Enhancing ethanol and methane production from rice straw by pretreatment with liquid waste from biogas plant

Energy Conversion and Management 178 (2018) 290–298

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Enhancing ethanol and methane production from rice straw by pretreatment with liquid waste from biogas plant

T

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Forough Momayeza,b, Keikhosro Karimia,c, , Ilona Sárvári Horváthb a

Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran Swedish Centre for Resource Recovery, University of Borås, 501 90 Borås, Sweden c Industrial Biotechnology Group, Research Institute for Biotechnology and Bioengineering, Isfahan University of Technology, Isfahan 84156-83111, Iran b

A R T I C LE I N FO

A B S T R A C T

Keywords: Bioethanol Biomethane Liquid anaerobic digestion Dry anaerobic digestion Enzymatic hydrolysis Thermal pretreatment

Effluent of biogas digestate, a waste stream with serious environmental problems, was used to pretreat rice straw in order to improve biofuel production. To investigate the effect of components presented in this waste stream and compare the results, water was also applied at the same conditions for the pretreatment. The straw was pretreated at 130, 160, and 190 °C for 30 and 60 min and subjected to enzymatic hydrolysis, simultaneous saccharification and fermentation (SSF), liquid anaerobic digestion (L-AD), and dry anaerobic digestion (D-AD). The highest improvements in hydrolysis and ethanol yield were 100 and 125%, achieved from the straw pretreated with effluent of biogas digestate at 190 °C for 60 min. The best methane yield was obtained through L-AD and D-AD of straw pretreated at 190 °C and 30 min with effluent of biogas digestate, resulting in 24 and 26% enhancements in produced methane. However, treatment with water had no significant effect on methane yield. Compositional and FTIR analyses indicated hemicellulose omission through the treatment under severe conditions. Furthermore, SEM images showed major enhancement in straw porosity by the pretreatment.

1. Introduction World energy consumption is growing rapidly and supplying the demand from traditional energy sources is not sustainable. The utilization of non-renewable energy sources has resulted in greenhouse gas emissions, leading to global warming and climate change. These facts prompted researchers to develop alternative energy-producing technologies, through which bioenergy has attracted substantial attention [1–4]. Among the various biofuels, biogas is a major green fuel, especially in rural areas. Biogas is produced through microbial decomposition of organic matter under oxygen-free conditions. The composition of produced gas depends on different parameters, but typically consists of 35–75% CH4, 25–65% CO2, 1–5% H2, and small quantities of ammonia, water vapor, halides, and hydrogen sulfide [5,6]. According to the total solid content, anaerobic digestion (AD) plants can be classified into dry AD (D-AD) and liquid AD (L-AD). Generally, the total solid content in DAD is greater than 15%, while it is lower than 15% for L-AD [7,8]. Usually L-AD has shorter reaction time and higher reaction rate in comparison to D-AD; however, the decreased reactor volume, leading to reduced energy demand for heating, together with minimizing the need for material handling and decreased energy loss make D-AD an

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advantageous process over L-AD [9,10]. Bioethanol, the most important liquid biofuel, is widely used as a fuel or gasoline enhancer worldwide. The high oxygen content of ethanol as an oxygenate liquid decreases the amount of required additive, causes better combustion of hydrocarbons and reduces the emissions of aromatic compounds and CO [11]. This nonpetroleum liquid can also be used as a clean-burning fuel in dedicated engines. According to strict environmental laws, which limited fossil fuel utilization, the demand for bioethanol as a fuel is increasing rapidly [12]. The entire elimination of acid rain caused by sulfur dioxide and 80% reduction in carbon emission are other advantages of using bioethanol instead of gasoline [13]. Different renewable substrates can be used for biogas and bioethanol production. In contrast with first-generation biofuels produced from food crops, the production of second-generation biofuels uses nonedible resources. These materials cannot be used for human consumption and much is not digestible by animals, either [14,15]. Lignocellulosic materials, as the main organic source on the earth, are considered the most viable renewable energy feedstock for the production of second-generation biofuels. Huge amounts of lignocellulosic residues pile up from forestry, municipal, agricultural, and other activities. This biomass mainly is composed of three polymers, i.e.,

Corresponding author at: Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran. E-mail address: [email protected] (K. Karimi).

https://doi.org/10.1016/j.enconman.2018.10.023 Received 2 August 2018; Received in revised form 9 October 2018; Accepted 10 October 2018 Available online 18 October 2018 0196-8904/ © 2018 Elsevier Ltd. All rights reserved.

