Enhancing Ethanol and Methane Production from Rice Straw by Biogas Digestate Pretreatment

Contents lists available at ScienceDirect Energy Conversion and Management journal homepage wwwelseviercomlocateenconman Enhancing ethanol and methane production from rice straw by pretreatment with liquid waste from biogas plant Forough Momayezab Keikhosro Karimiac Ilona Sárvári Horváthb a Department of Chemical Engineering Isfahan University of Technology Isfahan 8415683111 Iran b 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 8415683111 Iran A R T I C L E I N F O Keywords Bioethanol Biomethane Liquid anaerobic digestion Dry anaerobic digestion Enzymatic hydrolysis Thermal pretreatment A B S T R A C T 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 LAD and dry anaerobic digestion DAD The highest improvements in hydrolysis and ethanol yield were 100 and 125 achieved from the straw pre treated with effluent of biogas digestate at 190 C for 60 min The best methane yield was obtained through LAD and DAD 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 condi tions 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 utili zation of nonrenewable energy sources has resulted in greenhouse gas emissions leading to global warming and climate change These facts prompted researchers to develop alternative energyproducing tech nologies through which bioenergy has attracted substantial attention 14 Among the various biofuels biogas is a major green fuel especially in rural areas Biogas is produced through microbial decomposition of organic matter under oxygenfree conditions The composition of pro duced gas depends on different parameters but typically consists of 3575 CH4 2565 CO2 15 H2 and small quantities of ammonia water vapor halides and hydrogen sulfide 56 According to the total solid content anaerobic digestion AD plants can be classified into dry AD DAD and liquid AD LAD Generally the total solid content in D AD is greater than 15 while it is lower than 15 for LAD 78 Usually LAD has shorter reaction time and higher reaction rate in comparison to DAD 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 DAD an advantageous process over LAD 910 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 ad ditive causes better combustion of hydrocarbons and reduces the emissions of aromatic compounds and CO 11 This nonpetroleum li quid can also be used as a cleanburning fuel in dedicated engines According to strict environmental laws which limited fossil fuel utili zation 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 bioe thanol production In contrast with firstgeneration biofuels produced from food crops the production of secondgeneration biofuels uses non edible resources These materials cannot be used for human consump tion and much is not digestible by animals either 1415 Lig nocellulosic materials as the main organic source on the earth are considered the most viable renewable energy feedstock for the pro duction of secondgeneration biofuels Huge amounts of lignocellulosic residues pile up from forestry municipal agricultural and other ac tivities This biomass mainly is composed of three polymers ie httpsdoiorg101016jenconman201810023 Received 2 August 2018 Received in revised form 9 October 2018 Accepted 10 October 2018 Corresponding author at Department of Chemical Engineering Isfahan University of Technology Isfahan 8415683111 Iran Email address karimicciutacir K Karimi Energy Conversion and M anagem ent 178 2018 290298 Available online 18 October 2018 01968904 2018 Elsevier Ltd All rights reserved T 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 1618 However the first step the microbial andor enzymatic hydrolysis is difficult due to the re calcitrant structure and inherent characteristics of lignocelluloses 1619 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 dif ferent compositional and structural properties of biomass aiming at enhancing bioenergy production Cellulose crystallinity the extent of cellulose polymerization accessible surface area hemicellulose acet ylation 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 162021 Despite many benefits of the pretreatment step the high cost connected to this process is still a big challenge facing commercialization of secondgeneration biofuels Hence the development of ecofriendly and lowcost 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 pene tration 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 pre treatment 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 bio mass leading to enhanced digestibility during the following enzymatic hydrolysis or anaerobic digestion 1922 Using an appropriate pre treating agent that is cheap enough and does not make problems in subsequent steps is the greatest concern among these methods Conse quently there is a need to find a suitable waste liquid for the pre treatment which decreases the variable process cost together with re ducing environmental problems 23 The digestate residue produced in the biogas process is usually se parated into filtrate and biofiber phases This separation is the first step during the processing of this digestate residue The biofiber fraction is generally used as animal bedding or composted for horticultural ap plications By increasing the solid content of this phase transportation and storage handling are much easier The filtrate ie the liquid por tion is typically applied as a lowvalue 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 2426 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 2729 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 herein after 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 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 bio conversions For better comprehension the effect of pretreatment temperature as well as pretreatment time was also evaluated To de termine 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 diges tion 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 LAD systems and just a few publications