Bioresource Technology 124(2012)379-386 Contents lists available at SciVerse ScienceDirect 88恶 Bioresource Technology ELSEVIER journal homepage:www.elsevier.com/locate/biortech Comparison of solid-state to liquid anaerobic digestion of lignocellulosic feedstocks for biogas production Dan Brown,Jian Shi,Yebo Li* Department of Food,Agricultural,and Biological Engineering.The Ohio State University/Ohio Agricultural Research and Development Center,1680 Madison Ave,Wooster, OH 44691-4096.United States HIGH LIGH TS Solid state anaerobic digestion(SS-AD)of eight types of lignocellulosic biomass. Liquid anaerobic digestion (L-AD)of eight biomass feedstocks was compared with SS-AD. No significant difference in methane yield between SS-AD and L-AD. Volumetric biogas productivity of SS-AD was 2-to 7-fold greater than that with L-AD. Methane yields from crop residues were higher than those from woody biomass. ARTICLE IN FO ABSTRACT Article history: Lignocellulosic biomass feedstocks (switchgrass,corn stover,wheat straw,yard waste,leaves,waste Received 20 April 2012 paper,maple,and pine)were evaluated for methane production under liquid anaerobic digestion(L- Received in revised form 11 August 2012 AD)and solid-state anaerobic digestion(SS-AD).No significant difference in methane yield between L- Accepted 13 August 2012 Available online 22 August 2012 AD and SS-AD.except for waste paper and pine,were found.However,the volumetric productivity was 2-to 7-fold greater in the SS-AD system compared with the L-AD system,except for paper.Methane yields from corn stover,wheat straw,and switchgrass were 2-5 times higher than those from yard waste. Keywords: Solid-state anaerobic digestion maple,and pine biomass.Waste paper had a methane yield of only 15 L/kg VS caused by souring during Biogas SS-AD due to organic overloading.Pine also had very low biogas yield of 17 L/kg VS,indicating the need Corn stover for pretreatment prior to SS-AD.The findings of this study can guide future studies to improve the Switchgrass efficiency and stability of SS-AD of lignocellulosic biomass. Lignocellulosic biomass 2012 Elsevier Ltd.All rights reserved. 1.Introduction methanogenic bacteria involved in AD have a low growth rate and are sensitive to inhibitors such as low pH caused by excessive Anaerobic digestion (AD)is a naturally occurring phenomenon concentrations of volatile fatty acids (VFAs).Thus,maintaining a in which organic matter is decomposed by an assortment of mi- balance of the four phases (hydrolysis,acidogenesis,acetogenesis crobes in an oxygen-free environment to produce biogas,composed and methanogenesis)of the AD process is essential.The effect of primarily of methane(CH4)and carbon dioxide(CO2)(Frigon and composition of lignocellulosic biomass on methane yield has al- Guiot,2010).Although the initial applications of AD were for stabil- ready been studied extensively:however,most of these studies ization and treatment of waste sludge,AD can also be a source of were limited to liquid AD (L-AD)which operates at a total solids renewable energy (Yu and Schanbacher,2010).AD not only (TS)content of 15%or less even though solid-state AD (SS-AD). provides an alternative source of energy but is also an alternative which is generally operated at a TS content of 15%or higher route to divert organic wastes and reduce greenhouse gas emissions (Guendouz et al.,2010),would be ideal for feedstocks such as agri- from landfills(Mata-Alvarez et al.,2000;Frigon and Guiot,2010). cultural and municipal solids wastes due to their availability and The presence of lignin,the crystallinity of cellulose,and limited low moisture content(Yu and Schanbacher,2010).SS-AD has many surface availability reduce the biodegradability of lignocellulosic advantages over L-AD including a smaller reactor volume for the biomass,making the hydrolysis step one of the bottlenecks that same solids loading;fewer moving parts:lower energy input for limit the production of methane (Frigon and Guiot,2010).The heating and mixing:easier to handle end product;and a greater acceptance of inputs containing glass,plastics,and grit (Li et al.. Corresponding author.Tel.:+1 330 263 3855. 2011).Problems in L-AD,such as floating and stratification of fats, E-mail address:li.851@osu.edu (Y.Li). fibers,and plastics,are not present in SS-AD(Chanakya et al.,1999). 0960-8524/-see front matter2012 Elsevier Ltd.All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.08.051
Comparison of solid-state to liquid anaerobic digestion of lignocellulosic feedstocks for biogas production Dan Brown, Jian Shi, Yebo Li ⇑ Department of Food, Agricultural, and Biological Engineering, The Ohio State University/Ohio Agricultural Research and Development Center, 1680 Madison Ave., Wooster, OH 44691-4096, United States highlights " Solid state anaerobic digestion (SS-AD) of eight types of lignocellulosic biomass. " Liquid anaerobic digestion (L-AD) of eight biomass feedstocks was compared with SS-AD. " No significant difference in methane yield between SS-AD and L-AD. " Volumetric biogas productivity of SS-AD was 2- to 7-fold greater than that with L-AD. " Methane yields from crop residues were higher than those from woody biomass. article info Article history: Received 20 April 2012 Received in revised form 11 August 2012 Accepted 13 August 2012 Available online 22 August 2012 Keywords: Solid-state anaerobic digestion Biogas Corn stover Switchgrass Lignocellulosic biomass abstract Lignocellulosic biomass feedstocks (switchgrass, corn stover, wheat straw, yard waste, leaves, waste paper, maple, and pine) were evaluated for methane production under liquid anaerobic digestion (LAD) and solid-state anaerobic digestion (SS-AD). No significant difference in methane yield between LAD and SS-AD, except for waste paper and pine, were found. However, the volumetric productivity was 2- to 7-fold greater in the SS-AD system compared with the L-AD system, except for paper. Methane yields from corn stover, wheat straw, and switchgrass were 2–5 times higher than those from yard waste, maple, and pine biomass. Waste paper had a methane yield of only 15 L/kg VS caused by souring during SS-AD due to organic overloading. Pine also had very low biogas yield of 17 L/kg VS, indicating the need for pretreatment prior to SS-AD. The findings of this study can guide future studies to improve the efficiency and stability of SS-AD of lignocellulosic biomass. 2012 Elsevier Ltd. All rights reserved. 