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reuse a waste stream obtained from a biogas plant as a pretreatment medium aiming to enhance methane production from lignocellulosic biomass. The presence of different effective substances can make this waste a promising candidate for the chemical pretreatment process to break down the recalcitrant structure of lignocellulosic biomass and increase the bioethanol and biogas production in subsequent bioconversions. For better comprehension, the effect of pretreatment temperature as well as pretreatment time was also evaluated. To determine the impact of the compositional elements presented in the BLW, straw was also treated with water under the same conditions. The pretreated straw was then subjected to enzymatic hydrolysis and the amount of liberated sugars was measured. Moreover, anaerobic digestion and simultaneous saccharification and fermentation (SSF) were conducted to investigate the influence of the pretreatment on methane and ethanol production yields. In most previous studies regarding pretreatments, the effects on the subsequent biogas production was only determined in L-AD systems, and just a few publications were found in the literature investigating the D-AD of the pretreated biomass. In this study, both L-AD and D-AD processes were hence applied, and the results were compared. Moreover, to investigate the pretreatment effects on structural and chemical characterization of feedstock, complementary analyses were done. Changes in the composition of treated rice straw compared to that in raw straw, crystallinity index before and after treatment, and changes in straw morphology after pretreatment were studied through compositional analysis, Fourier transformation infrared (FTIR) spectrometry, and scanning electron microscopy (SEM), respectively.

cellulose, hemicellulose, and lignin. Through the hydrolysis of cellulose and hemicellulose, monomeric sugars are liberated that can be either converted to ethanol by fermentation or biogas by anaerobic digestion. These processes have made lignocellulosic materials an appropriate substrate for bioenergy production [16–18]. However, the first step, the microbial and/or enzymatic hydrolysis, is difficult due to the recalcitrant structure and inherent characteristics of lignocelluloses [16,19]. Thus, an efficient pretreatment process is needed to accelerate the biomass degradation and improve the yield of biogas or bioethanol produced in the subsequent steps. The main goal of the lignocelluloses pretreatment is to change different compositional and structural properties of biomass, aiming at enhancing bioenergy production. Cellulose crystallinity, the extent of cellulose polymerization, accessible surface area, hemicellulose acetylation degree, and the presence of lignin and hemicellulose have been found to be the most important features of lignocelluloses that affect the hydrolysis and should be altered by pretreatment. There are several studies on feedstock characterization, after applying physical, chemical, and biological pretreatments [16,20,21]. Despite many benefits of the pretreatment step, the high cost connected to this process is still a big challenge facing commercialization of second-generation biofuels. Hence, the development of eco-friendly and low-cost pretreatment methods in biogas and bioethanol production is still required. Liquid hot water pretreatment have been applied for the enhancement of the degradability of lignocellulosic biomass since many years. The penetration of water into the biomass structure results in the hydrolysis of cellulose as well as the partial removal of hemicellulose and lignin. Hence, the accessible surface area would increase after hot water pretreatment, enhancing the performance of the subsequent process steps towards either ethanol or biogas production. Furthermore, there is no need for addition of chemicals or for the use of a corrosion resistant reactor, making this pretreatment well appreciated. However, due to the milder conditions, the liquid hot water pretreatment is not efficient enough for most kind of lignocellulosic biomass [17]. Regarding other pretreatment methods, chemical methods, including the application of bases, acids, and ionic liquid, are found to be the most promising ones and hence have gained substantial interest in industry. These methods alter the chemical and physical characteristics of lignocellulosic biomass, leading to enhanced digestibility during the following enzymatic hydrolysis or anaerobic digestion [19,22]. Using an appropriate pretreating agent that is cheap enough and does not make problems in subsequent steps is the greatest concern among these methods. Consequently, there is a need to find a suitable waste liquid for the pretreatment, which decreases the variable process cost together with reducing environmental problems [23]. The digestate residue produced in the biogas process is usually separated into filtrate and bio-fiber phases. This separation is the first step during the processing of this digestate residue. The bio-fiber fraction is generally used as animal bedding or composted for horticultural applications. By increasing the solid content of this phase, transportation and storage handling are much easier. The filtrate, i.e., the liquid portion, is typically applied as a low-value fertilizer onto farmland or is recycled back into the AD process. However, handling liquid fertilizer is very difficult, and in most cases, using it without additional treatment creates many environmental risks, including nitrogen pollution [24–26]. According to the biogas feedstock and process conditions, the composition of digestate varies. However, some studies have indicated the presence of volatile fatty acids as well as nitrogen, phosphorus, potassium, calcium, and sulfur elements in the digestate residue [27–29]. The presence of these elements in various forms has made the digestate an attractive waste for diverse purposes. In this study, the liquid portion of biogas digestate, called hereinafter biogas liquid waste (BLW), was used for the pretreatment of rice straw, to open up the rigid and compact structure of this widely available but mainly unused biomass. According to our knowledge, there were no other studies reported in the literature that apply and

2. Material and methods 2.1. Raw materials The rice straw was obtained from a farm located in Lenjan, Isfahan Province, Iran. The collected straw was ball milled and then screened through 20- and 80-mesh screens to achieve appropriate particle sizes of 0.177–0.840 mm. The straw was then stored at room temperature until further use. 2.2. Pretreatment procedure The digestate residue, obtained from an anaerobic digester plant, treating the organic fraction of municipal solid waste (Borås Energi och Miljö AB, Borås, Sweden) was centrifuged (4000 rpm for 20 min) and then the supernatant, called “biogas liquid waste (BLW)” below, was used for the pretreatments. Table 1 presents the characterization of digestate used in this study. To compare the efficiency of using this liquid fraction, pretreatments were also performed with hot water as control, applying the same treatment conditions. On the bases of previous investigation [30], 16.2 g (dry weight) rice straw was mixed with 93 mL of liquid fraction of biogas digestate (or water) in 150 mL stainless steel reactors (Swagelok, USA). These tubular reactors were then placed in an oil bath at the desired temperatures of 130, 160, or Table 1 Biogas digestate characterization.

