ultrasonic pretreatment was 87% and 91%, respectively. Also, in the absence of ozone pretreatment, the highest yields of biomethane and biogas occurred at a 6.4% concentration of NaOH and a temperature of 135 °C; however, in the presence of ozone, the yield of biomethane and biogas produced was greater and the inhibitory effect of sodium hydroxide also occurs in higher amounts. Experiments have shown that ozonation increases biomethane production rather than increasing biogas production (hence the ratio of methane to biogas).

Biomethane production from waste activated sludge promoted by sludge incineration bottom ash: The distinctive role of metal cations and inert fractions

Sludge incineration bottom ash (SA), a solid waste generated by incineration of waste activated sludge (WAS), has been demonstrated as an inexpensive additive to increase biomethane production from anaerobic digestion (AD) of WAS. However, how SA improved methanogenic performance of a WAS digester remains elusive. Here, we addressed this question by fractionating the SA into accessible leachate (SA-L) and inert residue (SA-R) and investigating their individual effects. The cumulative biomethane production was increased by 6.7%, 20.2% and 39.6% with addition of SA-L, SA-R and SA, respectively. Mechanistic study showed that release of organic-binding metals (Ca and Fe) from SA dissolution suppressed volatile fatty acids production by increasing the apparent activation energy (AAE) and decreasing the surface binding sites for hydrolytic/acidogenic enzymes during WAS hydrolysis-acidogenesis, while trace elements in SA-L promoted metabolism of methanogens (Methanothermobacter and Methanosarcina). In contrast, the gypsum/silicate-cored SA-R facilitated hydrolysis-acidogenesis with reduced AAE but drastically inhibited methanogenesis due to competition of sulfate-reducing bacteria Thermodesulfovibrio. The comparative analysis of KEGG-based functional genes indicated that the enhanced methane metabolism and reductive CO2 fixation pathways with SA addition could result from the release of trace elements to support key enzyme activities.

Mathematical Analysis and Update of ADM1 Model for Biomethane Production by Anaerobic Digestion

Biomethane is a renewable product that can directly substitute its fossil counterpart, although its synthesis from residual biomasses has some hurdles. Because of the complex nature of both biomasses and the microbial consortia involved, innovative approaches such as mathematical modeling can be deployed to support possible improvements. The goal of this study is two-fold, as we aimed to modify a part of the Anaerobic Digestion Model No. 1 (ADM1), describing biomethane production from activated sludge, matching with its actual microbial nature, and to use the model for identifying relevant parameters to improve biomethane production. Firstly, thermodynamic analysis was performed, highlighting the direct route from glucose to biomethane as the most favorable. Then, by using MATLAB® and Simulink Toolbox, we discovered that the model fails to predict the microbiological behavior of the system. The structure of the ADM1 model was then modified by adding substrate consumption yields in equations describing microbial growth, to better reflect the consortium behavior. The updated model was tested by modifying several parameters: the coefficient of decomposition was identified to increase biomethane production. Approaching mathematical models from a microbiological point of view can lead to further improvement of the models themselves. Furthermore, this work represents additional evidence of the importance of informatics tools, such as bioprocess simulations to foster biomethane role in bioeconomy.

The effect of reduced graphene oxide addition on methane production from municipal organic solid waste

AbstractBACKGROUNDConductive materials have become the focus of recent studies to accelerate and stabilize the conversion of organic wastes to methane in anaerobic digestion processes. In this study, the effect of the addition of reduced graphene oxide (rGO) on biogas/biomethane production from municipal organic solid wastes in anaerobic batch reactors was investigated. In this context, the effect of loading ratios (0, 0.5, 1 and 2 gVS inoculum (gVS nutrient)−1) and rGO addition at different concentrations (0, 10, 20 and 30 mg L−1) was examined during a 45‐day study period.RESULTSThe highest biogas production reached a level of 816 ± 14 mL (gVS)−1 when substrate/inoculum ratio was 1 and 20 mg L−1 rGO was added (50% more than the group without rGO added). Addition of 30 mg L−1 rGO resulted in 667 ± 12 mL (gVS)−1 corresponding to 23% higher biogas production compared to the control reactor without rGO. The highest cumulative biomethane production was observed to be 525 ± 20 mL (gVS)−1 in the reactor with 20 mg L−1 rGO added in which substrate/inoculum ratio was 1. In the reactors with 10 and 30 mg L−1 and without rGO addition biomethane productions were 448 ± 21, 401 ± 13 and 323 ± 23 mL (gVS)−1, respectively.CONCLUSIONSResults revealed that the addition of rGO increased biomethane production from municipal organic solid wastes. It is concluded that rGO addition has the potential to be applied for real‐scale facilities within the scope of renewable energy production and circular economy. © 2021 Society of Chemical Industry

Enhanced biomethane production by co-digestion of mixed sewage sludge and dephenolised two-phase olive pomace.