were found in the literature investigating the DAD of the pretreated biomass In this study both LAD and DAD processes were hence applied and the results were compared Moreover to investigate the pretreatment effects on structural and chemical characterization of feedstock com plementary 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 2 Material and methods 21 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 80mesh screens to achieve appropriate particle sizes of 01770840 mm The straw was then stored at room temperature until further use 22 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 pre vious investigation 30 162 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 Parameter Value Parameter Value Total solid 4 Acetic acid 0454 gL Volatile solid 23 Propionic acid 0051 gL pH 83 Isobutyric acid 0005 gL Total nitrogen 4300 mgkg Butyric acid 0004 gL Ammonium 3300 mgkg Isovaleric acid 0029 gL Total phosphorus 035 kgton Valeric acid 0007 gL Total potassium 11 kgton Total calcium 083 kgton Total sulfur 028 kgton F Momayez et al Energy Conversion and M anagem ent 178 2018 290298 291 190 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 coldwater bath to reduce the temperature and pressure The pretreated materials were collected and each sample was then separated into li quid 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 con centration was less than 1 g L1 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 23 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 L1 solids in ci trate buffer at pH of 5 01 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 Substrate concentration g L F 111 100 1 where the constant 111 stands for the dehydration factor converting glucan to glucose monomers and F is the glucan fraction in the bio mass Furthermore untreated and treated straw samples were subjected to SSF processing aiming ethanol production An amount of 50 g L1 of substrate was added to 50 mmol L1 sodium citrate buffer containing fermentation nutrients 5 g L1 yeast extract 35 g L1 K2HPO4 75 g L1 NH42SO4 075 g L1 MgSO47H2O and 1 g L1 CaCl22H2O 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 FPUg substrate cellulase enzymes and 1 g L1 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 pro duced ethanol The ethanol yields were then calculated according to Eq 2 33 Ethanol yield Ethanol produced g L Substrate concentration g L F 111 051 100 2 where the factor 051 was set for theoretical conversion of glucose to ethanol 111 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 24 Liquid anaerobic digestion LAD According to the method provided by Brown et al 34 025 g of each substrate was well mixed with a sufficient amount of inoculum to obtain an substratetoinoculum ratio SI ratio of 05 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 digestion period To obtain the amount of methane produced by the inoculum a blank reactor containing inoculum without any substrate was also performed All LAD 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 25 Dry anaerobic digestion DAD An amount of 025 g of untreated or treated straw was mixed with appropriate amount of inoculum to achieve an SI 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 LAD setups The reactors were then incubated at 37 C in the same incubator as the LAD setups and were manually shaken regularly All DAD setups were performed in duplicates The accumulated methane pro duction was monitored as described above 26 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 FPUmL according to the procedure reported by Adney and Baker 36 Twostep acid hydrolysis method according to the NREL Laboratory Analytical Procedure 37 was applied to determine the feedstock composition ie glucan xylan galactan mannan and arabinan acid soluble lignin and acidinsoluble lignin To determine the contents of monomeric sugars liberated during acid hydrolysis highperformance liquid chromatography HPLC Waters 2695 Waters Corporation USA was used A lead IIbased column Aminex HPX87P BioRad USA with two MicroGuard deashing cartridges followed by Micro Guard CarboP guard column BioRad USA was used to separate monomeric sugars at 85 C using ultrapure water at a flow rate of 06 mL min1 as eluent The glucose and the ethanol concentrations were also determined by HPLC Waters 2695 Waters Corporation USA using a hydrogen ionbased ionexchange column Aminex HPX87H BioRad USA at 60 C with a MicroGuard cationH guard column BioRad USA and 5 mM H2SO4 at 06 mL min1 as eluent A gas chromatograph PerkinElmer USA equipped with a packed column 60 18 OD 80100 Mesh PerkinElmer USA and a thermal conductivity detector PerkinElmer USA was used to de termine 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 struc ture of the treated and untreated straw were investigated by FTIR spectrometer TENSOR 27 FTIR Bruker USA equipped with a uni versal attenuated total reflection ATR accessory and deuteratedtri glycine sulfate DTGS detector The spectra were obtained from 600 to 4000 cm1 with an average of 60 scans and 4 cm1 resolution The surface morphology changes after treatment were qualitatively studied by SEM analysis Freezedried straw was coated with gold Emitech Sputter Coater SC7640 Quorum Technologies UK and then images were taken by SEM Zeiss Germany with 500 times magnifi cation at 13 kV 27 Statistical analysis Analysis of variance ANOVA was performed for statistical vali dation of the achieved results SAS software was used for the estimation and comparison of confidence intervals and significant difference F Momayez et al Energy Conversion and M anagem ent 178 2018 290298 292 between the treatments Factors with a probability P value less than 005 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 sam ples 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 31 Enzymatic hydrolysis of treated and untreated straw The glucose yields of all samples obtained after 24 and 48 h enzy matic 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 Ac cording 