291

Parameter

Value

Parameter

Value

Total solid Volatile solid pH Total nitrogen Ammonium Total phosphorus Total potassium Total calcium Total sulfur

4% 2.3% 8.3 4300 mg/kg 3300 mg/kg 0.35 kg/ton 1.1 kg/ton 0.83 kg/ton 0.28 kg/ton

Acetic acid Propionic acid Isobutyric acid Butyric acid Isovaleric acid Valeric acid

0.454 g/L 0.051 g/L 0.005 g/L 0.004 g/L 0.029 g/L 0.007 g/L

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digestion period. To obtain the amount of methane produced by the inoculum, a blank reactor containing inoculum without any substrate was also performed. All L-AD setups were performed in duplicates. The accumulated methane production was monitored by taking gas samples from the headspace of the reactors, which were then analyzed by gas chromatography.

190 °C and heated for the specified duration times of 30 or 60 min. At the end of the heating period, the reactors were put immediately in a cold-water bath to reduce the temperature and pressure. The pretreated materials were collected, and each sample was then separated into liquid and solid phases by centrifugation (4000 rpm for 20 min). Samples from the liquid phase were then analyzed by HPLC to determine the amount of presented sugars. Since the measured total sugar concentration was less than 1 g L−1 in all samples, only the solid fraction was used for further investigations. These solid residues were first dried at room temperature for four days and then stored in airtight plastic bags until using in subsequent experiments.

2.5. Dry anaerobic digestion (D-AD) An amount of 0.25 g of untreated or treated straw was mixed with appropriate amount of inoculum to achieve an S/I ration of 1 (VS basis) and a total solid content (TS) of 21%. The materials were mixed well and then loaded into 118 mL glass reactors, using the method provided by Brown et al. [34]. Blank reactors were prepared and the anaerobic conditions were achieved using procedures similar to those of the L-AD setups. The reactors were then incubated at 37 °C in the same incubator as the L-AD setups and were manually shaken regularly. All D-AD setups were performed in duplicates. The accumulated methane production was monitored as described above.

2.3. Enzymatic hydrolysis and simultaneous saccharification and fermentation (SSF) The enzymatic hydrolysis of untreated straw as well as treated samples were carried out in shaking flasks using 25 g L−1 solids in citrate buffer at pH of 5 ± 0.1. Cellulase enzyme (Cellic CTec2, Novozymes, Denmark), corresponding to 20 FPU per gram of straw, was added to each flask, and then they were put in a water bath at 45 °C and 125 rpm for 48 h. To measure the released glucose, liquids samples were periodically taken. The glucose yields were calculated based on Eq. (1) [31]:

Enzymatic hydrolysis yield (%)= Produced glucose (g / L) × 100 Substrate concentration (g /L) × F × 1.11

2.6. Analytical methods The total solids (TS) of substrate and inoculum were measured by oven drying at 105 °C until constant weight was achieved. The dried residues were then heated in the furnace at 575 °C for 24 h in order to measure the volatile solid (VS) content [35]. The enzyme activity was measured as 95 (FPU)/mL, according to the procedure reported by Adney and Baker [36]. Two-step acid hydrolysis method, according to the NREL Laboratory Analytical Procedure [37], was applied to determine the feedstock composition (i.e., glucan, xylan, galactan, mannan, and arabinan), acidsoluble lignin, and acid-insoluble lignin. To determine the contents of monomeric sugars liberated during acid hydrolysis, high-performance liquid chromatography (HPLC) (Waters® 2695, Waters Corporation, USA) was used. A lead (II)-based column (Aminex HPX-87P, Bio-Rad, USA) with two Micro-Guard deashing cartridges, followed by MicroGuard Carbo-P guard column (Bio-Rad, USA), was used to separate monomeric sugars at 85 °C using ultrapure water at a flow rate of 0.6 mL min−1 as eluent. The glucose and the ethanol concentrations were also determined by HPLC (Waters® 2695, Waters Corporation, USA) using a hydrogen ion–based ion-exchange column (Aminex HPX-87H, Bio-Rad, USA) at 60 °C with a Micro-Guard cation-H guard column (Bio-Rad, USA) and 5 mM H2SO4 at 0.6 mL min−1 as eluent. A gas chromatograph (PerkinElmer, USA) equipped with a packed column (60′ × 1.8″ OD, 80/100, Mesh, PerkinElmer, USA) and a thermal conductivity detector (PerkinElmer, USA) was used to determine biogas composition. Nitrogen gas with a flow rate of 20 mL/ min at 60 °C employed as carrier gas. Changes in the chemical bonds, crystallinity, and molecular structure of the treated and untreated straw were investigated by FTIR spectrometer (TENSOR 27 FT-IR, Bruker, USA), equipped with a universal attenuated total reflection (ATR) accessory and deuterated-triglycine sulfate (DTGS) detector. The spectra were obtained from 600 to 4000 cm−1 with an average of 60 scans and 4 cm−1 resolution. The surface morphology changes after treatment were qualitatively studied by SEM analysis. Freeze-dried straw was coated with gold (Emitech Sputter Coater SC7640, Quorum Technologies, UK) and then images were taken by SEM (Zeiss, Germany) with 500 times magnification at 13 kV.