1. Introduction Anaerobic digestion (AD) is a naturally occurring phenomenon in which organic matter is decomposed by an assortment of microbes in an oxygen-free environment to produce biogas, composed primarily of methane (CH4) and carbon dioxide (CO2) (Frigon and Guiot, 2010). Although the initial applications of AD were for stabilization and treatment of waste sludge, AD can also be a source of renewable energy (Yu and Schanbacher, 2010). AD not only provides an alternative source of energy but is also an alternative route to divert organic wastes and reduce greenhouse gas emissions from landfills (Mata-Alvarez et al., 2000; Frigon and Guiot, 2010). The presence of lignin, the crystallinity of cellulose, and limited surface availability reduce the biodegradability of lignocellulosic biomass, making the hydrolysis step one of the bottlenecks that limit the production of methane (Frigon and Guiot, 2010). The methanogenic bacteria involved in AD have a low growth rate and are sensitive to inhibitors such as low pH caused by excessive concentrations of volatile fatty acids (VFAs). Thus, maintaining a balance of the four phases (hydrolysis, acidogenesis, acetogenesis and methanogenesis) of the AD process is essential. The effect of composition of lignocellulosic biomass on methane yield has already been studied extensively; however, most of these studies were limited to liquid AD (L-AD) which operates at a total solids (TS) content of 15% or less even though solid-state AD (SS-AD), which is generally operated at a TS content of 15% or higher (Guendouz et al., 2010), would be ideal for feedstocks such as agricultural and municipal solids wastes due to their availability and low moisture content (Yu and Schanbacher, 2010). SS-AD has many advantages over L-AD including a smaller reactor volume for the same solids loading; fewer moving parts; lower energy input for heating and mixing; easier to handle end product; and a greater acceptance of inputs containing glass, plastics, and grit (Li et al., 2011). Problems in L-AD, such as floating and stratification of fats, fibers, and plastics, are not present in SS-AD (Chanakya et al., 1999). 0960-8524/$ - see front matter 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.08.051 ⇑ Corresponding author. Tel.: +1 330 263 3855. E-mail address: li.851@osu.edu (Y. Li). Bioresource Technology 124 (2012) 379–386 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
380 D.Brown et aL/Bioresource Technology 124 (2012)379-386 Feedstocks for AD are influenced by accessibility and availabil- et al.,2012).Well mixed materials were loaded into 1-L glass reac- ity due to costs associated with collection and transportation(Fri- tors and incubated in a walk-in incubation room for up to 30 days gon and Guiot,2010:Li et al,2011).Lignocellulosic biomass can be at 37t 1 C.Duplicate reactors were run for each condition.Inocu- harvested at a high TS content,which results in lower transporta- lum without any feedstock addition was used as a control.Biogas tion costs per unit of solids compared to low TS feedstocks.Crop was collected in a 5-L Tedlar gas bag (CEL Scientific,Santa Fe residues,such as corn stover and wheat straw,are widespread Springs,CA,USA)attached to the outlet of the reactor.Biogas com- through much of the US.Switchgrass has the potential to be grown position and volume were measured every 2-3 days during the 30- in areas where common crops experience low production,such as day SS-AD. areas currently in the Conservation Reserve Program through the Natural Resources Conservation Service.Yard waste,which in- 2.3.Liquid anaerobic digestion (L-AD) cludes grass,leaves,and branches,is a major lignocellulosic source generated from households,municipalities,and landscaping com- Each feedstock was mixed with deionized(DI)water and efflu- panies.Although yard waste can be composted,the stored energy ent to obtain a mixture of 5%TS and an F/E ratio of 0.5 based on dry is lost in the form of respiration heat(Koch et al.,2010).The use of volatile solids (VS).The mixtures were loaded in 2-L glass jars and yard waste,leaves,maple,and pine may improve the overall eco- incubated in a walk-in incubation room on an Innova model 2300 nomics of SS-AD due to the low or zero cost associated with acquir- platform shaker (New Brunswick Scientific;Enfield,CT.USA)at ing these feedstocks (Jagadish et al..1998).Waste paper is paper 150 rpm for up to 30 days at 37+1C.Duplicate reactors were that either cannot be recycled and/or is used in some other way run for each condition.Inoculum without any feedstock addition such as a landfill cover.Paper is treated both chemically and ther- was used as a control.Biogas generated was collected in a 5-L Ted- mally during the paper-making process,effectively serving as a lar gas bag(CEL Scientific,Santa Fe Springs,CA,USA)attached to pretreatment and allows the breakdown of cellulose to occur.A the outlet of the reactor.Biogas composition and volume were number of these lignocellulosic biomass sources have been tested measured every 2-3 days during the 30-day L-AD. for methane production from L-AD systems (Turick et al.,1991: Gunaseelan,1997);however,the suitability of lignocellulosic bio- 2.4.Enzyme hydrolysis of lignocellulosic feedstocks mass as feedstocks and factors affecting methane production dur- ing SS-AD have not been studied. Cellulase (Spezyme CP)(Lot#4900857805,protein content Therefore,the objectives of the present study were to 82 mg/ml with activity of approximately 50 FPU/ml),and Multifect determine:(1)the methane yield and volumetric productivity of xylanase(Lot#4901063047,protein content 27 mg/ml with activ- switchgrass,corn stover,wheat straw,yard waste,leaves,maple, ity of approximately 25000 OSX/ml).were obtained from Genen- pine,and waste paper in L-AD and SS-AD systems using a single cor,now DuPont Industrial Biosciences (Palo Alto,CA,USA).All source of inoculum:and(2)the effect of the composition and enzy- enzymatic hydrolysis experiments were run in duplicate following matic digestibility of lignocellulosic biomass on methane yield the National Renewable Energy Laboratory(NREL)Laboratory Ana- from SS-AD. lytical Procedures(LAP)(Selig et al.,2008)at 50C with 0.05 M cit- rate buffer,pH 4.8,solid loading of 2%,and cellulase loading of 15 FPU/g solids supplemented with xylanase at a loading of 3750 OSX/ 2.Methods g solids on a rotary shaker (150 rpm for 72 h).The hydrolysate sample was filtered through a 0.2 um nylon membrane filter for 2.1.Feedstock and inoculum sugar analysis by HPLC as described in Section 2.5.The overall glu- cose and xylose yields of enzymatic hydrolysis (i.e.,enzymatic Corn stover,wheat straw,and switchgrass were collected in digestibility)were defined according to Cui et al.