Days Of Anaerobic Digestion Research Articles

Overcoming yard waste recalcitrance through four different liquid hot water pretreatment techniques – Structural evolution, biogas production and energy balance

Liquid hot water (LHW) pretreatment is considered as a key step in reducing biomass recalcitrance and improving its accessibility to microorganisms during anaerobic digestion. In the present study, a comprehensive comparison among hot air oven, hot water bath, autoclave and microwave (MW) pretreated yard waste (YW) was conducted. The effect of treatment temperature and duration for all the four LHW pretreatment techniques were optimized with respect to the organic matter solubilisation. The results showed that all four LHW pretreatment techniques have a positive effect on organic matter solubilisation. The highest organic matter solubilisation was found in case of MW pretreatment for a treatment duration of 100 s at a temperature of 140 °C. Therefore, batch biochemical methane potential (BMP) of untreated and MW pre-treated YW was carried out for this treatment condition. The BMP of MW pretreated YW resulted in an improvement in methane yield to 364.5 ± 11 mL/g VS on 26th day of anaerobic digestion, which was 1.1 times higher than that of untreated (328.9 ± 15 mL/g VS on 45th day of anaerobic digestion). The FTIR and XRD analysis confirmed that LHW pretreatment technique removed amorphous constituents due to the polymerization of polysaccharides present in YW. In addition, SEM analysis confirmed that the LHW pretreatment led irregular and distorted structure of biomass which improved cellulose accessibility to anaerobic bacteria. A positive energy balance confirmed the possible applicability of MW pretreatment in real life projects.

Compositional changes of rice straw fibers after pretreatment with diluted acetic acid: Towards enhanced biomethane production

In the present study, diluted acetic acid was used for the pretreatment of rice straw in order to improve its bioconversion into biomethane. After pretreatment with 2% acetic acid for 24 h, cellulose and hemicellulose significantly reduced by 8.9% and 18.1%, respectively. The degradation of cellulose and hemicellulose resulted in significant increase of relative lignin ratio by 14.9%. Cumulative biomethane production after 28 days of anaerobic digestion of straw pretreated with 2%, 4% and 6% for 24 h was 24.3%, 15.2%, and 18.1%, respectively, higher than the untreated sample. Longer pretreatment for 36 and 48 h showed the same trend with relatively low biomethane yield. Thus, 24 h pretreatment with 2% acetic acid improved biomethanation over other pretreatments. Scanning electron microscopy and Fourier transform infrared spectroscopy confirmed the partial degradation of rice straw and modification of lignin structure by un-bonding the ether chain within lignin molecules as a result of pretreatment. Pretreatment with 2% acetic acid for 24 h resulted in the maximum biomethane energy yield of 6.29 MJ kg−1 dry straw, 47.3% energy conversion efficiency and the maximum net profit of US$ 106.4 ton−1 dry straw. In conclusion, application of 2% acetic acid for 24 h is a promising approach for high lignocellulose degradation efficiency at mild conditions due to the relatively short pretreatment time and enhanced biomethanation.

Energy production potential of phytoremediation plant biomass: Helianthus annuus and Silybum marianum

The use of the biomass of two plant species (Helianthus annuus and Silybum marianum) generated during the phytoremediation of a trace element contaminated soil was studied for the production of clean and renewable energy in laboratory experiments. Biogas production potential, aerobic biodegradability and calorific values of the plants were determined and compared with the results of the plants grown in non-contaminated soil to evaluate the effect of trace elements contamination on their potential use as feedstock for bioenergy production. The biogas production potential of the aerial vegetative biomass of S. marianum (194 - 223 ml g−1) was found to be higher than that from H. annuus (134 - 154 ml g−1), with no significant differences between non-contaminated and contaminated soils (average CH4 concentration: 63.1% for S. marianum and 65.7% H. annuus, respectively). The greatest biogas production was obtained from the seeds of H. annuus (356 - 473 ml g−1; 70.7% of CH4) after 9 days of anaerobic digestion. Also, the aerial vegetative biomass of H. annuus had higher calorific values than that of S. marianum (17.1 and 14.3 MJ kg−1, respectively). Combustion was the most efficient method for energy production compared to anaerobic digestion, while the presence of low concentrations of trace elements in plant biomass did not limit the potential energy recovery. These findings show the suitability of bioenergy production as an added value alternative for the management of plant biomass coming from phytoremediation processes.