(1)

where the constant 1.11 stands for the dehydration factor converting glucan to glucose monomers, and F is the glucan fraction in the biomass. Furthermore, untreated and treated straw samples were subjected to SSF processing aiming ethanol production. An amount of 50 g L−1 of substrate was added to 50 mmol L−1 sodium citrate buffer containing fermentation nutrients (5 g L−1 yeast extract, 3.5 g L−1 K2HPO4, 7.5 g L−1 (NH4)2SO4, 0.75 g L−1 MgSO4·7H2O, and 1 g L−1 CaCl2·2H2O). The pH of all samples was adjusted to 5, using 2 M NaOH solution. Flocculation strain of Saccharomyces cerevisiae (CCUG 53310, Culture Collection, University of Gothenburg, Sweden) was used for fermentation. The yeast biomass was prepared according to the method provided by Karimi et al. [32]. Finally, 20 FPU/g substrate cellulase enzymes and 1 g L−1 S. cerevisiae were added. The SSF process was conducted by placing the flasks in a shaking bath at 37 °C and 120 rpm for 48 h. Liquid samples were taken to determine the amount of produced ethanol. The ethanol yields were then calculated according to Eq. (2) [33]:

Ethanol yield (%)= Ethanol produced (g / L) × 100 Substrate concentration (g /L) × 1.11 × F × 0.51

(2)

where the factor 0.51 was set for theoretical conversion of glucose to ethanol, 1.11 was used for hydration of glucan to glucose, and F was the glucan fraction in the biomass. All enzymatic hydrolysis and SSF setups were run in triplicates. 2.4. Liquid anaerobic digestion (L-AD) According to the method provided by Brown et al. [34], 0.25 g of each substrate was well mixed with a sufficient amount of inoculum to obtain an substrate-to-inoculum ratio (S/I) ratio of 0.5 (volatile solids (VS) basis). Deionized water was then added to all samples to achieve 5% TS in each reactor. The reactors were sealed with butyl rubber and aluminum caps. In order to provide anaerobic conditions, each glass bottle was purged with a mixture of 80% nitrogen and 20% carbon dioxide for 2 min. Finally, the reactors were incubated at 37 °C in an incubator for 30 days and manually shaken regularly during the

2.7. Statistical analysis Analysis of variance (ANOVA) was performed for statistical validation of the achieved results. SAS software was used for the estimation and comparison of confidence intervals and significant difference 292

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the yield of glucose formation after the optimal condition. Furthermore, the glucose yield obtained at the best conditions in this study was close to those reported by other studies. Hsu et al. [39] reported a maximum glucose yield of 83%, achieved after enzymatic hydrolysis of rice straw pretreated with 1% (w/w) sulfuric acid at 160 °C or 180 °C for 1–5 min. In another study, the effect of three different pretreatments (acid hydrolysis, ammonia fiber explosion, and acid-catalyzed steam explosion) on the digestibility of rice straw was investigated [40]. The results showed that the highest glucose yield was 70.5%, obtained from the straw treated with 10% sulfuric acid at 160 °C for 10 min. Furthermore, the effects of a combination pretreatment using screw press and four different ionic liquids on rice straw was investigated by Sriariyanun et al. [41]. They have found that the maximum glucose yield of 87% could be achieved when the straw was treated by a combination of screw press and 1-ethyl-3-methylimidazolium acetate, respectively.

between the treatments. Factors with a probability (P value) less than 0.05 were classed as significant. 3. Results and discussion Pretreatments were conducted with the liquid fraction of digestate residue (biogas liquid waste), as well as water, at 130, 160, and 190 °C and for 30 and 60 min. After the pretreatment, the solid fraction was separated, dried, and used in the experiments. The effects of different pretreatment conditions were evaluated by enzymatic hydrolysis of untreated and pretreated straw. Furthermore, SFF processes were also performed to determine pretreatment effects on the production of ethanol. Additionally, liquid anaerobic digestion and dry anaerobic digestion assays were conducted using untreated and pretreated samples as substrates. Complementary analysis aiming to obtain changes in the composition and structure of different treated samples compared to those of untreated straw were also performed.