(2011). October 2009 from farms operated by the Ohio Agricultural Re- search and Development Center (OARDC)in Wooster,and Jackson, 2.5.Analytical methods OH.Fallen tree leaves were collected in October 2010 from the OARDC Wooster campus.Yard waste containing leaves and tree The extractive content of feedstock was measured according to branches was obtained in June 2011 from the OARDC Wooster the NREL Laboratory Analytical Procedure (Sluiter et al.,2008a). campus.Pine and maple wood with bark were obtained in June The extractive-free biomass and solid fractions before and after 2011 from the OARDC Wooster campus.The shredded waste paper digestion were further fractionated using a two-step hydrolysis used was classified as alternative daily cover for landfills and col- method based on the NREL Laboratory Analytical Procedure(Sluit- lected from the Solid Waste Management facility of Wayne County. er et al.,2008b).Monomeric sugars (glucose,xylose,galactose OH.These feedstocks were oven-dried at 40C for 48 h in a convec- arabinose,and mannose)and cellobiose in the acid hydrolysate tion oven (Precision Thelco Model 18,Waltham,MA)to obtain a were measured by an HPLC instrument(Shimadzu LC-20AB,MD. moisture content of less than 10%,ground with a hammer mill USA)equipped with a Biorad Aminex HPX-87P column and a (Mackisik,Parker Ford,PA)to pass through a 5 mm screen,and refractive index detector (RID).A deionized water flow rate of stored in air tight containers. 0.6 ml/min was used as the mobile phase.The temperature of Effluent from a mesophillic liquid anaerobic digester in Colum- the column and detector were maintained at 80 and 55 C,respec- bus,OH fed with municipal waste water sludge and food waste tively.The Standard Methods for the Examination of Water and was used as inoculum.The inoculum was kept in air-tight buckets Wastewater were used to analyze the TS and VS contents of feed- at 4C.Prior to use,the inoculum was acclimated at 37C for stocks,inoculum,and digestate taken at the beginning and end of 1 week. the AD process(Eaton et al.,2005).Total carbon and nitrogen con- tents in feedstock,effluent and digestion materials were deter- 2.2.Solid-state anaerobic digestion (SS-AD) mined using an elemental analyzer (Elementar Vario Max CNS, Elementar Americas,Mt.Laurel,NI.USA).VFAs and alkalinity were Each lignocellulosic biomass feedstock was mixed with the measured using a two-step titration method(McGhee,1968).Sam- inoculum using a hand-mixer(Black Decker,250-watt mixer, ples for pH,VFA,and alkalinity measurements were prepared by Towson,MD,USA)to obtain a feedstock to effluent (F/E)ratio of diluting a 5-g sample with 50 ml of DI water.The mixture was ana- 3(based on dry volatile solids)and TS content of 18-19%(Liew lyzed using an auto-titrator(Mettler Toledo,DL22 Food Beverage
Feedstocks for AD are influenced by accessibility and availability due to costs associated with collection and transportation (Frigon and Guiot, 2010; Li et al., 2011). Lignocellulosic biomass can be harvested at a high TS content, which results in lower transportation costs per unit of solids compared to low TS feedstocks. Crop residues, such as corn stover and wheat straw, are widespread through much of the US. Switchgrass has the potential to be grown in areas where common crops experience low production, such as areas currently in the Conservation Reserve Program through the Natural Resources Conservation Service. Yard waste, which includes grass, leaves, and branches, is a major lignocellulosic source generated from households, municipalities, and landscaping companies. Although yard waste can be composted, the stored energy is lost in the form of respiration heat (Koch et al., 2010). The use of yard waste, leaves, maple, and pine may improve the overall economics of SS-AD due to the low or zero cost associated with acquiring these feedstocks (Jagadish et al., 1998). Waste paper is paper that either cannot be recycled and/or is used in some other way such as a landfill cover. Paper is treated both chemically and thermally during the paper-making process, effectively serving as a pretreatment and allows the breakdown of cellulose to occur. A number of these lignocellulosic biomass sources have been tested for methane production from L-AD systems (Turick et al., 1991; Gunaseelan, 1997); however, the suitability of lignocellulosic biomass as feedstocks and factors affecting methane production during SS-AD have not been studied. Therefore, the objectives of the present study were to determine: (1) the methane yield and volumetric productivity of switchgrass, corn stover, wheat straw, yard waste, leaves, maple, pine, and waste paper in L-AD and SS-AD systems using a single source of inoculum; and (2) the effect of the composition and enzymatic digestibility of lignocellulosic biomass on methane yield from SS-AD. 2. Methods 2.1. Feedstock and inoculum Corn stover, wheat straw, and switchgrass were collected in October 2009 from farms operated by the Ohio Agricultural Research and Development Center (OARDC) in Wooster, and Jackson, OH. Fallen tree leaves were collected in October 2010 from the OARDC Wooster campus. Yard waste containing leaves and tree branches was obtained in June 2011 from the OARDC Wooster campus. Pine and maple wood with bark were obtained in June 2011 from the OARDC Wooster campus. The shredded waste paper used was classified as alternative daily cover for landfills and collected from the Solid Waste Management facility of Wayne County, OH. These feedstocks were oven-dried at 40 C for 48 h in a convection oven (Precision Thelco Model 18, Waltham, MA) to obtain a moisture content of less than 10%, ground with a hammer mill (Mackisik, Parker Ford, PA) to pass through a 5 mm screen, and stored in air tight containers. Effluent from a mesophillic liquid anaerobic digester in Columbus, OH fed with municipal waste water sludge and food waste was used as inoculum. The inoculum was kept in air-tight buckets at 4 C. Prior to use, the inoculum was acclimated at 37 C for 1 week. 2.2. Solid-state anaerobic digestion (SS-AD) Each lignocellulosic biomass feedstock was mixed with the inoculum using a hand-mixer (Black & Decker, 250-watt mixer, Towson, MD, USA) to obtain a feedstock to effluent (F/E) ratio of 3 (based on dry volatile solids) and TS content of 18–19% (Liew et al., 2012). Well mixed materials were loaded into 1-L glass reactors and incubated in a walk-in incubation room for up to 30 days at 37 ± 1 C. Duplicate reactors were run for each condition. Inoculum without any feedstock addition was used as a