Highest Methane Yield Research Articles

Boosting thermophilic anaerobic digestion with conductive materials: Current outlook and future prospects

Thermophilic anaerobic digestion (TAD) can provide superior process kinetics, higher methane yields, and more pathogen destruction than mesophilic anaerobic digestion (MAD). However, the broader application of TAD is still very limited, mainly due to process instabilities such as the accumulation of volatile fatty acids and ammonia inhibition in the digesters. An emerging technique to overcome the process disturbances in TAD and enhance the methane production rate is to add conductive materials (CMs) to the digester. Recent studies have revealed that CMs can promote direct interspecies electron transfer (DIET) among the microbial community, increasing the TAD performance. CMs exhibited a high potential for alleviating the accumulation of volatile fatty acids and inhibition caused by high ammonia levels. However, the types, properties, sources, and dosage of CMs can influence the process outcomes significantly, along with other process parameters such as the organic loading rates and the type of feedstocks. Therefore, it is imperative to critically review the recent research to understand the impacts of using different CMs in TAD. This review paper discusses the types and properties of CMs applied in TAD and the mechanisms of how they influence methanogenesis, digester start-up time, process disturbances, microbial community, and biogas desulfurization. The engineering challenges for industrial-scale applications and environmental risks were also discussed. Finally, critical research gaps have been identified to provide a framework for future research.

CO2 methanation on γ-Al2O3 nanoparticles from waste aluminium: Effect of Ba promoter on sulfur impurities

The inferior activity of γ-Al2O3 (GAL1) from the waste aluminium for CO2 methanation was improved using Ba promoter. Ba promoted CO2 conversion to ∼82% with 100% methane selectivity at 400 °C (Ni-Ba/GAL1), compared to ∼10% conversion without Ba (Ni/GAL1). Acid digestion of waste aluminium formed residual Al2(SO4)3 at ∼0.5% atomic weight, forming the detrimental NiAl2O4 under reaction conditions. Ba inhibited NiAl2O4 formation by producing BaAl2O4, increasing Ni-Ba/γ-Al2O3 stability for 50 h at 400 °C. In-situ DRIFTS analysis revealed that NiAl2O4 caused CO2 methanation via CO2 dissociation into CO, followed by high temperature hydrogenation to CH4. Ba promotes catalytic activity and stability of alumina with sulfur impurities by maintaining Nio oxidation states to catalyze CO2 hydrogenation to formate at low temperatures, producing a high methane yield. In addition to transforming waste aluminium into a valuable material, investigation on γ-Al2O3 catalytic activity expands its potential use as a catalyst for CO2 methanation.

Semi-continuous anaerobic co-digestion of thermal and thermal-alkali processed organic fraction of municipal solid waste: Methane yield, energy analysis, anaerobic microbiome

The semi-continuous anaerobic co-digestion (AcoD) of thermal and thermal-alkali pretreated organic fraction of municipal solid waste (OFMSW) and sewage sludge (SS) was studied under varying hydraulic retention times (HRT) and organic loading rates (OLR Three semi-continuous digesters were operated under control (non-pre-treated), thermally pretreated (125 °C), and thermal-alkali pretreated (125°C-3g/L NaOH) conditions at variable OLRs at 2.5, 4.0, 5.1, and 7.6 kgVS/m3.d and corresponding HRTs of 30, 20, 15, and 10 days. The 10 and 43% higher methane yield (0.445 m3/kgVS) and 11 and 57% higher VS removal (52%) was achieved for thermal-alkali pretreated digester at 5.1 kgVS/m3.d OLR over thermally pretreated (0.408 m3/kgVS, 45% VS removal) and control digesters (0.310 m3/kgVS, 33% VS removal), respectively. Thermal and thermal-alkali digesters failed on increasing the OLR to 7.6 kgVS/m3.d, whereas the control digester becomes upset at 5.1 kgVS/m3.d OLR. The metagenomic study revealed that Firmicutes, Bacteroidetes, Chloroflexi, Euryarchaeota, Proteobacteria, and Actinobacteria were the predominant bacterial population, whereas Methanosarcina and Methanothrix dominated the archaeal community. Energy balance analysis revealed that thermal alkali pretreatment showed the highest positive energy balance of 114.6 MJ/ton with an energy ratio of 1.25 compared with thermally pretreated (81.5 MJ/ton) and control samples (−46.9 MJ/ton). This work pave the way for scaleup of both thermal and thermal-alkali pre-treatment at 125 °C to realize the techno-economic and energy potential of the process.