In this study, co-digestion of mixed sewage sludge from a wastewater treatment plant (WWTP) and partially dephenolised two-phase olive pomace (DOP) as a co-substrate was addressed with the aim of improving the biodigestibility of both substrates. The introduction of DOP into WWTP anaerobic digester facilities could significantly increase biomethane production and enhance the sustainability of both activities. An improvement in the system's performance was supported by stability parameters: total alkalinity increased and stabilised with the addition of 5% v/v DOP, and the specific energy loading rate was maintained at 0.177 ± 0.03 d-1, which indicated better buffer capacity and stability in the bioreactor, and the possibility of enhancing the organic loading rate. In terms of average daily biogas production rate, an increase of 39% was achieved, up to 0.39 ± 0.11 L L-1d-1. Moreover, there was a 40% and 37% improvement in specific methane production and methane production rate, respectively, up to 0.28 ± 0.02 L CH4 g TVS-1 and 0.26 ± 0.08 L L-1d-1. In addition, the proposed strategy leads to an energy saving of 20,328.6 kWh year-1 at the WWTP as a result of the electric energy production surplus, corresponding to an annual saving of €3293.23.

The impact of crude glycerol from biodiesel production and its trace element content on biomethane production in a batch experiment: modelling as a step towards impartial routine comparison of results

In this study, crude glycerol from the biodiesel industry was tested as a co-substrate in biogas production. To investigate the influence of crude glycerol and the underlying trace element (TE) content on the efficiency of biomethane production, a batch experiment using Automatic Methane Potential Test System (AMPTS II) was carried out. The single addition of crude glycerol significantly contributed only to the total content of K (14.4%), Si (17.3%), and P (11.6%), whereas the contributions of other metals were within the range of other substrates. The addition of crude glycerol increased biomethane production, however, its utilization beyond 1% of total volume resulted in prolonged lag phase and final cessation of biomethane production. The negative effects of inorganic salts present in crude glycerol were reflected in progressively diminishing parts of glycerol and methanol being utilized in its anaerobic digestion, posing serious problems for daily routine use. A nonlinear least square regression analysis was performed to fit the Gompertz, Logistic, Transfer, and Richards models to biomethane production. The most suitable model was the Richards model, exhibiting the best fit to the experimental curves for complex substrates. Glycerol fractions remaining after biodiesel production have to be pre-tested for their negative effects on the content of TEs and inorganic salts, lag phase in biogas production, before they are used as co-substrates in biogas production phase.

Sequential valorization strategies for dairy wastewater and water hyacinth to produce fuel and fertilizer

AbstractTo overcome the obstacles of treatment of dairy wastewater (DW) under anaerobic mono‐digestion and sustainable disposal of harvested water hyacinth (WH) a novel sequential valorization strategy was adopted to obtain fuel and fertilizer. Initially, DW and WH were subjected for anaerobic co‐digestion to elucidate the synergistic effect of the substrates to improve the process efficiency and increased biomethane production. The results indicated that the co‐digestion mixture (DW: WH‐1:1) has resulted in biomethane yield of 504.38 L/Kg VSfed which is 1.28‐fold and 3.14‐fold higher as compared to the mono‐digestion of WH (431.71 L/Kg VSfed) and DW (160.59 L/Kg VSfed), respectively. Further, the residual digestate was subjected for slow pyrolysis (320 ± 15°C) to obtain bio‐oil and biochar. The obtained bio‐oil shown superior fuel characteristics as compared to direct WH bio‐oil resulting in density, viscosity, and calorific value of 1.21 ± 0.04 g/ml, 1.8 ± 0.05 cSt and 1766.82 ± 11.53 KJ/Kg, respectively. The biochar residue from pyrolysis was amended to the soil to determine its bio‐fertilizing property. The soil properties in terms of bulk density, porosity, organic carbon, and pH were enhanced. Tomato cultivation in biochar amended soil shown comparable vegetative growth to that of digestate and urea thereby advocating the replacement of chemical fertilizers.Practical ApplicationWith recent increase in environmental concerns and requirement to meet the prescribed limits for effluent discharge from the industries, especially from food processing for the protection of aquatic bodies, there is a requirement of adoption of cost effective technology that could provide both economic and ecological sustainability. In this study, a sequential valorization strategy was adopted for efficient management of dairy wastewater and harvested water hyacinth from the water resources. The resultant products biogas, bio‐oil and biochar could be potentially used as fuel and fertilizer. Circular bio‐economy is in need of time which could be attainable by the implementation of such strategies in large scale industries for efficient waste management and resource recovery. This will not only benefit the environment but also to the society by galvanizing the sustainable development technology.