3.2. Simultaneous saccharification and fermentation (SSF) 3.1. Enzymatic hydrolysis of treated and untreated straw The glucose yields of all samples obtained after 24 and 48 h enzymatic hydrolysis were calculated according to Eq. (1), and the results are shown in Fig. 1. Increasing pretreatment temperature and prolonging pretreatment time significantly enhanced the achieved yield of glucose. The highest glucose yield of 76% was obtained from the sample pretreated with BLW at 190 °C for 60 min, which was double so much as the glucose yield of 38% from untreated straw (Fig. 1). According to the ANOVA results, changing the pretreatment liquid from hot water to the BLW additionally improved the yield of glucose. The straw treated at 160 °C for 60 min with the BLW had the highest improvement of 34% in glucose yield, compared to that of straw treated with hot water at the same conditions (Fig. 1). At higher temperatures, difference between the obtained yields from BLW treatment compared with hot water treatment decreased. The highest glucose yield of 68% was obtained after hot water treatment at 190 °C for 60 min. For the improvement of rice straw hydrolysis, Imman et al. [38] used liquid hot water pretreatment at 140, 160, and 180 °C for 5, 10, and 20 min. They reported that pretreatment at 160 °C for 10 min resulted in the highest enzymatic digestibility yield of 71.2% from the solid residue. Increasing temperature and prolonging the pretreatment time could not improve

The ethanol yield achieved from treated and untreated straw samples within the SSF process were calculated based on the theoretical yield (Eq. (2)) and the results obtained after 48 h fermentation are presented in Fig. 2. The maximum ethanol yield of 71% was observed after pretreatment at 190 °C for 60 min using the BLW, while the ethanol yield from raw straw was 32%. This achievement is comparable to those reported in previous studies after different treatments of various lignocellulosic feedstocks [42–45]. In an earlier study, rice straw was soaked in 0.5% sulfuric acid for 20 h, and the mixture was then heated for 10 min at 15 bar in a sealed stainless steel reactor. Afterward, pretreated straw was utilized in SSF using different fungi as fermenting microorganisms at various enzyme loadings. Using 15 FPU/g dry matter and Rhizopus oryzae, the maximum ethanol yield was found to be 74% [32]. In another study, an ethanol yield of 57.5% was obtained by SSF after the pretreatment of rice straw with 1% NaOH at 40 °C for 30 min using microwave power of 700 W [46]. In agreement with the enzymatic hydrolysis results, the pretreatments with BLW were more effective than those using hot water. According to the ANOVA analysis, pretreatment time and temperature had a significant effect on the ethanol yield. Hence, in line with other studies [47,48], applying high-temperature pretreatment favored the

Fig. 1. Glucose yield of untreated and treated rice straw after 24 and 48 h enzymatic hydrolysis (W: Water, BLW: Biogas Liquid Waste). The dissimilar uppercase letters indicate the significant differences among the pretreatment conditions. There are no significant differences among the like-lettered groups.

Fig. 2. Ethanol yield of untreated and treated samples after 48 h simultaneous saccharification and fermentation (see Fig. 1 caption for the letters used). 293

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Fig. 3. Accumulated methane produced from raw as well (A) water (W) treated, (B) biogas liquid waste (BLW) treated rice straw after 3, 13, and 23 days L-AD (see Fig. 1 caption for other letters used).

improvement in the methane yield. In another study, the effects of pretreatment using an acetic-propionic acid mixture were investigated. Optimal conditions of 0.75 mol/L acid concentration, 1:20 solid-to-liquid ratio, and 2 h pretreatment time were found for enzymatic hydrolysis. Treated samples under these conditions were also investigated in L-AD assays, showing 35% enhancement in the cumulative methane yield, compared to that of untreated straw [53].

conversion of substrate to ethanol through the SSF process. Hot water treatment also performed well regarding the increase in ethanol yield after the treatments. Pretreatment at 190 °C for 60 min with water increased the yield to 63%. However, pretreatments using hot water or BLW at 130 °C for 30 min as well as hot water at 130 °C for 60 min had no remarkable effect on the ethanol yield. Consequently, these conditions were not severe enough to modify the recalcitrant structure of the straw.

3.4. Dry anaerobic digestion 3.3. Liquid anaerobic digestion Fig. 4 presents the results of cumulative methane obtained after 3, 13, and 34 days in D-AD assays. No further methane production was observed after 35 days. In agreement with the L-AD results, hot water treatment had only limited impact on the methane produced during the D-AD. Only a very slight increase in the methane yield could be detected after treatment with hot water at severe conditions, i.e., high temperature and long treatment time (Fig. 4a). Similar to the results obtained during L-AD, the maximum methane yield of 190 NmL/gVS was achieved after pretreatment with the BLW at 190 °C for 30 min. This corresponds to 26% increase, compared to that of the untreated straw (150 NmL/gVS). Similar to the results obtained in the L-AD system, increasing pretreatment time at 190 °C led to a decrease in the methane yield (Fig. 3 vs Fig. 4). Comparing the methane yields obtained through L-AD versus D-AD, it was concluded that generally higher yields could be achieved in the LAD process than in the D-AD process. The highest methane potential in L-AD was 241 mL/gVS, while the methane yield from the same sample in D-AD was only 190 NmL/gVS. Moreover, as expected, longer retention time was needed for D-AD, compared to L-AD. This could be due to the water limitation presented in D-AD and high viscosity, causing limitations in mass transfer of substrate and intermediates within the system [7,9]. However, one important benefit of D-AD is reduction of reactor volume. Hence, the produced methane can be calculated considering the unit of reactor volume of the digester [54]. According to these calculations, in this study, the volumetric methane productivities (Lmethane/Lworking volume) of straw samples pretreated at the optimal conditions and then subjected to L-AD versus D-AD processes were 3.34 and 5.93 Lmethane/Lworking volume, respectively. This considerable enhancement in volumetric productivity showed the benefit of using D-AD instead of L-AD for substrates with high solid contents, meaning that a smaller reactor would be enough for an equal solid loading. However, the D-AD process has some operational difficulties, including loading and uploading high solid content feedstock, which accompanied with