Anaerobic Co-digestion of sewage sludge and organic fraction of municipal solid waste: Focus on mix ratio optimization and synergistic effects

The utilization of municipal solid waste (MSW) and sewage sludge (SS) as a source of renewable energy is crucial in achieving sustainable and integrated MSW management. SS and organic fraction of municipal solid waste (OFMSW) can be anaerobically digested to produce methane for energy. However, anaerobic digestion of specific substrates is challenging with respect to substrate characteristics. The problem of mono-digestion can be mitigated by co-digestion of these two major organic wastes because of their complementary characteristics. Moreover, there is a lack of studies on optimization of different mix ratios of organic fraction of municipal solid waste (OFMSW) and sewage sludge (SS) based on total solids (TS). The study aims to optimize the mix ratio for anaerobic co-digestion of OFMSW and SS. The study further elucidates synergistic effects associated with the co-digestion process. Different mix ratios of SS and OFMSW (0:100, 20:80, 40:60, 60:40, 80:20, 100:0) at 5, 7.5 and 10% TS were assessed for biomethane potential assessment. The results showed that with an increase in SS in the mix ratio feed the methane yield increased by 91% and 50% as compared to mono digestion of sewage sludge and OFMSW respectively at TS 7.5%. Based on the kinetic analysis, it was observed that the lag phase reduced for 60:40 mix ratio leading to higher rate of biodegradation. Positive synergistic effects were observed for 40:60, 60:40 and 80:20 mix ratio of SS:OFMSW based on co-digestion impact factor value. Response surface modelling was used to get the optimized mix ratio and TS %. The optimum mix ratio with the highest methane yield (388 ml/gVS added) was 70:30 (SS: OFMSW) at TS 7.5% with a desirability value of 0.98. These findings demonstrate that co-digesting SS and OFMSW is a preferable alternative for harnessing renewable energy and managing organic waste in a sustainable manner.

Energy-saving pretreatments affect pelagic Sargassum composition and DNA metabarcoding reveals the microbial community involved in methane yield.

Sargassum spp. flood the Caribbean coastline, causing damage to the local economy and environment. Anaerobic digestion (AD) has been proposed as an attractive option for turning macroalgae into valuable resources. Sargassum spp. has a complex composition that affects the microbial composition involved in AD which generates a low methane yield. This study aimed to improve the methane yield of pelagic Sargassum, using different energy-saving pretreatments and identifying the microbial community associated with methane production. We applied different energy-saving pretreatments to algal biomass and assessed the methane yield using a biomethane potential (BMP) test. The microbial communities involved in the AD of the best- and worst-performing methanogenic systems were analyzed by high-throughput sequencing. The results showed that pretreatment modified the content of inorganic compounds, fibers, and the C:N ratio, which had a strong positive correlation with BMP. The water washing pretreatment resulted in the best methane yield, with an increase of 38%. DNA metabarcoding analysis revealed that the bacterial genera Marinilabiliaceae_uncultured, DMER64, Treponema, and Hydrogenispora, as well as the archaea genera Methanosarcina, RumEn_M2, Bathyarchaeia, and Methanomassiliicocus, dominated the microbial community with a high methane yield. This study is the first to demonstrate the microbial community structure involved in the AD of Sargassum spp. The pretreatments presented in this study can help overcome the limitations associated with methane yield.