Clean and efficient utilization of coal combined with corn straw by synergistic biodegradation

To establish a process for cleaning and efficiently utilizing solid residues and residual liquid after co-fermentation of coal and corn straw, three coal samples combined with corn straw were used. Combustion tests were carried out using the solid residue to compare the changes in calorific value, slagging characteristics and air pollutant emission before and after fermentation. Biogas production experiments were performed using acclimated residual liquid to establish the biomethane production characteristics. Results show that the total calorific value of solid residue after co-fermentation of bituminous coal D, bituminous coal C and corn straw increased by 8.16% and 8.08%, respectively, compared to original samples. The slagging index also decreased during the combustion of solid residues. The diversity of bacterial community structure was the main causes for the significant reduction in air pollutant emissions from the combustion of solid residues. Acclimation experiments improved the removal rate of hemicelluloses from corn straw and increased biomethane production by 45.86%, 10.66% and 60.47%, respectively. This work can improve the clean conversion efficiency of coal and straw, not only reducing environmental pollution, but also transforming waste into clean energy, which has a positive impact on the environment and allows the sustainable use of resources.

Weak-base pretreatment to increase biomethane production from wheat straw.

Weak-base pretreatment of wheat straw was investigated for its ability to improve biomethane production. Anaerobic digestion (AD) was performed on wheat straw pretreated with 3%, 5%, or 7% Na2CO3 as a weak base. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR) spectra demonstrated disruption of lignocellulosic structures by pretreatment. In the 5% Na2CO3 treatment group, cellulose and hemicellulose were retained effectively, with efficient removal of lignin. The removal rates of cellulose, hemicellulose, and lignin were 27.9%, 20.4%, and 31.0%, respectively, after 5% Na2CO3 pretreatment. The methane content (53.3-77.3%) was improved in the 5% Na2CO3 treatment group, with maximum methane production (307.9L/kg VS) that was 41.6% higher than that of the untreated sample. Cellulose and hemicelluloses were degraded 59.3% and 56.3% after AD. It took 20days to reach 80% of the maximum cumulative methane production for the 5% Na2CO3 pretreatment group, which was 4days faster than the untreated group. These results indicate that 5% Na2CO3 pretreatment improve the lignocellulose structure of wheat straw, allowing better biodegradability of wheat straw in AD for increased biogas production, enhanced methane content, and decreased digestion time.

Biogas from Anaerobic Digestion: Power Generation or Biomethane Production?

Biogas is a fuel obtained from organic waste fermentation and can be an interesting solution for producing electric energy, heat and fuel. Recently, many European countries have incentivized the production of biomethane to be injected into natural gas grids or compressed and used as biofuel in vehicles. The introduction of an upgrading unit into an existing anaerobic digestion plant to convert biogas to biomethane may have a strong impact on the overall energy balance of the systems. The amount of biomethane produced may be optimized from several points of view (i.e., energy, environmental and economic). In this paper, the mass and energy fluxes of an anaerobic digestion plant were analyzed as a function of the biogas percentage sent to the upgrading system and the amount of biomethane produced. A numerical model of an anaerobic digestion plant was developed by considering an existing case study. The mass and energy balance of the digesters, cogeneration unit, upgrading system and auxiliary boiler were estimated when the amount of produced biomethane was varied. An internal combustion engine was adopted as the cogeneration unit and a CO2 absorption system was assumed for biogas upgrading. Results demonstrated that the energy balance of the plant is strictly dependent on the biomethane production and that an excess of biomethane production makes the plant totally dependent on external energy sources. As for the environmental impact, an optimal level of biomethane production exists that minimizes the emissions of equivalent CO2. However, high biomethane subsides can encourage plant managers to increase biomethane production and thus reduce CO2 savings.

Enhanced biomethane production from anthracite by application of an electric field

The utilization of biogenic coalbed methane (CBM) meets the increasing demand of energy and reduces the impact of greenhouse gases on the environment. Application of an electric field is a promising technology to increase biomethane production from various complex organic compounds. Here, the effects of an applied voltage (0.6 V, 1.2 V, and 1.8 V) on methane production due to coal biodegradation, as well as the structure of microflora and intermediates were studied by anaerobic culture, pyrosequencing, and GC–MS. The maximum methane production was observed at 1.2 V with an average of 63.0 μmol/g of coal, which was 134% higher than the control without electrodes (26.9 μmol/g of coal). The CO2 yield was negatively correlated with the methane yield, suggesting that more carbon in coal has been transferred to methane after application of an electric field. The pyrosequencing results showed that the abundance of bacteria with extracellular electron transfer abilities, such as Soehngenia, Desulfovibrio, and Deferrisoma, increased after applying a 1.2 V electric field. Moreover, the structure of the fungal communities changed significantly, especially the abundance of Mortierella, Thielavia, Humicola, and Plectosphaerella. According to the GC-MS results, most of the products detected at the end of the cultivation period in the control were not detected in the cultures cultivated at 1.2 V. New organic compounds such as alcohols, unsaturated fatty acids, and amines were produced in the cultures cultivated at 1.2 V. These results suggest that application of an electric field changes the structure of the microflora, which promotes extracellular electron transfer and the biodegradation of organic compounds, thus enhancing methane production from coal. Furthermore, the distinct organic products detected after cultivation and the significant changes in microbial communities suggest that the degrading metabolism of organic matter in coal might be altered due to the application of an electric field. Based on the results, a biogenic methane production system implemented by application of an electric field was proposed and described.