control. Biogas was collected in a 5-L Tedlar gas bag (CEL Scientific, Santa Fe Springs, CA, USA) attached to the outlet of the reactor. Biogas composition and volume were measured every 2–3 days during the 30- day SS-AD. 2.3. Liquid anaerobic digestion (L-AD) Each feedstock was mixed with deionized (DI) water and effluent to obtain a mixture of 5% TS and an F/E ratio of 0.5 based on dry volatile solids (VS). The mixtures were loaded in 2-L glass jars and incubated in a walk-in incubation room on an Innova model 2300 platform shaker (New Brunswick Scientific; Enfield, CT, USA) at 150 rpm for up to 30 days at 37 ± 1 C. Duplicate reactors were run for each condition. Inoculum without any feedstock addition was used as a control. Biogas generated was collected in a 5-L Tedlar gas bag (CEL Scientific, Santa Fe Springs, CA, USA) attached to the outlet of the reactor. Biogas composition and volume were measured every 2–3 days during the 30-day L-AD. 2.4. Enzyme hydrolysis of lignocellulosic feedstocks Cellulase (Spezyme CP) (Lot#4900857805, protein content 82 mg/ml with activity of approximately 50 FPU/ml), and Multifect xylanase (Lot#4901063047, protein content 27 mg/ml with activity of approximately 25000 OSX/ml), were obtained from Genencor, now DuPont Industrial Biosciences (Palo Alto, CA, USA). All enzymatic hydrolysis experiments were run in duplicate following the National Renewable Energy Laboratory (NREL) Laboratory Analytical Procedures (LAP) (Selig et al., 2008) at 50 C with 0.05 M citrate buffer, pH 4.8, solid loading of 2%, and cellulase loading of 15 FPU/g solids supplemented with xylanase at a loading of 3750 OSX/ g solids on a rotary shaker (150 rpm for 72 h). The hydrolysate sample was filtered through a 0.2 lm nylon membrane filter for sugar analysis by HPLC as described in Section 2.5. The overall glucose and xylose yields of enzymatic hydrolysis (i.e., enzymatic digestibility) were defined according to Cui et al. (2011). 2.5. Analytical methods The extractive content of feedstock was measured according to the NREL Laboratory Analytical Procedure (Sluiter et al., 2008a). The extractive-free biomass and solid fractions before and after digestion were further fractionated using a two-step hydrolysis method based on the NREL Laboratory Analytical Procedure (Sluiter et al., 2008b). Monomeric sugars (glucose, xylose, galactose, arabinose, and mannose) and cellobiose in the acid hydrolysate were measured by an HPLC instrument (Shimadzu LC-20AB, MD, USA) equipped with a Biorad Aminex HPX-87P column and a refractive index detector (RID). A deionized water flow rate of 0.6 ml/min was used as the mobile phase. The temperature of the column and detector were maintained at 80 and 55 C, respectively. The Standard Methods for the Examination of Water and Wastewater were used to analyze the TS and VS contents of feedstocks, inoculum, and digestate taken at the beginning and end of the AD process (Eaton et al., 2005). Total carbon and nitrogen contents in feedstock, effluent and digestion materials were determined using an elemental analyzer (Elementar Vario Max CNS, Elementar Americas, Mt. Laurel, NJ, USA). VFAs and alkalinity were measured using a two-step titration method (McGhee, 1968). Samples for pH, VFA, and alkalinity measurements were prepared by diluting a 5-g sample with 50 ml of DI water. The mixture was analyzed using an auto-titrator (Mettler Toledo, DL22 Food & Beverage 380 D. Brown et al. / Bioresource Technology 124 (2012) 379–386
D.Brown et al/Bioresource Technology 124(2012)379-386 381 Analyzer,Columbus,OH,USA).The VFA/alkalinity ratio was calcu- which are easily degradable and can potentially contribute to bio- lated using the empirical formula to determine the risk of acidifi- gas generation(Tong et al.,1990;Zheng et al.,2009). cation,a measure of the process stability (Anderson and Yang. 1992) Biogas volume and composition were determined every 2- 3.2.Biogas production 3 days.The volume of biogas collected in a 5-L Tedlar bag was mea- Fig.1a shows the daily methane yield during the 30-day SS-AD sured with a drum-type gas meter(Ritter,TG 5,Germany)and the composition of biogas(CO2.CH4.N2.and O2)was analyzed using a period.Corn stover and wheat straw had significantly higher peak daily methane yields (~12 L/kg VS)compared to the rest of the GC(Agilent Technologies,HP 6890,DE,USA)equipped with a Ther- feedstocks.Leaves,yard waste,and waste paper had peaks be- mal Conductivity Detector at 200C and a 10-ft stainless steel col- tween 8.7 L/kg VS(switchgrass)and 6.5 L/kg VS(maple)and pine umn 45/60 Molecular Sieve 13X.Helium was used as a carrier gas had a significantly lower peak of 2.5 L/kg VS.Different feedstocks at a flow rate of 5.2 ml/min.The temperature of the column oven peaked at different times.Waste paper peaked at day 4;switch- was initially programmed at 40C for 4 min,then elevated to 60 C at 20C/min and held for 5 min. grass,leaves,yard waste,and maple all peaked at day 6;corn stover peaked at day 8:wheat straw peaked at day 9;and pine Methane yield expressed in L/kg VSfeedstock was calculated as the peaked at day 13.Waste paper had two significant peaks and volume of methane gas produced per kg of VS loaded into the reac- tor at start-up corrected by subtracting the methane yield obtained stopped producing methane by day 13. Fig.1b shows the daily methane yield observed during the 30- from the control reactor with only inoculum then adjusted linearly (Angelidaki et al.,2009).Volumetric methane productivity of ligno- day L-AD period.Waste paper achieved the highest daily methane yields of 19.2 L/kg VS.The rest of the feedstocks had peaks between cellulosic biomass is expressed in Vmethane/Vwork:volume of meth- 11.6 L/kg VS(wheat straw)and 6.3 L/kg VS(yard waste and maple). ane gas produced (Vmethane)per unit working volume of reactor Yard waste peaked at day 4;corn stover,wheat straw,and maple (Vwork). peaked at day 7:waste paper peaked at day 11:pine at day 13: and leaves and switchgrass at day 14.However,these peaks are 2.6.Statistical analysis not as well defined,that is,do not show clear increases in daily methane yield over a couple of days as the peaks in SS-AD,since Data were analyzed using PROC GLM for means and variances a much smaller amount of lignocellulosic biomass was used in L- and statistical significance was determined by analysis of variance AD.The high concentration of lignocellulosic biomass in SS-AD (ANOVA)using SAS software (Version 8.1,SAS Institute Inc.,Cary. may have resulted in the enrichment of hydrolytic and acidogenic NC,USA)with a threshold p-value of 0.05 