Co-digestion of mushroom compost with switchgrass using solid-state anaerobic digester

Spent mushroom compost (SMC) already broken down into smaller particles by fungal action is an ideal material for producing biogas. Two cycles of five solid-state anaerobic digesters (SS-ADs) with different mix-ratio of SMC and switchgrass (SG) were operated at feedstock-to-effluent ratio of 2 at a temperature 35 ± 2°C. The total solids concentration of the digester was kept at ∼17%. Initial biogas production observed during the start-up of the digester confirmed the presence of readily available extractives for digestion. In the first cycle, the highest methane yield was observed in SMC 0 (0% SMC + 100% SG) of 28.82 l/kg VS/d and the lowest yield was observed in SMC 4 (100% SMC + 0% SG) as 10.32 l/kg VS/d. The substrate containing 100% SG (SMC 0) recorded the highest cumulative biogas yield of 295.43 l/kg VS in 63 days. The digesters with higher SMC fraction showed lower methane production, low pH value and high volatile fatty acids content upon decomposition. The SS-ADs having SMC/SG of 50 : 50 showed more than 2 times methane production in comparison with SS-ADs having SMC as sole substrate. An estimation of volumetric productivity also established a linear relationship with the SMC/SG ratio.

An anaerobic dynamic membrane bioreactor (AnDMBR) system enhanced the biogas conversion efficiency and stability of mesophilic codigestion with waste activated sludge and food waste

To enhance the substrate conversion efficiency and further improve the reactor stability of codigestion with waste activated sludge (WAS) and food waste (FW), a new type of anaerobic dynamic membrane bioreactor (AnDMBR) was designed and operated in parallel with a conventional continuous stirred tank reactor (CSTR) for approximately 500 days. The results showed that a higher methane yield (330–350 mL/g COD) was realized in the AnDMBR than in the CSTR (96–326 mL/g COD) under the same organic loading rates (OLRs) in the range of 8.16–22.8 g COD/L/d, which was consistent with the high methane production, specific methanogenic activity, coenzyme F420 concentration, and stable pH in the AnDMBR. Furthermore, the dynamic membrane performance of the AnDMBR was compared with that of the CSTR system; the measured characteristics, including the particle diameter and suspended solid concentration in the permeate or discharge sludge, suggested that the substrate and microorganisms can both be effectively intercepted by the dynamic membrane during long-term periodic operation. 16S rRNA high-throughput sequencing analysis indicated that the microbiological community structures in the AnDMBR and CSTR were similar, but the relative abundances of dominant bacteria (Treponema and Lutispora) and archaea (Methanoculleus) in the AnDMBR were both higher than those in the CSTR. This difference in relative abundance is the main reason why the AnDMBR enhanced the biogas conversion efficiency and stability of mesophilic codigestion with WAS and FW, other than substrate retention by the dynamic membrane. A comparison of the microbial metabolism functions of the AnDMBR and CSTR showed that the proportions of carbohydrate/coenzyme transport and metabolism and signal transduction mechanisms in the AnDMBR with stainless steel mesh were significantly improved after long-term parallel operation.

Enhancement of biomethane potential of brown sludge by pre-treatment using vortex based hydrodynamic cavitation

Novel, non-thermal and economically benign pre-treatment process was developed for enhancing valorisation potential of brown sludge generated by dairy industry wastewater treatment plant (WWTP). Vortex-based hydrodynamic cavitation (HC) device was used to quantify influence of pretreatment by measuring biomethane potential (BMP) of untreated and treated brown sludge. Pre-treatment parameters, primarily, pressure drop and number of passes through the cavitation device were varied to quantify influence on BMP. BMP tests were performed at 39 °C containing 5% of total solids in each reactors using an automatic BMP measurement system containing 15 reactors with each volume of 500 mL fitted with overhead stirrer. HC treatment increased the soluble chemical oxygen demand (sCOD) by more than 25% which increased the BMP. HC treatment was able to push the BMP of treated sludge to more than 80% of the theoretical BMP. Volatile solids (VS) removal was more than 65%. Highest methane yield was 376 mL/g-VS of sludge. The methodology and results presented here show significant potential to valorise brown dairy sludge via vortex based hydrodynamic cavitation.