Adding benefits to phytoremediation of sugar mill effluent by growing water hyacinth (Eichhornia crassipes): Evaluation of biomass for biogas production

In this experiment, we assessed the phytoremediation potential of water hyacinth (Eichhornia crassipes) for pollutants removal from sugar mill effluent amended at different concentrations and further biogas production from its grown biomass co-digested with cow dung in a laboratory scale anaerobic digester. The results showed that the maximum values of kinetic growth rate (2.56gg-1d-1), total chlorophyll content (4.10±0.10mg/gfwt.) and fresh plant biomass (393.87±4.67g/Kg) of E. crassipes were achieved in 75% concentration of sugar mill effluent after 60 days of phytoremediation experiments. Also, the enrichment factor (Ef) and bioaccumulation factor (Bf) of heavy metals were greater than or equal to 1 in the roots and leaves of E. crassipes which indicated efficient elimination of these metals from the sugar mill effluent. Significant values of cumulative biogas production (5195 ml) and predicted by modified Gompertz kinetic model (5238.71 ml) were found after 15 days of anaerobic digestion at 40°C with maximum reduction of COD (83.11%) of the substrate slurry. The plot of log(COD) vs. t (HRT) suggested good fitness of first order kinetic equation (R2= 0.9746) for reduction of co-substrate COD. The different kinetic parameters of COD reduction for biogas production viz., a, xc and k were noted as 6096.12, 7.73 and 0.26, with R2 value of 0.99, respectively. The findings of this study concluded that E. crassipes can be used for the phytoremediation of heavy metals and other pollutants most efficiently in 75% concentration of the sugar mill effluent. Additionally, the biomass of E. crassipes grown during phytoremediation can be used for enhanced biogas production.

Effect of COD/SO42- Ratio on Anaerobic Digestion of Penicillin Bacterial Residues

In order to assess the sulfate-induced inhibition of anaerobic digestion of antibiotic manufacturing bio-waste. The effect of COD/SO42- ratio on biogas production potential and substrate utilization characteristics during the acidogenic phase of anaerobic digestion of penicillin bacterial residues were investigated through batch experiments. The results obtained indicated that biogas production was gradually enhanced after 10 days of anaerobic digestion. However, the maximum cumulative methane production probably exceeded about 208 mL·g-1(on TS basis) since COD/SO42-≥3. Because adaptive acclimation, more than 71% COD removal, which may have been converted to methane, was achieved. Consequently, at COD/SO42-≤1.5, methane production was suppressed by 49% and 100% when the organics and SO42- removal rates were less than 17% and 5%, respectively. This indicated that methanogens and sulfate-reducing bacteria were inhibited at high sulfate loading rates. In addition, the COD balance analysis revealed that less than 9.1% of the COD was converted to methane. However, 5.0%-9.0% of the COD was used for sulfate reduction. This means that methanogens are more susceptible than sulfate-reducing bacteria to sulfide-induced inhibition. The S balance showed that the reduced sulfate was mainly present as sulfide in the digester. A small fraction of it was present as hydrogen sulfide in the biogas. The analysis of substrate utilization characteristics during the acidogenic phase revealed that methanation of soluble protein was initiated after methanation of soluble carbohydrate.

Pretreatment of waste activated sludge by peracetic acid oxidation for enhanced anaerobic digestion

This article studied the sludge reduction and supernatant changes during peracetic acid (PAA) oxidation and investigated subsequent anaerobic biodegradation. PAA oxidation treatment effectively leads to a disintegration of sewage sludge. The results suggested that the sludge solid concentration and volatile solid concentration decreased by 24.5 and 39.0% after 120 min, respectively, when the PAA dose was 30 mg PAA/g SS. The VSS to SS ratio dropped from 0.85 to 0.71. The concentration of supernatant soluble chemical oxygen demand, protein, total phosphorus, polysaccharide, and ammonia nitrogen increased by 530, 177, 166, 165, and 222%, respectively. Biogas production was enhanced when PAA was employed as a pretreatment. Compared with the control system, the cumulative biogas production increased by 20.0% after 28 days of anaerobic digestion (AD). The biogas production rate in digester with pretreated sludge reached the maximum on day 8 at 87mL/d, but the peak value was obtained on day 11 at 74mL/d in digester with the raw sludge, which indicated that PAA pretreatment could decrease the retention time of AD and increase biogas yield. © 2018 American Institute of Chemical Engineers Environ Prog, 37: 1901–1907, 2018

