Organosolv pretreatment was used to improve solid-state anaerobic digestion (SSAD) for methane production from three different lignocellulosic substrates (hardwood elm, softwood pine, and agricultural waste rice straw). Pretreatments were conducted at 150 and 180°C for 30 and 60 min using 75% ethanol solution as an organic solvent with addition of sulfuric acid as a catalyst. The statistical analyses showed that pretreatment temperature was the significant factor affecting methane production. Optimum temperature was 180°C for elmwood while it was 150°C for both pinewood and rice straw. Maximum methane production was 152.7, 93.7, and 71.4 liter per kg carbohydrates (CH), which showed up to 32, 73, and 84% enhancement for rice straw, elmwood, and pinewood, respectively, compared to those from the untreated substrates. An inverse relationship between the total methane yield and the lignin content of the substrates was observed. Kinetic analysis of the methane production showed that the process followed a first-order model for all untreated and pretreated lignocelluloses.
Worldwide concerns about the limitations of fossil resources, rising crude oil prices, and greenhouse gas (GHG) emissions have led researchers to seek alternative clean and renewable energy sources, for example, biofuels [
Biogas, produced during anaerobic digestion (AD) processes, can be used as a versatile source of energy to produce heat and electricity, either separate or combined, and to propel vehicles. The production of biogas offers other advantages, such as controlling organic waste, reducing greenhouse gas emissions, and producing another economically viable fertilizer [
Although different factors, for example, the crystallinity of cellulose and the accessible surface area, may play important roles in the bioconversion of lignocelluloses, the presence of lignin is apparently the most important factor affecting biodegradability [
The main objective of this study was to improve the performance of solid-state anaerobic digestion of three different types of lignocelluloses, that is, elmwood, pinewood, and rice straw, by applying organosolv pretreatments using ethanol under varying conditions. The effects of the pretreatment parameters, that is, temperature and duration time, on the methane yield were determined by solid-state batch anaerobic digestion assays. In addition, the kinetics of the degradation process was investigated for both untreated and pretreated substrates.
Elm, a hardwood, pine, a softwood, and rice straw, an agricultural waste, were used as substrates for biogas production. Elmwood and pinewood were obtained from the forest of Isfahan University of Technology (Isfahan, Iran), and rice straw (Sazandegi cultivar, Isfahan, Iran) was sourced from a field in Lenjan Province, Iran. Both elmwood and pinewood were debarked, cut into smaller pieces, and milled to obtain chips of less than 2 cm. The wood chips and the rice straw were partly ball-milled and screened to achieve powder with particle sizes between 295 and 833
Effluent of a 7000 m3 mesophilic anaerobic digester (Isfahan Municipal Sewage Treatment, Isfahan, Iran) was used as inoculum for the batch digestion assays. Due to its low TS content, the inoculum was centrifuged at 4500 rpm for 30 min to obtain the desirable TS content for the SSAD. The supernatant was discharged, and the remaining sludge was mixed to obtain a homogenous inoculum for SSAD. The inoculum was kept at 37°C for one week for stabilization.
Ethanol as an organic solvent together with sulfuric acid as catalyst was used for the pretreatments. A predetermined amount of each feedstock was mixed with 75% (v/v) aqueous ethanol solution supplemented with 1% w/w (based on dry mass) sulfuric acid to obtain a solid-to-liquid ratio of 1 : 8 (based on dry mass). The pretreatments were carried out in a 500 mL high-pressure stainless steel batch reactor [
The untreated and pretreated elmwood, pinewood, and rice straw (1 g dry mass) were mixed with a predetermined amount of inoculum and deionized water to achieve a feed-to-inoculum ratio (F/I) (based on volatile solids (VS) content) of 3 and initial TS content of 21%. Sealable 118 mL glass reactors were used for the anaerobic digestion assays. Anaerobic conditions were provided by purging the reactors with nitrogen gas for about 2 min, and the reactors were then incubated in a convection oven at mesophilic conditions (
The kinetics of the anaerobic digestion process was also evaluated using a first-order kinetic model (
Total solid (TS) and volatile solid (VS) contents of the feedstocks and inoculum were measured by drying the samples at 105°C followed by heating the dried residues at 575°C to a constant weight [
Methane and carbon dioxide produced during the anaerobic digestions were analyzed by a gas chromatograph (Sp-3420A, TCD detector, Beijing Beifen Ruili Analytical Instrument Co., China) equipped with a packed column (3 m length and 3 mm internal diameter, stainless steel, Porapak Q column, Chrompack, Germany). The carrier gas was nitrogen at a flow rate of 45 mL/min. The column, injector, and detector temperatures were 40, 100, and 150°C, respectively. A pressure-tight syringe (VICI, Precision Sampling, Inc., USA) with a volume of 250
All biogas yields were presented at standard conditions.