Fig. 3 reports the cumulative methane production obtained after 3, 13, or 23 days in the L-AD assays. Measurements after 23 days of digestion period indicated no further methane production. As shown in Fig. 3(a), pretreatment with hot water was not successful, resulting in no increase in the methane yield. All samples treated with hot water had principally the same biomethane potential as the untreated one. Jiang et al [49] indicated that the treatment of giant reed with only hot water had limited effect on the methane yield. Under the best conditions (190 °C and 10 min), the cumulative methane yield was increased by 30%. However, an increment of 63% in the cumulative methane yield was obtained when the biomass was pretreated with 20 g/L NaOH at room temperature for 24 h. In another study, changes in the biomethane yield of sunflower oil cake after hydrothermal pretreatment were investigated [50]. Only 6.5% increase in the methane yield from the samples treated at 100 °C for 4 h, compared with samples soaked with water at room temperature for 4 h, was obtained. On the other hand, the straw treated with the BLW showed a better performance during the L-AD assays. The highest methane production of 241 NmL/gVS was achieved after pretreatment at 190 °C and for 30 min. This corresponds to 24% increase compared to the yield of methane (194 NmL/gVS) from untreated straw (Fig. 3b). However, increasing the pretreatment time from 30 to 60 min at 190 °C resulted in a decrease in the methane yield. This might be caused by inhibitory compounds, which could have formed under the more severe pretreatment condition. Mirmohamadsadeghi et al. [51] also reported the highest methane yield from rice straw pretreated at 150 °C for 1 h. They indicated that severe pretreatment conditions resulted in methane yield reduction due to the formation of inhibitory compounds. Various chemical treatment methods were investigated previously, aiming to improve biogas production from rice straw during L-AD. Zhang et al. [52] reported that a combined treatment, including grinding, heating, and addition of ammonia, resulted in only a 17.5% 294

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Fig. 4. Accumulated methane produced from raw (A) water (W) treated, (B) biogas liquid waste (BLW) treated rice straw after 3, 13, and 34 days D-AD (see Fig. 1 caption for other letters used).

factor playing role in biomass recalcitrance. The obtained higher glucose and ethanol yields (Figs. 1 and 2, respectively) despite the higher remaining xylan content after treatments with BLW can be explained by no strict relation between the hemicellulose content and glucose and ethanol yields. Moreover, as the pretreatment medium changes, the way of effecting the biomass structure varies. Besides xylan content, the arrangement and location of xylan are also important factors. Rearrangement of xylan location in biomass, especially the one surrounded the cellulose fibers, could reduce the recalcitrant behavior of biomass. This was observed in a number of studies on pretreatment of lignocelluloses. Safari et al. [58] obtained no more hemicellulose in the substrate structure of pinewood after pretreatment with 0.5% H2SO4 at 140 °C for 5 min. Nevertheless, no improvement in glucose concentration was achieved in the following enzymatic hydrolysis. In another study, Liu et al. [59] investigated the effect of dilute sulfuric acid, ammonium hydroxide, and ionic liquid pretreatments on the characteristics and digestibility of three different switchgrass substrates. They have observed that although the xylan content of the dilute acid pretreated substrate was reduced to 0.8% for one of the switchgrass types, the highest glucose yield was achieved from a substrate treated with ionic liquid, still containing 22.8% xylan. Compared to that obtained after the treatment with hot water, less acid-insoluble lignin (AIL) was presented in the straw after treatment with the BLW. Furthermore, no significant changes were observed in

major problems in large-scale continuous digesters. The feeding of this process is not easily possible by usual pumping, and special reactor design is essential for high solid content feedstocks [34,55]. 3.5. Effect of pretreatment on straw composition Table 2 reports the TS and VS contents of untreated and treated straw together with solid recovery determined after treatment at different conditions. As is shown, there were no significant differences in the TS and VS contents of the samples. Table 3 shows the chemical composition of treated and untreated straw. Temperature of 190 °C resulted in an increase of the glucan content and a decrease in the xylan content. Treatment with hot water at 190 °C for 60 min led to the 80% removal of xylan. Hence, the sample treated at these conditions contained the maximum amount of glucan in the solids. This result was in accordance with previous studies, in which almost 80% hemicellulose removal was reported through the hot water treatment of corn stover, sugarcane bagasse, and wheat straw [47,56,57]. Treatment by the BLW at severe conditions also led to significant removal of xylan. The xylan content of 18% in raw straw was decreased to 10.6% by pretreatment with the BLW at 190 °C for 60 min, leading to an increase in the glucan content by 21%. The xylan content is an important factor in decreasing the accessibility of cellulose for enzymatic hydrolysis. However, it is not the only Table 2 TS, VS, and solid recovery of raw and treated rice straw. Pretreatment conditions