PRODUÇÃO DE BIOGAS POR DEJETO DE BOVINOS LEITEIROS SUBMETIDOS A PRÉ-TRATAMENTO QUÍMICO

Alkali and acid chemical pre-treatment for lignocellulosic biomass can affect the amount of energy harvested from it during the anaerobic digestion process. This study aimed to assess the biogas production of dairy cattle manure subjected to acid and alkaline pre-treatment. This assay was performed in two phases, which consisted of 10 days of pre-treatment and 120 days of anaerobic digestion. The substrates for pre-treatment and anaerobic digestion phase were calculated to attain 5% of total solids (TS). Pre-treatment phase was conducted during 10 days and the following experimental treatments were used: Control (Water); 2.5% of hydrochloric acid (HCl-2.5%), 2.5% of dolomite limestone (DI-2.5%), 5.0% of hydrochloric acid (HCl-5.0%) and 5.0% of dolomite limestone (DI-5.0%). After this phase, pH of all substrates had their values corrected to the start of the anaerobic digestion phase. Higher volatile solids reduction was attained by treatment Dl-5.0% whose biogas production and biogas yield per kg of waste was significantly higher compared to control treatment. Cattle manure pre-treated with 5.0% of dolomite limestone for 10 days incremented the biogas yield per kg of waste.

Enhanced Biogas Production and Dewaterability from Sewage Sludge with Alkaline Pretreatment at Mesophilic and Thermophilic Temperatures

This study investigated the biogas production and dewaterability of sewage sludge with alkaline pretreatment at mesophilic and thermophilic temperatures. The total suspended solids (TSS) and volatile suspended solids (VSS) of raw sludges were 21.1 ± 2.3 and 16.2 ± 1.5 g L−1, respectively. Raw sludges were pretreated at uncontrolled, pH 8, pH 10, and pH 12 under mesophilic (Mu, M8, M10, and M12) and thermophilic (Tu, T8, T10, and T12) conditions, respectively. All the pretreatments last 6 days. The pH of pretreated sludges was adjusted to the pH 7.0 prior to inoculating with mesophilic anaerobic digested sludge and undergoing 60 days of anaerobic digestion. The ultimate biogas yield of Mu, M8, M10, M12, Tu, T8, T10, and T12 was 296.8, 384.8, 339.9, 323.1, 376.6, 322.4, 271.5, and 258.1 mL g−1-VSadded, respectively. Both the pH of alkali treatment and temperature of thermal treatment affect the performance of anaerobic digestion. High hydrolysis pH (pH 10 and pH 12) resulted in high Na+ concentration (over 4000 mg L−1), and Na+ inhibitory effect reduced the ultimate biogas yield. The normalized capillary suction time (NCST) found in the treatments of M8 and Tu were 11.8 ± 1.1 to 23.4 ± 1.7 and 27.9 ± 5.4 to 111.8 ± 1.7 s g−1-TSS, respectively. The results suggest that both the pH of alkali treatment and temperatures of mild thermal treatment affect the performance of anaerobic digestion and sludge pretreated at pH 8.0 under mesophilic conditions could achieve high biogas yield and adequate dewaterability of digested sludge.

Valorization of tomato pomace by sequential lycopene extraction and anaerobic digestion

Tomato pomace, a major byproduct of tomato paste production, is an abundant solid waste stream from food processing in California. Tomato pomace is a rich source of lycopene, a red carotenoid and antioxidant, and lignocellulose, the recalcitrant but energy-rich polysaccharide matrix that comprises plant cell walls. Harvesting both of these co-products could add substantial value to the pomace and potentially reduce waste. In this study, lycopene was extracted from tomato pomace using a mixed organic solvent approach. Yields of lycopene from the tomato pomace tended to be higher than most literature values reported for raw tomatoes, and consistent with many reported values for lycopene in tomato pomace and other products. However, review of the current literature indicates that reported lycopene content of tomatoes products varies by roughly two orders of magnitude, which suggests a need for investigation of the factors responsible for this unusually wide range. After lycopene extraction, direct bioconversion to methane via anaerobic digestion and pretreatment with the ionic liquid 1-ethyl-3-methylimidazolium acetate ahead of anaerobic digestion were explored. Under certain conditions, especially 100 °C for 1 h, pretreatment was beneficial to enzymatic digestion of cellulose. Extraction resulted in a statistically significant reduction in methane yield compared to raw pomace after 90 days of anaerobic digestion. However, supplementation of extracted pomace with the non-lycopene-containing aqueous fraction from the extraction is expected to restore the methane yield to that of raw pomace based on measured values for chemical and biochemical oxygen demand. Ionic liquid pretreatment decreased methane production of extracted pomace.