Analysis of variance (ANOVA) using Minitab software v. 15 was performed to compare confidence intervals and significance between treatments. The factors were considered significant when the probability (
The inoculum obtained from the industrial biogas plant contained 5.7 and 2.7% TS and VS, respectively (Table
Composition analyses of the inoculum as well as the untreated versus pretreated feedstocks.
Samples | Pretreatment | TS content (%) | VS content (%) | Total lignin* (%) | Hemicellulose (%) | Cellulose (%) |
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Inoculum | — | 5.7 | 2.7 | ND | ND | ND |
Centrifuged | 11.7 | 5.3 | ND | ND | ND | |
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Elmwood | Untreated | 95.5 | 94.5 | 26.2 | 26.3 | 46.4 |
150°C, 0.5 h | 95.5 | 94.1 | 25.1 | 23.4 | 50.0 | |
150°C, 1 h | 95.5 | 93.8 | 23.4 | 21.5 | 53.3 | |
180°C, 0.5 h | 96.3 | 94.4 | 20.4 | 21.9 | 55.7 | |
180°C, 1 h | 94.9 | 93.6 | 19.1 | 21.3 | 58.1 | |
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Pinewood | Untreated | 95.1 | 95.2 | 26.8 | 28.0 | 44.5 |
150°C, 0.5 h | 95.3 | 94.6 | 27.8 | 20.2 | 51.3 | |
150°C, 1 h | 95.9 | 95.1 | 26.5 | 21.3 | 51.4 | |
180°C, 0.5 h | 96.5 | 95.5 | 22.1 | 18.5 | 58.4 | |
180°C, 1 h | 96.9 | 95.8 | 21.1 | 16.9 | 60.8 | |
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Rice straw | Untreated | 95.4 | 83.9 | 17.1 | 50.1 | 21.5 |
150°C, 0.5 h | 95.6 | 83.8 | 12.2 | 45.6 | 29.9 | |
150°C, 1 h | 95.7 | 83.6 | 13.4 | 45.3 | 28.7 | |
180°C, 0.5 h | 95.9 | 86.2 | 11.4 | 42.3 | 36.2 | |
180°C, 1 h | 96.0 | 84.7 | 10.6 | 42.2 | 35.3 |
ND = not determined.
*Sum of acid soluble lignin (ASL) and acid insoluble lignin (AIL) contents.
Elmwood, pinewood, and rice straw were subjected to organosolv pretreatment using ethanol prior to anaerobic digestion in order to improve the yield of biogas production. The untreated and pretreated materials were characterized, according to their TS, VS, lignin, cellulose, and hemicellulose contents, and results are summarized in Table
Total lignin contents of untreated elmwood and pinewood were 26.2 and 26.8%, respectively, which was much higher than that of untreated rice straw (17.1%).
The various components of the materials were differently affected by the pretreatments. Depending on the pretreatment conditions, the lignin contents were reduced by 4–27% for elmwood, by 1–21% for pinewood, and by 21–37% for rice straw. Increasing the severity of the pretreatment generally resulted in higher lignin removal. A relatively high portion of straw’s lignin (37.7%) was removed through pretreatment at 180°C for 60 min, resulting in a pretreated straw with carbohydrate content of over 77% of TS. On the other hand, the organosolv pretreatment of elmwood and pinewood, at 180°C for 60 min, resulted in 27% and 21% lignin removal, respectively, with corresponding CH contents of 72.7% and 72.5% of TS, respectively. In addition to delignification, parts of hemicelluloses were also removed due to the pretreatments. Higher hemicellulose removal was obtained in pretreated pinewoods (28–40%), compared to that in elmwood (11–19%) or straw (9–16%).