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 im provement 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 712 from the solid residue Increasing temperature and prolonging the pretreatment time could not improve 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 ww sulfuric acid at 160 C or 180 C for 15 min In another study the effect of three dif ferent pretreatments acid hydrolysis ammonia fiber explosion and acidcatalyzed steam explosion on the digestibility of rice straw was investigated 40 The results showed that the highest glucose yield was 705 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 1ethyl3methylimidazo lium acetate respectively 32 Simultaneous saccharification and fermentation SSF The ethanol yield achieved from treated and untreated straw sam ples 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 var ious lignocellulosic feedstocks 4245 In an earlier study rice straw was soaked in 05 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 FPUg dry matter and Rhizopus oryzae the maximum ethanol yield was found to be 74 32 In another study an ethanol yield of 575 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 pretreat ments 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 4748 applying hightemperature 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 likelettered 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 F Momayez et al Energy Conversion and M anagem ent 178 2018 290298 293 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 33 Liquid anaerobic digestion Fig 3 reports the cumulative methane production obtained after 3 13 or 23 days in the LAD assays Measurements after 23 days of di gestion period indicated no further methane production As shown in Fig 3a 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 in creased by 30 However an increment of 63 in the cumulative methane yield was obtained when the biomass was pretreated with 20 gL NaOH at room temperature for 24 h In another study changes in the biomethane yield of sunflower oil cake after hydrothermal pre treatment were investigated 50 Only 65 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 LAD assays The highest methane production of 241 NmLgVS was achieved after pretreatment at 190 C and for 30 min This corresponds to 24 increase compared to the yield of methane 194 NmLgVS from untreated straw Fig 3b However in creasing 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 pre treatment 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 LAD Zhang et al 52 reported that a combined treatment including grinding heating and addition of ammonia resulted in only a 175 improvement in the methane yield In another study the effects of pretreatment using an aceticpropionic acid mixture were investigated Optimal conditions of 075 molL acid concentration 120 solidtoli quid ratio and 2 h pretreatment time were found for enzymatic hy drolysis Treated samples under these conditions were also investigated in LAD assays showing 35 enhancement in the cumulative methane yield compared to that of untreated straw 53 34 Dry anaerobic digestion Fig 4 presents the results of cumulative methane obtained after 3 13 and 34 days in DAD assays No further methane production was observed after 35 days In agreement with the LAD results hot water treatment had only limited impact on the methane produced during the DAD Only a very slight increase in the methane yield could be de tected after treatment with hot water at severe conditions ie high temperature and long treatment time Fig 4a Similar to the results obtained during LAD the maximum methane yield of 190 NmLgVS 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 NmLgVS Similar to the results obtained in the LAD 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 LAD versus DAD it was concluded that generally higher yields could be achieved in the L AD process than in the DAD process The highest methane potential in LAD was 241 mLgVS while the methane yield from the same sample in DAD was only 190 NmLgVS Moreover as expected longer reten tion time was needed for DAD compared to LAD This could be due to the water limitation presented in DAD and high viscosity causing limitations in mass transfer of substrate and intermediates within the system 79 However one important benefit of DAD is reduction of reactor volume Hence the produced methane can be calculated con sidering the unit of reactor volume of the digester 54 According to these calculations in this study the volumetric methane productivities LmethaneLworking volume of straw samples pretreated at the optimal conditions and then subjected to LAD versus DAD processes were 334 and 593 LmethaneLworking volume respectively This considerable en hancement in volumetric productivity showed the benefit of using DAD instead of LAD for substrates with high solid contents meaning that a smaller reactor would be enough for an equal solid loading However the DAD process has some operational difficulties including loading and uploading high solid content feedstock which accompanied with 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 LAD see Fig 1 caption for other letters used F Momayez et al Energy Conversion and M anagem ent 178 2018 290298 294 major problems in largescale 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 3455 35 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 dif ferent 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 475657 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 106 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 accessi bility of cellulose for enzymatic hydrolysis However it is not the only factor playing role in biomass recalcitrance The obtained higher glu cose 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 Re arrangement of xylan location in biomass especially the one sur rounded 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 05 H2SO4 at 140 C for 5 min Nevertheless no improvement in glucose concentra tion 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 