Biorefinery of Safflower Seeds in a Sequential Process for Effective Use of the Substrate for Biofuel Production

Biofuels are a promising alternative to fossil fuels that addresses the associated environmental concerns and satisfies growing global energy demand. In this study, safflower seeds were used to produce multiple biofuels with an integrated biorefinery perspective. The safflower oil was used for biodiesel production while the resultant seed cake was employed in subsequent ethanol production. Pretreatment of the residual solid is necessary in order to improve the ethanol production yield. Contrasting costly chemical pretreatments, the seed cake was instead pretreated using incomplete anaerobic digestion to remove hemicellulose, increase cellulose accessibility for fermentation, and produce methane. The seed cake produced 124.8 ml/g-VS of methane after 20 days of anaerobic digestion. The digestate (biologically pretreated seed cake) produced 92 g ethanol/kg solid, in stark contrast to 42 g ethanol/kg solid from untreated seed cake. This ethanol was later utilized in biodiesel production. The safflower oil produced 85.9% biodiesel via ethanolic transesterification at optimum conditions. From the integrated biorefinery perspective, the seed cake from 1 kg of safflower seeds produced 70 l of methane at NTP and 51.0 g of ethanol—surpassing the 47.6 g of ethanol stoichiometrically necessary to complete transesterification of safflower oil. Thus, this ethanol could be recycled, largely decreasing the need for an external stream of ethanol for biodiesel production. The incomplete anaerobic digestion is an effective and environmentally friendly way to improve the ethanol production from the safflower seed cake with biomethane as an additional valuable byproduct.

Evaluation of Mixing Effects on Solid-state Anaerobic Digestion Performance of Dairy Manure and Sawdust Bedding Mixtures

The influence of mixing on biogas production and organic material removal performance of solid state anaerobic digestion of dairy manure and sawdust bedding mixtures was evaluated using 22 L volume lab-scale digesters. After 45 days of anaerobic digestion at 37°C, cumulative methane yield of unmixed test unit (73.1 N·mL/g-VS) was almost 1.3 times of that of mixed one (56.3 N·mL/g-VS). The biodegradable volatile solids removal rate of unmixed test unit was 67%, which was almost 28% greater than mixed one. Our results reveal that unmixed condition is better than mixed one in terms of biogas production and organic material reduction.

Sewage sludge pretreatment by microwave irradiation combined with activated carbon fibre at alkaline pH for anaerobic digestion

This research focuses on the effects of microwave-assisted activated carbon fibre (ACF) (MW-ACF) treatment on sewage sludge at alkaline pH. The disintegration and biodegradability of sewage sludge were studied. It was found that the MW-ACF process at alkaline pH provided a rapid and efficient process to disrupt the microbial cells in the sludge. The results suggested that when irradiated at 800 W MW for 110 s with a dose of 1.0 g ACF/g solid concentration (SS) at pH 10.5, the MW-ACF pretreatment achieved 55% SS disintegration, 23% greater than the value of MW alone (32%). The concentration of total nitrogen, total phosphorus, supernatant soluble chemical oxygen demand, protein, and polysaccharide increased by 60%, 144%, 145%, 74%, and 77%, respectively. An increase in biogas production by 63.7% was achieved after 20 days of anaerobic digestion (AD), compared to the control. The results indicated that the MW-ACF pretreatment process at alkaline pH provides novel sludge management options in disintegration of sewage sludge for further AD.

High-Rate Anaerobic Side-Stream Reactor (ASSR) Processes to Minimize the Production of Excess Sludge.