Organosolv pretreatments in four different conditions were performed on the three different lignocellulosic materials, and the methane yields of the pretreated and untreated materials were then measured through batch SSAD assays. The accumulated methane productions obtained during 55 days of digestion from the untreated and pretreated materials are shown in Figure
Accumulated methane production from SSAD of untreated and pretreated (a) elmwood, (b) pinewood, and (c) rice straw in different pretreatment conditions. The symbols represent the untreated substrates (◆), the substrates pretreated at 150°C for 0.5 h (▲), at 150°C for 1 h (×), at 180°C for 0.5 h (+), and at 180°C for 1 h (■).
Methane production yields from all of the substrates were generally improved by the pretreatments in all conditions. The highest methane yield of 152.7 L·kg−1CH was obtained from rice straw pretreated at 150°C for 1 h (Table
The accumulated methane yields obtained after 55 days of anaerobic digestion from untreated and pretreated lignocellulosic substrates together with the specific rate constants and the regression coefficients calculated from the first-order kinetic model fitting.
Sample | Pretreated conditions | CH4 (L·kg−1CH) |
|
|
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Elmwood | Untreated |
|
0.054 | 0.975 |
150°C, 0.5 h |
|
0.063 | 0.934 | |
150°C, 1 h |
|
0.066 | 0.914 | |
180°C, 0.5 h |
|
0.062 | 0.961 | |
180°C, 1 h |
|
0.097 | 0.937 | |
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Pinewood | Untreated |
|
0.066 | 0.973 |
150°C, 0.5 h |
|
0.094 | 0.981 | |
150°C, 1 h |
|
0.073 | 0.933 | |
180°C, 0.5 h |
|
0.080 | 0.979 | |
180°C, 1 h |
|
0.065 | 0.962 | |
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Rice straw | Untreated |
|
0.081 | 0.943 |
150°C, 0.5 h |
|
0.084 | 0.946 | |
150°C, 1 h |
|
0.088 | 0.918 | |
180°C, 0.5 h |
|
0.078 | 0.991 | |
180°C, 1 h |
|
0.068 | 0.984 |
Among the untreated samples, the highest methane yield, 115.9 L·kg−1CH, was obtained from rice straw, which had the lowest lignin content among the substrates utilized in this study. The digestion of untreated elmwood and pinewood resulted in methane yields of 54.2 and 38.7 L·kg−1CH, respectively. The presence of pores in the structure of hardwoods which facilitate microorganisms’ accessibility might be responsible for the higher yield obtained from elmwood in comparison to that from pinewood [
The fitting of kinetics data on the first-order model for all of the substrates is shown in Table
The effect of lignin content on final methane yield was investigated by comparing methane yield as a function of the materials’ lignin content (Figure
Relationship between lignin content and total methane yield from lignocellulosic substrates (untreated and pretreated elmwood, pinewood, and rice straw).
Organosolv pretreatment prior to SSAD was an efficient process for improvement of methane production from different types of lignocellulosic materials; however, its effectiveness greatly depended on the type of lignocelluloses. The pretreatment process was more effective on softwood than on hardwood or agricultural waste. Moreover, hardwood needed more severe conditions to be able to achieve maximum improvement during the subsequent batch digestion assays. Lignin content was among the most important factors negatively affecting the methane production from all of the investigated lignocellulosic substrates.
The authors declare that there is no conflict of interests regarding the publication of this paper.
All experiments and paper preparation were performed by Safoora Mirmohamadsadeghi. The coauthors supervised the experiments and helped with paper preparation.
The authors are grateful for financial support from the Region Västra Götaland and the Institute of Biotechnology and Bioengineering, Isfahan University of Technology.