microbes in early stages of the digestion,and thus led to high VS destruction and,subsequently,a more apparent peak of biogas 3.Results and discussion production than for L-AD. The total methane yield of the lignocellulosic biomass during 3.1.Composition of inoculum and lignocellulosic biomass 30 days of SS-AD and L-AD are presented in Fig.2a.Compared to SS-AD,total methane yields from L-AD were similar with the Table 1 shows the TS,VS,C/N ratio,extractives,lignin,cellulose, exception of waste paper and pine.Methane yields of paper and hemicellulose,pH,and VFA/alkalinity ratio,of the inoculum and pine were significantly lower in the SS-AD compared to the L-AD. lignocellulosic biomass tested in this study.The C/N ratio of the It is speculated that the highly digestible waste paper quickly leaves of 26.1 was considerably lower than of the other materials. caused acidification and inhibition to methanogenesis during SS- The cellulose content was highest in waste paper(60.0%):but low- AD and led to a methane yield of only 15 L/kg VS;however,due est in leaves (11.7%).Switchgrass had the highest hemicellulose to the lower organic loading and higher buffering capacity in L- content,which is the sum of the xylan,galactan,arabinan,and AD,a much higher methane yield of 312.4 L/kg VS was achieved mannan,among the tested feedstocks.The highest lignin content In contrast,pine,a soft wood notorious for its resistance to micro- was found in pine.The paper's lignin content was very low at bial degradation,resulted in a very low biogas yield for SS-AD com- 2.1%owing to the lignin removal during the thermo-chemical pa- pared with that from the other feedstocks,indicating the need for per-making process.Leaves contained the highest amount of pretreatment prior to feeding to SS-AD. extractives.In general,extractives include compounds such as free Volumetric productivities of lignocellulosic biomass in both sugars,oligosaccharides,and organic acids (Chen et al.,2007) L-AD and SS-AD are shown in Fig.2b.Comparison of volumetric Table 1 Characteristics of inoculum and lignocellulosic biomass. Parameters Inoculum Corn stover Switchgrass Wheat straw Leaves Yard waste Maple Waste paper Pine Total solids ( 7.7±0.0 91.6±03 93.0±03 90.0±0.3 93.0±0.1 943±0.1 93.6±0.5 94.2±02 93.0±0.1 Volatile solids 4.1±0.0 89.0±0.1 89.9±0.3 83.4±0.3 86.9±02 91.7±0.2 92.2±0.3 83.9±0.2 90.5±0.3 C:N ratio 3.0±0.0 746±5.1 898±0.1 60.0±00 26.1±00 553±32 1225±18 323.5±12.6 933±2.4 Extractives ( ND 65±0.1 11.9±0.3 17.0±0.1 35.1±1.1 14.7±0.5 5.1±0.6 9.9±1.1 14.2±0.6 Lignin(%泸 ND 17.1±0.3 17.8±0.2 15.2±0.1 22.8±1.2 22.1±0.9 22.0±0.3 2.1±0.1 28.3±0.6 Glucan ( ND 39.2±1.1 32.3±0.5 33.1±0.7 11.7±2.3 27.4±1.3 36.5±0.6 60.0±0.1 26.0±0.4 Xylan ( ND 15.3±0.6 16.7±0.1 15.2±0.3 42±0.5 9.0±0.7 9.2±0.2 8.7±0.5 4.4±0.3 Galactan ( ND 0.0±0.0 0.0±0.0 0.0±0.0 2.1±03 0.9±0.1 0.0±0.0 0.0±0.0 39±0.5 Arabinan ( ND 1.6±0.4 2.0±0.2 15±0.1 18±0.4 1.1±0.1 09±0.2 0.0±0.0 15±0.2 Mannan ( ND 0.0±0.0 0.0±0.0 10±0.1 1.0±0.0 1.2±0.1 12±02 22±0.0 4.4±0.6 pH 7.8±0.0 ND ND ND ND ND ND ND ND VFA/alkalinity ratio 1.00±021 ND ND ND ND ND ND ND ND a Data shown are the average and standard deviation based on duplicate runs:ND=not determined. b based on the total solids of sample. based on the total weight of sample. mg HAceq/mg CaCO3
Analyzer, Columbus, OH, USA). The VFA/alkalinity ratio was calculated using the empirical formula to determine the risk of acidifi- cation, a measure of the process stability (Anderson and Yang, 1992). Biogas volume and composition were determined every 2– 3 days. The volume of biogas collected in a 5-L Tedlar bag was measured with a drum-type gas meter (Ritter, TG 5, Germany) and the composition of biogas (CO2, CH4, N2, and O2) was analyzed using a GC (Agilent Technologies, HP 6890, DE, USA) equipped with a Thermal Conductivity Detector at 200 C and a 10-ft stainless steel column 45/60 Molecular Sieve 13X. Helium was used as a carrier gas at a flow rate of 5.2 ml/min. The temperature of the column oven was initially programmed at 40 C for 4 min, then elevated to 60 C at 20 C/min and held for 5 min. Methane yield expressed in L/kg VSfeedstock was calculated as the volume of methane gas produced per kg of VS loaded into the reactor at start-up corrected by subtracting the methane yield obtained from the control reactor with only inoculum then adjusted linearly (Angelidaki et al., 2009). Volumetric methane productivity of lignocellulosic biomass is expressed in Vmethane/Vwork: volume of methane gas produced (Vmethane) per unit working volume of reactor (Vwork). 2.6. Statistical analysis Data were analyzed using PROC GLM for means and variances and statistical significance was determined by analysis of variance (ANOVA) using SAS software (Version 8.1, SAS Institute Inc., Cary, NC, USA) with a threshold p-value of 0.05. 3. Results and discussion 3.1. Composition of inoculum and lignocellulosic biomass Table 1 shows the TS, VS, C/N ratio, extractives, lignin, cellulose, hemicellulose, pH, and VFA/alkalinity ratio, of the inoculum and lignocellulosic biomass tested in this study. The C/N ratio of the leaves of 26.1 was considerably lower than of the other materials. The cellulose content was highest in waste paper (60.0%); but lowest in leaves (11.7%). Switchgrass had the highest hemicellulose content, which is the sum of the xylan, galactan, arabinan, and mannan, among the tested feedstocks. The highest lignin content was found in pine. The paper’s lignin content was very low at 2.1% owing to the lignin removal during the thermo-chemical paper-making process. Leaves contained the highest amount of extractives. In general, extractives include compounds such as free sugars, oligosaccharides, and organic acids (Chen et al., 2007) which are easily degradable and can potentially contribute to biogas generation (Tong et al., 1990; Zheng et al., 2009). 