High-rate anaerobic side-stream reactor (ASSR) processes were developed to minimize excess sludge production during wastewater treatment. New ASSRs were operated in 2.5-day solids retention time (SRT), much shorter than 10-day SRT used by the commercial sludge reduction process. The 2.5-day was selected based on literature review and preliminary studies, showing that maximum solublization of key floc components, such as divalent cations, extracellular polymeric substances (EPS), and protease, occur within 2 to 3 days of anaerobic digestion. The laboratory reactor study showed that 2.5-day ASSR systems produced approximately 60 and 20% less sludge than the control (no ASSR) and the 10-day ASSR, respectively. The experimental systems showed acceptable effluent quality, despite minimal sludge wastage. This was possible because sludge EPS were continuously released/degraded and regenerated as sludge underwent recirculation between ASSR and the aerobic basin. The results supported that the activated sludge process incorporating small ASSRs significantly decrease the production of excess sludge during wastewater treatment.

Ultrasonic and Thermal Pretreatments on Anaerobic Digestion of Petrochemical Sludge: Dewaterability and Degradation of PAHs.

Effects of different pretreatment methods on sludge dewaterability and polycyclic aromatic hydrocarbons (PAHs) degradation during petrochemical sludge anaerobic digestion were studied. Results showed that the total biogas production volume in the thermal pretreatment system was 4 and 5 times higher than that in the ultrasound pretreatment and in the control system, and the corresponding volatile solid removal efficiencies reached 28%, 15%, and 8%. Phenanthrene, paranaphthalene, fluoranthene, benzofluoranthene, and benzopyrene removal rates reached 43.3%, 55.5%, 30.6%, 42.9%, and 41.7%, respectively, in the thermal pretreatment system, which were much higher than those in the ultrasound pretreatment and in the control system. Moreover, capillary suction time (CST) of sludge increased after pretreatment, and then reduced after 20 days of anaerobic digestion, indicating that sludge dewaterability was greatly improved after anaerobic digestion. The decrease of protein and polysaccharide in the sludge could improve sludge dewaterability during petrochemical sludge anaerobic digestion. This study suggested that thermal pretreatment might be a promising enhancement method for petrochemical sludge solubilization, thus contributing to degradation of the PAHs, biogas production, and improvement of dewaterability during petrochemical sludge anaerobic digestion.

Effect of Combined Microwave-Ultrasonic Pretreatment of Real Mixed Sludge on the Enhancement of Anaerobic Digester Performance

The anaerobic biodegradability of combined microwave-ultrasonic pretreated thickened excess activated sludge (PTEAS) mixed with raw primary sludge (PS) was investigated in this study. The pretreatment resulted in the enhancement of mesophilic anaerobic digester performance which in turn improved biogas production capacity and quality, total and volatile solid reduction, dewaterability, protein solubilisation and significant reduction of pathogens to produce class A biosolid. This study presented the results of two continuously stirred mesophilic anaerobic digesters charged with various proportions of a mixture of PTEAS and PS similar to the large-scale industrial practice. Digester 1 was charged with 75 % PTEAS and 25 % PS, while digester 2 was fed with 25 % PTEAS and 75 % PS. The methane production was 122 mL CH4/g total chemical oxygen demand for digester 2 after 20 days of anaerobic digestion. This amount further increased for both digesters with digestion time. The biogas quality in terms of methane to carbondioxide ratio (CH4/CO2) was significantly improved for digester 1 compared with digester 2 after 20 days of digestion. Volatile solid reduction of 76 and 57 % was achieved for digester 1 and digester 2 respectively after the same 20 days of digestion. The CH4/CO2 ratio reached 2.2:1 and 1.1:1 after 20 days of digestion for digester 1 and digester 2, respectively. Higher percentage of PTEAS increases the digestion kinetics, the methane production capacity and the biogas quality. Furthermore, total coliform reduction of 84 and 44 % was achieved for digester 1 and digester 2 respectively after 22 days of digestion. Hydrolysis rate and biochemical methane production were improved for both digesters based on the results of Gompertz kinetic model and the hydrolysis rate constants as determined by model fitting of the experimental data.

Extrusion as Pretreatment for Boosting Methane Production: Effect of Screw Configurations

A major barrier that limits degradation of the lignocellulosic materials during biogas production is the structure itself. Without pretreatment, enzymatic attack during hydrolysis is not effective and leads to a poor yield of biogas. Thus, in this study, the effect of extrusion as pretreatment on wheat straw and deep litter was evaluated. Five screw configurations of the extruder were tested, namely, mild kneading, long kneading, reverse, kneading and reverse, and kneading with reverse. Sugar availability and biogas potentials from extruded samples were examined. Energy consumption, barrel temperature, and residence time of samples during extrusion were measured. The results showed an increment in methane yields of about 4–29 and 1–16% of extruded samples after 28 and 90 days of anaerobic digestion, respectively. A strong positive correlation (R2 = 0.70) was observed between ultimate sugar availability and methane yields at 28 days. Increases in sugar availability (7–42%) accelerate degradation of the bio...