Total solid (%)

Volatile solid (%)

Solid recovery (%)

Pretreatment medium

Temperature (°C)

Time (min)

Water

130 160 190 130 160 190

30 30 30 60 60 60

92.16 94.46 95.67 93.65 94.36 93.83

± ± ± ± ± ±

0.13 0.33 0.66 0.2 0.79 0.12

85.25 87.73 86.89 86.60 87.10 84.55

± ± ± ± ± ±

0.14 0.19 0.86 0.06 0.65 0.32

92.08 84.33 63.33 85.58 78.67 58.83

Biogas Liquid Waste

130 160 190 130 160 190

30 30 30 60 60 60

93.54 93.77 92.98 94.31 94.17 94.47

± ± ± ± ± ±

0.42 0.81 0.24 0.48 0.91 0.04

85.79 86.37 82.93 86.65 86.03 84.09

± ± ± ± ± ±

0.41 0.53 0.47 0.78 0.69 0.04

88.50 81.50 76.33 85.83 80.33 69.75

Untreated Straw

93.11 ± 0.93

295

83.23 ± 0.18

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Table 3 Chemical composition of raw and treated rice straw.a Pretreatment conditions

Glucan (%)b

Xylan (%)

Arabinan (%)

AIL (%)

16.5 ± 0.2 17.7 ± 0.1 9.4 ± 0.2 17.1 ± 0.2 16.6 ± 0.1 3.4 ± 0.1

2.9 2.7 0.7 2.9 1.8 0.2

± ± ± ± ± ±

0.1 0.1 0.0 0.1 0.1 0.1

14.2 14.8 18.4 15.1 16.1 23.9

± ± ± ± ± ±

0.9 0.6 0.6 0.2 0.4 0.8

5.1 4.9 4.4 4.9 4.8 5.4

± ± ± ± ± ±

0.0 0.1 0.1 0.0 0.0 0.3

3.0 3.1 2.1 2.9 3.1 1.0

± ± ± ± ± ±

0.1 0.1 0.2 0.2 0.1 0.1

14.6 13.2 14.8 14.0 13.9 17.5

± ± ± ± ± ±

0.6 1.6 0.5 0.5 0.6 0.6

5.2 4.6 4.5 4.7 4.8 5.7

± ± ± ± ± ±

0.0 0.1 0.3 0.1 0.5 0.2

Pretreatment medium

Temperature (°C)

Time (min)

Water

130 160 190 130 160 190

30 30 30 60 60 60

37.7 40.2 50.7 38.9 41.9 52.9

Biogas Liquid Waste

130 160 190 130 160 190

30 30 30 60 60 60

38.5 ± 0.4 40.6 ± 0.2 43 ± 0.9 38.4 ± 1.7 39.8 ± 1.2 46.0 ± 0.6

16.8 17.6 14.0 16.8 16.4 10.6

Untreated Straw

–

–

37.9 ± 0.6

18.0 ± 0.1

a b

± ± ± ± ± ±

0.5 0.3 0.9 0.5 0.2 0.4

± ± ± ± ± ±

0.2 0.2 0.2 0.7 0.5 0.2

3.1 ± 0.1

ASL (%)

16.1 ± 0.2

4.4 ± 0.0

All analyses were triplicated and the averages of values are reported. The data are reported based on the oven dry weight of straw.

the content of acid-soluble lignin (ASL) after any pretreatment conditions. Treatments applying a temperature lower than 190 °C had a slight effect on the rice straw composition. Consequently, straw treated at lower temperature had almost the same composition as untreated straw. The better performance of straw samples pretreated at higher temperatures in the enzymatic hydrolysis, SSF, L-AD, and D-AD could be related to the removal of hemicellulose. Hemicellulose acts as a physical barrier, preventing accessibility of hydrolytic enzymes and microorganisms to cellulose [60]. However, it should be noticed that hemicellulose can be utilized for methane production in L-AD and D-AD [61,62]. The similar methane production determined from untreated straw and straw treated with hot water, even at 190 °C, may be due to the removal of major fraction of hemicellulose under these treatment conditions. Rearrangement and partial omission of hemicellulose resulting from the treatments with the BLW led to higher methane yield in both the L-AD and the D-AD assays. In contrast to the results obtained for methane yield from watertreated straw, an improvement in ethanol production was observed after treating at severe conditions. This improvement in ethanol production was also seen in the case of straw treated with the BLW. As S. cerevisiae, with the ability of utilizing only hexose sugars, is used for fermentation in this study, pretreatment conditions for the complete elimination of hemicelluloses without formation of inhibitory compounds is desirable.