The Influence of the Ultrasound Disintegration of Microalgal–Bacterial Granular Sludge on Anaerobic Digestion Efficiency

It has been proven that the biocenosis of microalgae and bacteria improves the chemical properties of biomass for its use in anaerobic digestion. However, this anaerobic digestion can be limited by the strong, compact, and complex structure of granulated biomass. Therefore, there is a need to search for an effective method for microalgal–bacterial granular sludge pretreatment, which has not been undertaken in previous scientific works. In this study, ultrasonic pretreatment was used to determine the effects of sonication on anaerobic digestion efficiency. Anaerobic digestion was performed in batch respirometric reactors. It was found that the ultrasonic pretreatment enhanced the biomass solubility; thus, the organic matter concentration increased more than six times compared to the variant without pretreatment. The study showed a positive effect of sonication on the kinetics of the anaerobic process and methane production. The highest methane yield was found in the variants in which the ultrasonication lasted from 150 s to 200 s, and this yield was from 534 ± 16 mL CH4/g VS to 561 ± 17 mL CH4/g VS. The data analysis confirmed strong correlations between the pretreatment time, the amount of biogas and methane production, and the gross energy gain. The highest net energy output and net energy gain were obtained for 150 s of sonication, and, respectively, were 4.21 ± 0.17 Wh/g VS and 1.19 ± 0.18 Wh/g VS.

Methane hydrate exploitation characteristics and thermodynamic non-equilibrium mechanisms by long depressurization method

Methane hydrate as a clean energy source has attracted worldwide attention. Yet, thermodynamic evolution of methane hydrate-bearing deposits during long depressurization are still unclear. In this study, five methane yields (1.17, 0.77, 0.41, 0.21 and 0.13 Ls/min) for exploitation were conducted to simulate long depressurization of thousands of minutes. The rates of both depressurization and hydrate dissociation are linearly enhanced by increasing methane yield. Icing is always observed when the pressure reaches approximately 2.4 MPa unless the hydrate dissociation already finished. The accelerating effect of icing on hydrate dissociation can be strengthened by high methane yields. Thermodynamic responses at different exploitation stages are different because of the varied combinations of some non-equilibrium processes including gas expansion, hydrate dissociation, icing and melting as well as heat transfer. In this study, there are four thermodynamic non-equilibrium phenomena, respectively are gas expansion production, non-isothermal heat transfer, hydrate stationary dissociation and ice isothermal melting. The thermodynamic controlling mechanism and influencing factors of different exploitation stages are analyzed qualitatively or quantitatively, which reveals the temperature response of methane hydrate exploitation during long depressurization process. These results are helpful for understanding complex coupling mechanism of phase change and heat and mass transfer in methane hydrate exploitation.

Integrated genome-centric metagenomic and metaproteomic analyses unravel the responses of the microbial community to ammonia stress

Ammonia is a major inhibitor in anaerobic digestion of nitrogen-rich organic wastes. In this study, integrated genome-centric metagenomic and metaproteomic analyses were used to identify the key microorganisms and metabolic links causing instability by characterizing the process performance, microbial community, and metabolic responses of key microorganisms during endogenous ammonia accumulation. The identification of 89 metagenome-assembled genomes and analysis of their abundance profile in different operational phases permitted the identification of key taxa (Firmicutes and Proteobacteria) causing poor performance. Metabolic reconstruction indicated that the key taxa had the genetic potential to participate in the metabolism of C2C5 volatile fatty acids (VFAs). Further investigation suggested that during Phase I, the total ammonia nitrogen (TAN) level was maintained below 2000 mg N/L, and the reactor showed a high methane yield (478.30 ± 33.35 mL/g VS) and low VFAs concentration. When the TAN accumulated to > 2000 mg N/L, acid accumulation, mainly of acetate, began to occur, and the methane yield gradually decreased to 330.44 mL/g VS (Phase II). During this phase, the VFA degradation functions of the community were mainly mediated by Firmicutes. Approximately 61.54% of significant differentially expressed proteins (DEPs) related to acetate metabolism in Firmicutes were down-regulated, which led to an increase in acetate concentration to 4897.91 ± 1558.96 mg/L. However, the reactor performance showed spontaneous recovery without any interference (Phase III), during which Firmicutes gradually adapted to the high ammonia conditions. Approximately 75% of the significant DEPs related to acetate metabolism of Proteobacteria were also up-regulated in Phase III compared with Phase II; thus, VFA-related metabolic functions of the community were enhanced, which resulted in a decrease in the total VFA concentration to 195.39 mg/L. When the TAN increased above 4000 mg N/L, the system gradually showed acid accumulation dominated by propionate, accompanied by a second decrease in methane yield (Phase IV). During this phase, the number of up-regulated and down-regulated proteins related to acetate metabolism of Firmicutes and butyrate/valerate metabolism of Proteobacteria was comparable with that of Phase III, indicating that the metabolic functions related to acetate, butyrate, and valerate of the microbial community were not significantly affected. However, for propionate metabolism, the expression activity of fumarate hydratase from Firmicutes and Proteobacteria was severely inhibited by ammonia, as shown by down-regulation ratios of 63.64% and 85.71%, respectively. No protein with the same function that was not inhibited by ammonia could be detected, and the fumarate degradation function of the microbial community was severely damaged, leading to blocked propionate metabolism and irreversible deterioration of reactor performance. This study has provided a new perspective on the microecological mechanisms of ammonia inhibition.