char acteristics and digestibility of three different switchgrass substrates They have observed that although the xylan content of the dilute acid pretreated substrate was reduced to 08 for one of the switchgrass types the highest glucose yield was achieved from a substrate treated with ionic liquid still containing 228 xylan Compared to that obtained after the treatment with hot water less acidinsoluble lignin AIL was presented in the straw after treatment with the BLW Furthermore no significant changes were observed in 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 DAD see Fig 1 caption for other letters used 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 30 9216 013 8525 014 9208 160 30 9446 033 8773 019 8433 190 30 9567 066 8689 086 6333 130 60 9365 02 8660 006 8558 160 60 9436 079 8710 065 7867 190 60 9383 012 8455 032 5883 Biogas Liquid Waste 130 30 9354 042 8579 041 8850 160 30 9377 081 8637 053 8150 190 30 9298 024 8293 047 7633 130 60 9431 048 8665 078 8583 160 60 9417 091 8603 069 8033 190 60 9447 004 8409 004 6975 Untreated Straw 9311 093 8323 018 F Momayez et al Energy Conversion and M anagem ent 178 2018 290298 295 the content of acidsoluble lignin ASL after any pretreatment condi tions 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 tem peratures in the enzymatic hydrolysis SSF LAD and DAD could be related to the removal of hemicellulose Hemicellulose acts as a phy sical barrier preventing accessibility of hydrolytic enzymes and mi croorganisms to cellulose 60 However it should be noticed that hemicellulose can be utilized for methane production in LAD and DAD 6162 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 re sulting from the treatments with the BLW led to higher methane yield in both the LAD and the DAD assays In contrast to the results obtained for methane yield from water treated straw an improvement in ethanol production was observed after treating at severe conditions This improvement in ethanol pro duction 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 com pounds is desirable 36 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 how ever 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 cm1 refers to cellulose I or crystalline cellulose with high re sistance to biological digestibility while the absorption band at 896 cm1 refers to cellulose II or amorphous cellulose with less re sistance to enzymatic and microbial hydrolysis Crystallinity index CI was calculated using the absorbance ratio of A1430A896 64 The absorption band at 1510 cm1 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 untreated and four treated samples after baseline correction and nor malization to the band at 1510 cm1 The calculated CI for untreated straw was 138 which was reduced to 123 or to 118 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 123 or 124 respectively Therefore higher amount of cellulose II compared to cellulose I presented in rice straw after pretreatments at these con ditions The absorption band at 1730 cm1 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 ob tained from the compositional analyses of the samples Table 3 The band position at about 3450 cm1 is attributed to OH stretching Re duced intensity at this band position indicates disruption of hydrogen bonds of cellulose A slight reduction in the intensity of peaks at the 16501560 cm1 bands was observed after pretreatment This reduc tion can be attributed to lignin removal during the pretreatment 66 Surface characterization as well as morphological features of un treated and treated straw was inferred from SEM images Fig 6 Raw straw and straw treated at lower temperature ie 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 Table 3 Chemical composition of raw and treated rice strawa Pretreatment conditions Glucan b Xylan Arabinan AIL ASL Pretreatment medium Temperature C Time min Water 130 30 377 05 165 02 29 01 142 09 51 00 160 30 402 03 177 01 27 01 148 06 49 01 190 30 507 09 94 02 07 00 184 06 44 01 130 60 389 05 171 02 29 01 151 02 49 00 160 60 419 02 166 01 18 01 161 04 48 00 190 60 529 04 34 01 02 01 239 08 54 03 Biogas Liquid Waste 130 30 385 04 168 02 30 01 146 06 52 00 160 30 406 02 176 02 31 01 132 16 46 01 190 30 43 09 140 02 21 02 148 05 45 03 130 60 384 17 168 07 29 02 140 05 47 01 160 60 398 12 164 05 31 01 139 06 48 05 190 60 460 06 106 02 10 01 175 06 57 02 Untreated Straw 379 06 180 01 31 01 161 02 44 00 a All analyses were triplicated and the averages of values are reported b The data are reported based on the oven dry weight of straw Wavenumber 1cm Absorption 500 1000 1500 2000 2500 3000 3500 4000 Untreated W190C 30 min BLW190C 30 min W190C 60 min BLW190C 60 min 896 1430 1730 3450 Fig 5 FTIR spectra of untreated and treated samples W Water BLW Biogas Liquid Waste F Momayez et al Energy Conversion and M anagem ent 178 2018 290298 296 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 avail ability of cellulose for microbial and enzymatic attacks BLW contains different weak organic acids including acetic pro pionic isobutyric butyric isovaleric and valeric acids Table 1 Weak acids at high temperature attack the hemicellulosic parts of lig nocelluloses 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 hemi cellulose 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 levelingoff 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 re moval however relocalization of lignin and disruption lignincarbo hydrate 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 relocalized lignin However more analysis is necessary to locate the lignin in the biomass 4 Conclusions This study investigated the possibility of using the biogas liquid waste ie the supernatant liquid phase obtained after centrifugation of digestion residue for the pretreatment of rice straw at various condi tions 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 twofold after the treatment Improvement in methane production via LAD and DAD was 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