Extracellular polymeric substances and dewaterability of waste activated sludge during anaerobic digestion.

Anaerobic digestion of waste activated sludge was conducted to gain insight into the mechanisms underlying change in sludge dewaterability during its anaerobic digestion. Unexpectedly, the results indicated that sludge dewatering properties measured by capillary suction time only deteriorated after 10 days of anaerobic digestion, after which dewaterability recovered and remained stable. The loosely bound extracellular polymeric substance (LB-EPS) content increased three-fold after 20 days of anaerobic digestion, and did not change significantly during the remaining 30 days. The tightly bound EPS (TB-EPS) content reduced slightly after 20 days of anaerobic digestion, and stabilized during the last 30 days. Polysaccharides (PS) and proteins (PN) content in LB-EPS increased after 10 days of anaerobic digestion. However, PS and PN contents in TB-EPS decreased slightly. The relationship analysis showed that only LB-EPS correlated with dewaterability of the sludge during anaerobic digestion.

Effects of Nickel Ions on Anaerobic Co-Digestion of Cow Dung and <i>Phragmites australis</i>

The effect of nickel ions on the anaerobic digestion ofphragmites australisand cow dung were investigated in batch experiments. Five levels of nickel ions were set as 0, 0.2, 0.8, 1.4 and 2.0 mg Ni/L. The results indicated that anaerobic digestion started on about 3rdday 26 days of anaerobic digestion, the highest cumulative biogas was 32.70 mL/g·TS-1when nickel concentration was 0.8 mg Ni/L. The nickel ions in certain concentration range significantly affected biogas production rate, and effectively shorten reactor startup period. Biogas which was produced fromphragmitescombined with cow dung provided a new insight for bioenergy production whose? process can be promoted by nickel ions addition.

Effect of endogenous hydrolytic enzymes pretreatment on the anaerobic digestion of sludge

In this study, the effects of endogenous amylase, endogenous protease and combined amylase/protease pretreatment of sludge were studied to enhance the efficiency of sludge anaerobic digestion. These enzymes were obtained from bacterial fermentation and bacteria were separated from the sludge. All treatments improved sludge solubilization and acidification but had little influence on the floc sizes. In terms of sludge solubilization and acidification amylase was better than protease or mixed enzyme. After 7 h endogenous amylase treatment, the supernatant soluble chemical oxygen demand and volatile fatty acids concentration increased by 78.2% and 129.6%, respectively. But, in terms of anaerobic biodegradability, the best result was obtained with combined enzyme treatment, biogas production increased by 23.1% compared to the control after 11 days of anaerobic digestion. Scanning electron micrographs observation and particle size analysis revealed that the most important mechanism for the enzyme treatment of sludge might be solubilization of extracellular polymeric substances.

Enhancement of methane yield from wheat straw, miscanthus and willow using aqueous ammonia soaking

The increasing demand for methane production cannot be satisfied by the use of anaerobic digestion only from waste/wastewater treatment. Perennial energy crops, such as miscanthus and willow, as well as agricultural residues can be considered as options for increasing the methane production through biomass digestion, due to their high organic content and biomass yield. These materials present a great potential, which is only limited by the rigid lignocellulosic structure. In this case, it is possible to apply a pretreatment step in order to achieve increased biogas production. In the present study, aqueous ammonia soaking (AAS) has been investigated as a method to disrupt the lignocellulosic structure and increase the methane yield of wheat straw, miscanthus and willow. Among the three biomasses tested, wheat straw and miscanthus were the most promising in terms of methane production, yielding around 200 and 230 ml of methane per gram of total solids. In all three cases, AAS resulted to an increase in methane yield of 37–41%, 25–27% and 94–162% for wheat straw, miscanthus and willow, respectively. A comparison of the methane yields after 20 and 50 days of anaerobic digestion revealed that AAS affected positively the methane production rate as well. AAS also resulted to a low solubilization of sugars, with a 15.4% and 8.9% increase in soluble xylose concentration in miscanthus and willow, respectively, and a 5% solubilization of glucose in AAS-pretreated miscanthus.