Improved biogas production from tobacco processing waste via biochar-assisted thermophilic anaerobic digestion

Tobacco processing generates large amounts of waste, which can potentially serve as feedstock for the biogas production by anaerobic digestion (AD) technology. In this study, the thermophilic AD of flue-cured tobacco leaf waste with a high solid content was conducted, and the effects of biochar on biogas production at different organic loads were evaluated. The fermentation performance and microbial community with biochar addition (BAT) and without biochar addition (WBA) were thoroughly compared to explore the biochar-mediated enhancement mechanisms. Biochar addition enhanced methane production by 1.12–1.20 fold at the tested organic loads. Fermenting a low organic load (seed sludge/feedstock ratio: 2.0) in the BAT had the highest methane yield (120.4 mL/g-volatile total solids). Biochar addition significantly shortened the lag period of methane production and increased the methane production rate with an increasing organic load. The cellulolytic and fermentative bacterial genus Hydrogenispora, polysaccharide-degrading and syntrophic acetate-oxidizing bacterial genus norank_o_MBA03, and hydrogenotrophic methanogens were crucial for thermophilic flue-cured tobacco leaf digestion. Biogas production was enhanced by Hydrogenispora and Methanoculleus enrichment. These results demonstrate effective methane production from flue-cured tobacco leaves via biochar-assisted thermophilic fermentation with a high solid content and provide a useful reference for tobacco waste treatment.

Ethanol and Methane Production from the Organic Fraction of Municipal Solid Waste in a Two-Stage Process

In the present study, the OFMSW was utilized for ethanol and methane production in a two-stage process. First, the OFMSW was fermented by adjusting different levels of total solids (10%, 20%, and 29%) and a yeast dose (6, 12, and 24 g/kgOFMSW) for bioaugmentation with Saccharomyces cerevisiae. A control treatment without bioaugmentation was used for benchmarking. In the self-fermentation of the OFMSW with its indigenous microbiota, lactic acid was mainly produced, followed by ethanol and acetic acid, where a maximum ethanol production of 113 ± 2 gCOD/kgTS, corresponding to 10.8 ± 0.4 g/L, at 20% of total solids was observed. The results showed that in all treatments with bioaugmentation, regardless of the content of total solids, the main metabolites were ethanol, lactic acid, and acetic acid, which represented more than 95% of the total metabolites. The ethanol concentration was increased by increasing the total solids and the yeast dose. The highest ethanol production was 407 ± 3 gCOD/kgTS with the bioaugmentation of 24 g/kgOFMSW and 20% of total solids, whereas the maximum ethanol concentration of 43.4 ± 0.1 g/L was obtained with bioaugmentation of 12 g/kgOFMSW and 29%. The methane potential of the raw OFMSW was 329 ± 5 NL-CH4/kgVS, whereas the solid digestates resulting from the fermentation exhibited similar or even higher methane yields than raw OFMSW. A maximum methane potential of 464 ± 2 NL-CH4/kgVS from the solid digestate obtained after fermentation of the OFMSW with bioaugmentation of 6 g/kgOFMSW of yeast and 10% of total solids was achieved. Thus, large amounts of ethanol and methane were achieved without the application of pretreatment and/or enzymatic hydrolysis, which implies a low-cost alternative for the production of biofuels.