Improvement of biomethane potential of sewage sludge anaerobic co-digestion by addition of “sherry-wine” distillery wastewater

Abstract Co-digestion of sewage sludge (SS) with other unusually treated residues has been reported as an efficient method to improve biomethane production. In this work, Sherry-wine distillery wastewater (SW-DW) has been proposed as co-substrate in order to increase biomethane production and as a breakthrough solution in the management of both types of waste. In order to achieve this goal, different SS:SW-DW mixtures were employed as substrates in Biomethane Potential (BMP) tests. The biodegradability and biomethane potential of each mixture was determined selecting the optimal co-substrate ratio. Results showed that the addition of SW-DW as a co-substrate improves the anaerobic digestion of SS in a proportionally way in terms of CODs and biomethane production The optimal co-substrates ratio was 50:50 of SS:SW-DW obtaining %VSremoval = 54.5%; YCH4 = 225.1 L CH4/kgsv or 154 L CH4/kgCODt and microbial population of 5.5 times higher than sole SS. In this case, %VSremoval = 48.1%; YCH4 = 183 L CH4/kgsv or 135 L CH4/kgCODt. The modified Gompertz equation was used for the kinetic modelling of biogas production with successful fitting results (r2 = 0.99). In this sense, at optimal conditions, the maximum productivity reached at an infinite digestion time was ( Y CH 4 MAX ) = 229 ± 5.0 NL/kgSV; the specific constant was K = 25.0 ± 2.3 NL/kgSV·d and the lag phase time constant was (λ) = 2.49 ± 0.19.

Effect of Particulate Disintegration on Biomethane Potential of Particle-Rich Substrates in Batch Anaerobic Reactor

An investigation of particle disintegration was carried out using batch anaerobic reactors and a particle-rich substrate from pig manure supernatant. Two types of samples were applied, one high in suspended particles (raw feed) and another low in suspended particle content (centrifuged feed). Both feeds were digested with and without cellulase enzyme addition to obtain a better understanding of particle degradation mechanisms. An automatic methane potential test system (AMPTS) was used to carry out batch reactions at 35 °C. The raw feed with high-suspended solids had higher biomethane potential than the centrifuged feed but the conversion rate and methane yield was lower. The addition of cellulase increased biomethane production rates in both high- and low-particle content samples enhancing yield by 54% and 40%, respectively and converting 69% and 87% of feed chemical oxygen demand (COD), respectively. This implies that the feed particles have high contents of cellulose. This is also the case for the smaller particles remaining after centrifugation. Comparisons of anaerobic digestion model no. 1 (ADM1) simulations with experimental data reveal that classifying substrate particles into a fast and a slow degrading fraction with separate disintegration kinetics fit the experimental data better than lumping all particles into one parameter.

Two-stage thermophilic anaerobic co-digestion of corn stover and cattle manure to enhance biomethane production

Two-stage thermophilic anaerobic co-digestion of cattle manure and corn stover was conducted to increase biomethane production. The first stage pre-digestion of corn stover was studied based on the following treatment variables: corn stover to liquid fraction of digestate (CS:LFD) ratio (1:7, 1:10, 1:13, 1:14), digestion temperature (55 °C, 60 °C) and digestion time (3, 7, 14 days). The reduction in lignin, cellulose and hemicellulose (LCH) was between 3.97% and 11.98%, which increased the biodegradability of corn stover. Corn stover pre-digested with a CS:LFD ratio of 1:10 at 55 °C for a period of 3 and 7 days was subjected to anaerobic co-digestion with cattle manure. The highest biomethane yield was observed on day 21 with a value of 357.41 mL/g volatile solids (VS) for untreated corn stover, 446.84 mL/g VS for corn stover pre-digested for 3 days and 518.58 mL/g VS for corn stover pre-digested for 7 days with LFD. The VS conversion efficiency for co-digestion of cattle manure with untreated corn stover, corn stover pre-digested for 3 days and 7 days was 42.8%, 43.3% and 51.8%, respectively, on day 21, which was higher than that (34.0%) of cattle manure only.