Untreated W(190°C, 30 min) BLW(190°C, 30 min) W(190°C, 60 min) 1730 BLW(190°C, 60 min)

1430

896

Absorption

3450

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (1/cm) Fig. 5. FTIR spectra of untreated and treated samples (W: Water, BLW: Biogas Liquid Waste).

untreated and four treated samples after baseline correction and normalization to the band at 1510 cm−1. The calculated CI for untreated straw was 1.38, which was reduced to 1.23 or to 1.18 for straw treated using the BLW at 190 °C for 30 or 60 min, respectively. The CI of straw treated with hot water at 190 °C for 30 or 60 min also declined to 1.23 or 1.24, respectively. Therefore, higher amount of cellulose II compared to cellulose I presented in rice straw after pretreatments at these conditions. The absorption band at 1730 cm−1 indicates acetyl groups connecting hemicellulose and lignin. Reduction of this band in the spectra of treated samples confirmed the removal of hemicellulose due to the treatments (Fig. 5), which is in accordance with the results obtained from the compositional analyses of the samples (Table 3). The band position at about 3450 cm−1 is attributed to OH stretching. Reduced intensity at this band position indicates disruption of hydrogen bonds of cellulose. A slight reduction in the intensity of peaks at the 1650–1560 cm−1 bands was observed after pretreatment. This reduction can be attributed to lignin removal during the pretreatment [66]. Surface characterization as well as morphological features of untreated and treated straw was inferred from SEM images (Fig. 6). Raw straw and straw treated at lower temperature, i.e., 130 °C (Fig. 6a, b, and c), had a crystalline and highly packed structure, whereas straw treated at more severe conditions (Fig. 6d, e, and f) had a porous and penetrable structure. Moreover, native straw is covered by a silica layer and this silica layer prevents enzymatic hydrolysis [67]. The results

3.6. Effect of treatment on straw structure The effects of the pretreatments on the straw structural changes were studied by FTIR and SEM, using four different treated samples and comparing them with untreated straw. Although FTIR analysis is not a precise method for compositional and quantitative analysis of various crystalline parts of samples; however, it can be used for a comparative study of cellulose crystallinity [63]. Depending on the hydrogen bonds, cellulose can exist as cellulose type I (typical crystalline form) and cellulose type II (regenerated or amorphous celluloses). The intensity of the absorption band at 1430 cm−1 refers to cellulose I, or crystalline cellulose, with high resistance to biological digestibility, while the absorption band at 896 cm−1 refers to cellulose II, or amorphous cellulose, with less resistance to enzymatic and microbial hydrolysis. Crystallinity index (CI) was calculated using the absorbance ratio of A1430/A896 [64]. The absorption band at 1510 cm−1 is the only band assigned to the aromatic components in lignin, which is the unique pure band and can be applied as internal reference band [65]. Fig. 5 presents the FTIR spectra of 296

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Fig. 6. Scanning electron micrographs (SEM) of (A) untreated rice straw, (B) treated with water (W) at 130 °C for 30 min, (C) treated with biogas liquid waste (BLW) at 130 °C for 30 min, (D) treated with BLW at 190 °C for 30 min), (E) treated with W at 190 °C for 60 min, (F) treated with BLW at 190 °C for 60 min.

Acknowledgments

obtained in this study show that during the treatment at 190 °C, the silica layer was destroyed. The fragmental structure of straw observed after pretreatment at severe conditions (Fig. 6d, e, and f) indicates also an increment in accessible surface area, leading to increased availability of cellulose for microbial and enzymatic attacks. BLW contains different weak organic acids, including acetic, propionic, isobutyric, butyric, isovaleric, and valeric acids (Table 1). Weak acids at high temperature attack the hemicellulosic parts of lignocelluloses. The organic acids penetrate to the biomass structure and hydrolyze or dissolve the hemicellulose that can open up the structure and improve digestibility [63]. In the current study, the pretreatment with BLW significantly reduced the hemicellulose content. Moreover, increasing temperature and prolonging pretreatment enhanced hemicellulose removal. More digestible and more accessible cellulose was obtained after the pretreatment. On the other hand, it was shown that the pretreatment with weak acids affects the cellulose properties. It can decrease cellulose crystallinity and degree of polymerization, called leveling-off degree of polymerization [63]. The results of this study indicated reduced crystallinity of the straw by pretreatment with BLW. Dilute acid pretreatments are not successful processes for lignin removal; however, re-localization of lignin and disruption lignin-carbohydrate bonds can occur, resulting in easily digestible carbohydrates [68]. BLW pretreatment did not significantly dissolve lignin parts of rice straw; thus, the improvements may be related to the re-localized lignin. However, more analysis is necessary to locate the lignin in the biomass.

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4. Conclusions This study investigated the possibility of using the biogas liquid waste, i.e., the supernatant liquid phase obtained after centrifugation of digestion residue, for the pretreatment of rice straw at various conditions. Pretreatments at similar conditions were also performed with hot water for comparison purpose. The pretreatment at the best conditions with this waste stream increased the enzymatic hydrolysis yield of straw from 38% to 76%. Furthermore, ethanol yield was enhanced over two-fold after the treatment. Improvement in methane production via L-AD and D-AD was 24% and 26%, respectively. However, the high volumetric productivity of methane produced by D-AD was a promising effects of the pretreatment process. Although water treatment increased glucose and ethanol yield, no significant effect was observed on methane yield.

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