Approach to anaerobic bio-degradation of natural and synthetic fabrics: Physico-chemical study of the alteration processes

In this paper, the mesophilic Biochemical Methane Potential of several fabrics was assessed at different Total Solid concentrations (1–4%TS). Physico-chemical techniques were applied to explore the arising structural changes on fibers during the anaerobic digestion process. Additionally, the modified Gompertz model was used to assess and compare the AD performance of the fabrics. In cellulose-based fibers the production of biogas was enhanced thanks to the easy solubilization of acetate, which is generated upon partial breakage of cellulose bonds. The crystallinity of vegetal fibers decreased significantly from day 19. The highest methane yields were attained for silk and wool fabrics at the lowest TS concentrations. Conformational changes in fibroin and keratin were detected. The highest degrees of degradation were observed in solid samples with lower solid concentrations. Accordingly, the maximum methane yields were reported in the reactors operating with lower TS.

Application of Anaerobic Co-digestion of Brewery by-Products for Biomethane and Bioenergy Production in a Biorefinery Concept

This study investigated the anaerobic co-digestion (AcoD) of brewery by-products for biomethane and bioenergy recovery, focusing on operational performance evaluation, kinetic analysis, microbial metataxonomic, and metabolic function prediction. The biochemical methane potential was conducted under mesophilic (35 °C) and methanogenic conditions (pH 7.5) by mixing brewery wastewater and sludge from the brewery wastewater treatment plant (1:1, v/v), following the addition (2.5 – 12.5 %, w/v) of brewer’s spent grains (BSG). The results demonstrate that the highest methane yield (88.02 mL CH4/g TVS) was obtained with 12.5 % BSG, which was 20.66-fold higher than the control reactor operated with wastewater and sludge (4.26 mL CH4/g TVS). The bioenergy recovery from biomethane could generate electricity (0.348 kWh/kg TVS) and heat (1556 MJ/kg TVS), avoiding greenhouse gas emissions (0.114 kg CO2-eq/kg TVS). The microbial community dynamics revealed a predominance of Halobacterota, Chloroflexi, and Euryarchaeota phylum. The genera Methanosaeta and Methanobacterium, and the Anaerolineaceae family predominated in the AcoD process. The metabolic function prediction showed the presence of genes (K01895, K00193, K00625, and K00925) associated with the direct activation of acetate in the acetoclastic pathway and methane production. Finally, the data obtained provide a perspective on using brewery by-products for bioenergy production in a biorefinery concept, reducing the environmental impacts and contributing to the circular bioeconomy transition of the beer industry.Graphical abstract

Possibilities for Anaerobic Digestion of Slaughter Waste and Flotates for Biomethane Production

Anaerobic digestion for biomethane production is an important tool regarding sustainable energy production. The objective of this study was to investigate the effects of the substrate composition and operating parameters on biomethane production during anaerobic digestion, focusing on the use of flotates and slaughterhouse waste as substrates with a high organic content. A novelty here was the use of a moving bed biofilm reactor (MBBR) with circulation pump for the anaerobic treatment of flotates, slaughter waste (SW), and their mixture. Flotates and waste from slaughterhouses offer a substrate with a high organic content. In this work, it was shown that both substrates provide a high biochemical methane potential (BMP). The highest methane yield was achieved by mixing both substrates. In continuous operation, special challenges arose, due to the high nitrogen and fat content of the substrates. These could be overcome by mixing the substrates and using a circulation pump in the reactor for improved back-mixing. As a result, the highest average methane yield of 0.65 NLCH4·gTS eli−1 was achieved in mesophilic operation at an organic loading rate (OLR) of 4.2 gTS·L−1·d−1.

Impact of natural degradation of the invasive alga Rugulopteryx okamurae on anaerobic digestion: Heavy metal pollution and kinetic performance

This study shows, for the first time, how the natural biodegradation of the Phaeophyceae Rugulopteryx okamurae (R.o.) affects its methane yield, by biochemical methane potential assays, and the methane production kinetics. Additionally, a mechanical (zeolite-assisted milling) and a thermal (120 °C, 45 min) pretreatments were assessed. The highest methane yield was obtained from the mechanically pretreated fresh ashore biomass (219 (15) NLCH4 kgVS−1), which presents the use of zeolite during milling as an economical alternative for heavy metal toxicity reduction. Moreover, no significant differences were observed between the other tests (with the exception of the lowest value obtained for the mechanically pretreated fresh R.o.). Low methane yields were linked to the heavy metal content. However, an increase of 28.5 % and 20.0 % in the k value was found for the untreated fresh R.o. biomass and fresh ashore biomass, respectively, when subjected to thermal pretreatment. Finally, an enhancement of 80.5 % in the maximum methane production rate was obtained for the fresh ashore biomass milled with zeolite compared to the untreated fresh ashore biomass.