3.2. Biogas production Fig. 1a shows the daily methane yield during the 30-day SS-AD period. Corn stover and wheat straw had significantly higher peak daily methane yields (12 L/kg VS) compared to the rest of the feedstocks. Leaves, yard waste, and waste paper had peaks between 8.7 L/kg VS (switchgrass) and 6.5 L/kg VS (maple) and pine had a significantly lower peak of 2.5 L/kg VS. Different feedstocks peaked at different times. Waste paper peaked at day 4; switchgrass, leaves, yard waste, and maple all peaked at day 6; corn stover peaked at day 8; wheat straw peaked at day 9; and pine peaked at day 13. Waste paper had two significant peaks and stopped producing methane by day 13. Fig. 1b shows the daily methane yield observed during the 30- day L-AD period. Waste paper achieved the highest daily methane yields of 19.2 L/kg VS. The rest of the feedstocks had peaks between 11.6 L/kg VS (wheat straw) and 6.3 L/kg VS (yard waste and maple). Yard waste peaked at day 4; corn stover, wheat straw, and maple peaked at day 7; waste paper peaked at day 11; pine at day 13; and leaves and switchgrass at day 14. However, these peaks are not as well defined, that is, do not show clear increases in daily methane yield over a couple of days as the peaks in SS-AD, since a much smaller amount of lignocellulosic biomass was used in LAD. The high concentration of lignocellulosic biomass in SS-AD may have resulted in the enrichment of hydrolytic and acidogenic microbes in early stages of the digestion, and thus led to high VS destruction and, subsequently, a more apparent peak of biogas production than for L-AD. The total methane yield of the lignocellulosic biomass during 30 days of SS-AD and L-AD are presented in Fig. 2a. Compared to SS-AD, total methane yields from L-AD were similar with the exception of waste paper and pine. Methane yields of paper and pine were significantly lower in the SS-AD compared to the L-AD. It is speculated that the highly digestible waste paper quickly caused acidification and inhibition to methanogenesis during SSAD and led to a methane yield of only 15 L/kg VS; however, due to the lower organic loading and higher buffering capacity in LAD, a much higher methane yield of 312.4 L/kg VS was achieved. In contrast, pine, a soft wood notorious for its resistance to microbial degradation, resulted in a very low biogas yield for SS-AD compared with that from the other feedstocks, indicating the need for pretreatment prior to feeding to SS-AD. Volumetric productivities of lignocellulosic biomass in both L-AD and SS-AD are shown in Fig. 2b. Comparison of volumetric Table 1 Characteristics of inoculum and lignocellulosic biomass.a Parameters Inoculum Corn stover Switchgrass Wheat straw Leaves Yard waste Maple Waste paper Pine Total solids (%) 7.7 ± 0.0 91.6 ± 0.3 93.0 ± 0.3 90.0 ± 0.3 93.0 ± 0.1 94.3 ± 0.1 93.6 ± 0.5 94.2 ± 0.2 93.0 ± 0.1 Volatile solids (%)b 4.1 ± 0.0 89.0 ± 0.1 89.9 ± 0.3 83.4 ± 0.3 86.9 ± 0.2 91.7 ± 0.2 92.2 ± 0.3 83.9 ± 0.2 90.5 ± 0.3 C:N ratioc 3.0 ± 0.0 74.6 ± 5.1 89.8 ± 0.1 60.0 ± 0.0 26.1 ± 0.0 55.3 ± 3.2 122.5 ± 1.8 323.5 ± 12.6 93.3 ± 2.4 Extractives (%)b ND 6.5 ± 0.1 11.9 ± 0.3 17.0 ± 0.1 35.1 ± 1.1 14.7 ± 0.5 5.1 ± 0.6 9.9 ± 1.1 14.2 ± 0.6 Lignin (%)b ND 17.1 ± 0.3 17.8 ± 0.2 15.2 ± 0.1 22.8 ± 1.2 22.1 ± 0.9 22.0 ± 0.3 2.1 ± 0.1 28.3 ± 0.6 Glucan (%)b ND 39.2 ± 1.1 32.3 ± 0.5 33.1 ± 0.7 11.7 ± 2.3 27.4 ± 1.3 36.5 ± 0.6 60.0 ± 0.1 26.0 ± 0.4 Xylan (%)b ND 15.3 ± 0.6 16.7 ± 0.1 15.2 ± 0.3 4.2 ± 0.5 9.0 ± 0.7 9.2 ± 0.2 8.7 ± 0.5 4.4 ± 0.3 Galactan (%)b ND 0.0 ± 0.0 0.0 ± 0.0 0.0 ± 0.0 2.1 ± 0.3 0.9 ± 0.1 0.0 ± 0.0 0.0 ± 0.0 3.9 ± 0.5 Arabinan (%)b ND 1.6 ± 0.4 2.0 ± 0.2 1.5 ± 0.1 1.8 ± 0.4 1.1 ± 0.1 0.9 ± 0.2 0.0 ± 0.0 1.5 ± 0.2 Mannan (%)b ND 0.0 ± 0.0 0.0 ± 0.0 1.0 ± 0.1 1.0 ± 0.0 1.2 ± 0.1 1.2 ± 0.2 2.2 ± 0.0 4.4 ± 0.6 pH 7.8 ± 0.0 ND ND ND ND ND ND ND ND VFA/alkalinity ratiod 1.00 ± 0.21 ND ND ND ND ND ND ND ND a Data shown are the average and standard deviation based on duplicate runs; ND = not determined. b based on the total solids of sample. c based on the total weight of sample. d mg HAceq/mg CaCO3. D. Brown et al. / Bioresource Technology 124 (2012) 379–386 381
382 D.Brown et aL/Bioresource Technology 124(2012)379-386 14 -◇-5 witchgrass Corn Stover ((Aea 12 ★-Wheat Straw Leaves 10 --Yard Waste -Waste Paper 8 +Maple -Pine o 30 Time(Days) 25 -Switchgrass ■-Corn Stover t一Wheat Straw Leaves -米-Yard Waste 15 Waste Paper ...Maple 10 Pine 5 0 10 30 Time(Days) Fig.1.Daily methane production during 30-day AD in (a)SS-AD and (b)L-AD. productivities(Lmethane/Lwork)of L-AD to SS-AD showed increases of 11.5,15.1,15.0,and 15.0,respectively.The C/N ratios of SS-AD of 240-730%ofr L-AD while that of paper decreased by 21%.The high- switchgrass,corn stover,wheat straw,yard waste,leaves,waste er volumetric productivities of SS-AD indicate a major advantage paper,pine,and maple were43.0.39.0.36.0.31.5,21.0.64.1, over L-AD as a smaller reactor volume is required for the same sol- 46.2.and 50.3,respectively.Addition of nitrogen in SS-AD may ids loading:however,due to the difficulty in continuous loading eliminate the nutrient limitation placed on the bacteria consortium and unloading of solid feedstock,continuous operation of large but will add cost at commercial scale.This leads to the need for co- scale solid state digesters is challenging.In order to compare the digestion of lignocellulosic biomass at commercial scale with a methane yields of lignocellulosic biomass under both L-AD and nitrogen-rich substrate. SS-AD,this research was conducted at batch mode,but further In the present study,pH and VFA/alkalinity ratios were used as development of SS-AD technologies with continuous or semi-con- indicators of reactor performance (Lossie and Putz,2008).Initial tinuous configurations is favored for commercial systems. and final pH and VFA/alkalinity ratios of both L-AD and SS-AD can be found in Table 2.Initial pH of the lignocellulosic biomass 3.3.Reactor characteristics in SS-AD ranged from 7.1 for leaves to 8.3 for wheat straw.The fi- nal pH values were all considered healthy (above 7)except for Organic loading,C/N ratio,and the strength and amount of inoc- reactors fed with waste paper which had a pH of 6.2,indicating ulum are crucial factors affecting the performance of AD.In case of acidification due to VFA accumulation.Initial VFA/alkalinity ratios an imbalanced digester caused by high organic loading and C/N ra- were below 0.8 except for leaves and the final VFA/alkalinity ratios tio,accumulation of VFA and low pH become inhibitory to metha- were below 0.7 except for waste paper which,when coupled with a nogenesis(Frigon and Guiot,2010). pH of 6.2,indicates souring within the reactor.The L-AD reactors A balanced C/N ratio is crucial for growth of bacteria.The C/N had initial and final pH values in the range of 7.5-7.9 and initial ratios of L-AD of switchgrass,corn stover,wheat straw,yard waste, and final VFA/alkalinity ratios in the range of 0.27-0.60,indicating leaves,waste paper,pine,and maple were 14.3,13.2,13.5,11.8, healthy reactors (Lossie and Puitz,2008)
productivities (Lmethane/Lwork) of L-AD to SS-AD showed increases of 240–730% ofr L-AD while that of paper decreased by 21%. The higher volumetric productivities of SS-AD indicate a major advantage over L-AD as a smaller reactor volume is required for the same solids loading; however, due to the difficulty in continuous loading and unloading of solid feedstock, continuous operation of large scale solid state digesters is challenging. In order to compare the methane yields of lignocellulosic biomass under both L-AD and SS-AD, this research was conducted at batch mode, but further development of SS-AD technologies with continuous or semi-continuous configurations is favored for commercial systems. 3.3. Reactor characteristics Organic loading, C/N ratio, and the strength and amount of inoculum are crucial factors affecting the performance of AD. In case of an imbalanced digester caused by high organic loading and C/N ratio, accumulation of VFA and low pH become inhibitory to methanogenesis (Frigon and Guiot, 2010). A balanced C/N ratio is crucial for growth of bacteria. The C/N ratios of L-AD of switchgrass, corn stover, wheat straw, yard waste, leaves, waste paper, pine, and maple were 14.3, 13.2, 13.5, 11.8, 11.5, 15.1, 15.0, and 15.0, respectively. The C/N ratios of SS-AD of switchgrass, corn stover, wheat straw, yard waste, leaves, waste paper, pine, and maple were 43.0, 39.0, 36.0, 31.5, 21.0, 64.1, 46.2, and 50.3, respectively. Addition of nitrogen in SS-AD may eliminate the nutrient limitation placed on the bacteria consortium but will add cost at commercial scale. This leads to the need for codigestion of lignocellulosic biomass at commercial scale with a nitrogen-rich substrate. In the present study, pH and VFA/alkalinity ratios were used as indicators of reactor performance (Lossie and Pütz, 2008). Initial and final pH and VFA/alkalinity ratios of both L-AD and SS-AD can be found in Table 2. Initial pH of the lignocellulosic biomass in SS-AD ranged from 7.1 for leaves to 8.3 for wheat straw. The fi- nal pH values were all considered healthy (above 7) except for reactors fed with waste paper which had a pH of 6.2, indicating acidification due to VFA accumulation. Initial VFA/alkalinity ratios were below 0.8 except for leaves and the final VFA/alkalinity ratios were below 0.7 except for waste paper which, when coupled with a pH of 6.2, indicates souring within the reactor. The L-AD reactors had initial and final pH values in the range of 7.5–7.9 and initial and final VFA/alkalinity ratios in the range of 0.27–0.60, indicating healthy reactors (Lossie and Pütz, 2008). Fig. 1. Daily methane production during 30-day AD in (a) SS-AD and (b) L-AD. 382 D. Brown et al. / Bioresource Technology 124 (2012) 379–386
D.Brown et al/Bioresource Technology 124(2012)379-386 383 a 350 口SS-AD 300 ☒Liquid AD 250 200 150 100 50 Corn Stover Wheat Switchgrass Yard Waste Leaves Waste Maple Pine Straw Paper b 16 OSS-AD 14 Liquid AD 10 8 Corn Stover Wheat Switchgrass Yard Waste Leaves Waste Maple Pine Straw Paper Fig.2.(a)Total methane yield and(b)volumetric productivity during 30-day L-AD and SS-AD. Table 2 Changes of pH and VFA/alkalinity ratio during 30-day L-AD and SS-AD. Materials SS-AD L-AD pH VFA/alkalinity ratio pH VFA/alkalinity ratio Initial Final Initial Final Initial Final Initial Final Switchgrass 8.0±0.0 8.3±0.0 0.77±0.09 0.48±0.03 7.8±0.0 7.6±0.1 0.53±0.03 0.31±0.01 Corn stover 8.1±0.2 8.5±0.0 0.63±020 0.48±0.01 7.8±0.0 7.4±0.1 0.60±0.01 0.35±0.00 Wheat straw 8.3±0.0 8.3±02 0.56±0.07 0.36±0.03 7.9±0.0 7.6±0.0 0.43±0.00 0.27±0.02 Yard waste 7.2±0.0 8.4±0.1 072±0.00 051±0.18 7.8±0.1 7.7±0.0 0.33±004 034±000 Leaves 7.1±0.0 8.3±0.0 1.02±0.08 0.70±0.04 7.5±0.1 7.6±0.2 0.53±0.04 0.34±0.01 Maple 7.6±0.1 8.3±0.0 0.68±0.03 0.35±0.06 7.6±0.1 7.7±0.1 0.47±0.08 0.36±0.04 Pine 7.2±0.0 82±0.0 0.76±0.06 0.70±0.12 7.6±0.1 7.6±0.1 0.53±0.06 0.34±0.03 Waste paper 7.9±0.0 6.2±0.0 0.45±0.07 3.24±0.03 7.9±0.0 7.5±0.0 0.50±0.03 0.35±0.00 a Data shown are the average and standard deviation based on duplicate runs. 3.4.First-order kinetic model of methane production As shown in Table 3,there is a linear relationship between log- arithmic methane production and reaction time in both L-AD and First-order kinetic models were used to characterize the meth- SS-AD of switchgrass,corn stover,wheat straw,leaves,yard waste, ane production of lignocellulosic biomass in L-AD (Chynoweth and maple,(r between 0.90 and 0.98)indicating that the methane et al.,1993:Jewell et al.,1993;Angelidaki et al.,2009).The first-or- production could be explained by the simple first-order kinetic der kinetic model can be linearized as In()=kt.where:t=time model.Pine showed a linear relationship in the L-AD(r=0.96) in days:Mu=methane yield obtained in 30 days,L/kg VS feedstock: but not in the SS-AD(r=0.77).This indicates a pattern of methane M=methane yield obtained at time t.L/kg VSreedstock:and production in SS-AD of pine that is not experienced in L-AD.It was M=methane yield potential that remained at time,M=Mu-Mr. observed that pine particles settled to the bottom of the reactor
3.4. First-order kinetic model of methane production First-order kinetic models were used to characterize the methane production of lignocellulosic biomass in L-AD (Chynoweth et al., 1993; Jewell et al., 1993; Angelidaki et al., 2009). The first-order kinetic model can be linearized as ln Mu M ¼ kt, where: t = time in days; Mu = methane yield obtained in 30 days, L/kg VS feedstock; Mt = methane yield obtained at time t, L/kg VSfeedstock; and M = methane yield potential that remained at time, M = MuMt. As shown in Table 3, there is a linear relationship between logarithmic methane production and reaction time in both L-AD and SS-AD of switchgrass, corn stover, wheat straw, leaves, yard waste, and maple, (r 2 between 0.90 and 0.98) indicating that the methane production could be explained by the simple first-order kinetic model. Pine showed a linear relationship in the L-AD (r 2 = 0.96) but not in the SS-AD (r 2 = 0.77). This indicates a pattern of methane production in SS-AD of pine that is not experienced in L-AD. It was observed that pine particles settled to the bottom of the reactor Fig. 2. (a) Total methane yield and (b) volumetric productivity during 30-day L-AD and SS-AD. Table 2 Changes of pH and VFA/alkalinity ratio during 30-day L-AD and SS-AD.a Materials SS-AD L-AD pH VFA/alkalinity ratio pH VFA/alkalinity ratio Initial Final Initial Final Initial Final Initial Final Switchgrass 8.0 ± 0.0 8.3 ± 0.0 0.77 ± 0.09 0.48 ± 0.03 7.8 ± 0.0 7.6 ± 0.1 0.53 ± 0.03 0.31 ± 0.01 Corn stover 8.1 ± 0.2 8.5 ± 0.0 0.63 ± 0.20 0.48 ± 0.01 7.8 ± 0.0 7.4 ± 0.1 0.60 ± 0.01 0.35 ± 0.00 Wheat straw 8.3 ± 0.0 8.3 ± 0.2 0.56 ± 0.07 0.36 ± 0.03 7.9 ± 0.0 7.6 ± 0.0 0.43 ± 0.00 0.27 ± 0.02 Yard waste 7.2 ± 0.0 8.4 ± 0.1 0.72 ± 0.00 0.51 ± 0.18 7.8 ± 0.1 7.7 ± 0.0 0.33 ± 0.04 0.34 ± 0.00 Leaves 7.1 ± 0.0 8.3 ± 0.0 1.02 ± 0.08 0.70 ± 0.04 7.5 ± 0.1 7.6 ± 0.2 0.53 ± 0.04 0.34 ± 0.01 Maple 7.6 ± 0.1 8.3 ± 0.0 0.68 ± 0.03 0.35 ± 0.06 7.6 ± 0.1 7.7 ± 0.1 0.47 ± 0.08 0.36 ± 0.04 Pine 7.2 ± 0.0 8.2 ± 0.0 0.76 ± 0.06 0.70 ± 0.12 7.6 ± 0.1 7.6 ± 0.1 0.53 ± 0.06 0.34 ± 0.03 Waste paper 7.9 ± 0.0 6.2 ± 0.0 0.45 ± 0.07 3.24 ± 0.03 7.9 ± 0.0 7.5 ± 0.0 0.50 ± 0.03 0.35 ± 0.00 a Data shown are the average and standard deviation based on duplicate runs. D. Brown et al. / Bioresource Technology 124 (2012) 379–386 383