Unveiling the synergy of Chlorella sp. and cattle manure co-digestion under high feeding load

The synergy in anaerobic co-digestion (AcoD) of cattle manure (CM) and Chlorella sp. (Ch) Under high feeding loads (50 g VS/L) was investigated with different mixing gradients in terms of organic constitute, process parameters, and microbial community structure. AcoD enhanced the methane yield of CM by 9.3%–22.8% and Ch by 3.9%–16.7%, compared with their mono-digestion. The suitable protein/carbohydrates ratio was the important reason for having such a synergetic effect between carbohydrates-rich and protein-rich feedstock. AcoD of CM and Ch at the ratio of 8:2 showed a high methane yield enhanced, the fastest methane production rate, and a high relative abundance of Methanosaeta, which may be an available matching strategy for further application. The relative abundance of acetoclastic methanogen decreased with the increase of total ammonia nitrogen (TAN), and the fast methane production rate is associated with the syntrophism and high relative abundance of Syner-01 and Methanosaeta.

Comparison of Anaerobic Co-Digestion of Buffalo Manure and Excess Sludge with Different Mixing Ratios under Thermophilic and Mesophilic Conditions

In this study, the main aim is to evaluate the mixing ratio of co-digestion of buffalo manure (BM) and excess sludge (ES) influenced for methane yield and digestate dewaterability. Five batch experiments with different BM and ES mixing ratios were carried out under thermophilic and mesophilic conditions. The methane yield of co-digestion of BM and ES increased by 10.1–73.5% under thermophilic conditions and 87.9–153.3% under mesophilic conditions, compared with the mono-anaerobic digestion of ES under the same conditions. Shannon and Chao1 indices showed that the bacterial species of the mesophilic digesters were more abundant than that of the thermophilic digesters. With the increase in the BM proportion in the substrate, the normalized capillary suction time (NCST) and total solids (TS) of sediment (centrifugal dewatering) increased. The NCST at thermophilic temperature (8.98–12.54 s∙g−1-TS) was greater than that at the mesophilic temperatures (5.45–12.32 s∙g−1-TS). However, the TS of sediment was not directly related to the digestion temperature. This study has shown that anaerobic co-digestion of BM and ES at the appropriate ratio (BM/ES = 1:1.5) has a significant meaning in a high methane yield.

Methane yield of paper industry waste in the presence of two compounds from alcohol and aldehyde groups during thermophilic anaerobic digestion

In this study, effect of two chemical compounds (i.e., 1-octanol and hexanal) respectively from the alcohol and aldehyde groups on thermophilic (55±2 °C) anaerobic process digesting the waste produced at a paper industry was investigated. In this scope, possible inhibition was monitored by the cumulative methane (CH4) yields in the batch reactors digesting the paper waste as the feedstock at concentrations of 0.005%, 0.05%, and 0.5% for each compound. Comparing the effects of the two different groups with the control reactor having only the paper waste as the substrate, the results revealed that adding 1-octanol and hexanal up to 0.05% concentrations had some synergistic effect on biogas yield (i.e., from 3% to 12% enhancement). Accordingly, the highest methane yields were 550 and 567 mL/g-VSfed, respectively on average in the presence of 1-octanol and hexanal at a concentration of 0.05% while the cumulative methane yield was observed as 490 mL/g-VSfed for the control reactor. With the exception of 1-octanol at 0.5%, adding both compounds at each investigated concentration was beneficial for the digestion in the batch process. Therefore, the selected alcohol and aldehyde sources did not cause the expected detrimental effect on the methanogens even at the maximum amounts added in this study. Nevertheless, since the effect of the chemical compounds on methane generation has been generally concentration-dependent, the toxic effects of 1-octanol and hexanal would be better observed at higher concentrations (>0.5%), especially when their threshold levels are exceeded in anaerobic reactors digesting paper wastes.