384 D.Brown et aL/Bioresource Technology 124(2012)379-386 Table 3 reported elsewhere for L-AD(Tong et al,1990).L-AD conversion Parameters of logarithmic plot of methane production versus time for different lignocellulosic biomass in SS-AD and L-AD. constants were slightly lower than those for SS-AD due to mixing during L-AD which resulted in a more even distribution of mi- Feedstock L-AD SS-AD crobes and VFAs. k R2 k R2 Switchgrass 0.0784 0.9323 0.0872 0.9328 3.5.Degradation of cellulose,hemicellulose,and extractives Corn Stover 0.0750 09274 0.0900 0.9313 Wheat Straw 0.0822 0.9136 0.0854 0.9052 Degradation of cellulose and hemicellulose during the 30-day Leaves 0.0775 09608 0.0927 0.9469 SS-AD of different lignocellulosic biomass feedstocks is presented Yard Waste 0.0845 0.9605 0.1236 0.9810 Waste Paper 0.0844 08582 03494 0.8717 in Fig.3.Samples were obtained both before and after 30 days of Maple 0.0838 0.9777 0.1153 0.9821 SS-AD and compositional analyses were performed.The composi- Pine 0.0933 0.9617 0.1083 0.7709 tion is expressed as the weight of each compound measured based on an initial loading level of 100g TS.The highest cellulose degra- dation of 40%was observed during SS-AD of waste paper:while a which may have created local areas of a much higher F/E ratio than similar trend was noted in hemicellulose degradation(Fig.3a).The desired and led to a shortage of microbes to break down the pine highest hemicellulose removal of 58%was noted with SS-AD of particles.The methane production of waste paper during L-AD waste paper.It should be noted that degradation of cellulose and exhibited a multi-phase profile(Fig.1b).It is speculated that the hemicellulose of waste paper in the SS-AD did not correlate to presence of fast and slow digestible components,for example,eas- methane yield since souring of the reactor occurred;therefore, ily digestible extractives,and relatively resistant structural carbo- the degradation components could have been transformed into hydrates in substrates may lead to two or more conversion VFAs without further conversion to methane and CO2. phases during the AD process (Jewell et al.,1993).The conversion Degradation of cellulose and hemicellulose was negatively re- constant (k)for the feedstocks in L-AD ranged from 0.075 to 0.093 lated to the lignin content in raw feedstocks.Higher cellulose and in SS-AD from 0.085 to 0.115,with the exception of the waste and hemicellulose degradation was associated with corn stover. paper reactor(k=0.349),which is not included due to souring. wheat straw,and switchgrass which contained less lignin while These conversion constants are in general agreement with those higher lignin-containing yard waste,leaves,maple,and pine had ④ 60 ▣Before 50 ▣After 40 30 20 10 Switchgrass Wheat Corn Stover Leaves Yard Waste Maple Waste Straw Paper Feedstock 18 16 ▣Before After 14 12 10 2 0+ Switchgrass Wheat Corn Stover Leaves Yard Waste Maple Pine Waste Straw Paper Feedstock Fig.3.Degradation of(a)cellulose and(b)hemicellulose during 30 day SS-AD (based on 100 g initial TS)
which may have created local areas of a much higher F/E ratio than desired and led to a shortage of microbes to break down the pine particles. The methane production of waste paper during L-AD exhibited a multi-phase profile (Fig. 1b). It is speculated that the presence of fast and slow digestible components, for example, easily digestible extractives, and relatively resistant structural carbohydrates in substrates may lead to two or more conversion phases during the AD process (Jewell et al., 1993). The conversion constant (k) for the feedstocks in L-AD ranged from 0.075 to 0.093 and in SS-AD from 0.085 to 0.115, with the exception of the waste paper reactor (k = 0.349), which is not included due to souring. These conversion constants are in general agreement with those reported elsewhere for L-AD (Tong et al., 1990). L-AD conversion constants were slightly lower than those for SS-AD due to mixing during L-AD which resulted in a more even distribution of microbes and VFAs. 3.5. Degradation of cellulose, hemicellulose, and extractives Degradation of cellulose and hemicellulose during the 30-day SS-AD of different lignocellulosic biomass feedstocks is presented in Fig. 3. Samples were obtained both before and after 30 days of SS-AD and compositional analyses were performed. The composition is expressed as the weight of each compound measured based on an initial loading level of 100 g TS. The highest cellulose degradation of 40% was observed during SS-AD of waste paper; while a similar trend was noted in hemicellulose degradation (Fig. 3a). The highest hemicellulose removal of 58% was noted with SS-AD of waste paper. It should be noted that degradation of cellulose and hemicellulose of waste paper in the SS-AD did not correlate to methane yield since souring of the reactor occurred; therefore, the degradation components could have been transformed into VFAs without further conversion to methane and CO2. Degradation of cellulose and hemicellulose was negatively related to the lignin content in raw feedstocks. Higher cellulose and hemicellulose degradation was associated with corn stover, wheat straw, and switchgrass which contained less lignin while higher lignin-containing yard waste, leaves, maple, and pine had Table 3 Parameters of logarithmic plot of methane production versus time for different lignocellulosic biomass in SS-AD and L-AD. Feedstock L-AD SS-AD k R2 k R2 Switchgrass 0.0784 0.9323 0.0872 0.9328 Corn Stover 0.0750 0.9274 0.0900 0.9313 Wheat Straw 0.0822 0.9136 0.0854 0.9052 Leaves 0.0775 0.9608 0.0927 0.9469 Yard Waste 0.0845 0.9605 0.1236 0.9810 Waste Paper 0.0844 0.8582 0.3494 0.8717 Maple 0.0838 0.9777 0.1153 0.9821 Pine 0.0933 0.9617 0.1083 0.7709 Fig. 3. Degradation of (a) cellulose and (b) hemicellulose during 30 day SS-AD (based on 100 g initial TS). 384 D. Brown et al. / Bioresource Technology 124 (2012) 379–386
D.Brown et aL/Bioresource Technology 124(2012)379-386 385 Table 4 Relationship between methane yield and lignin content and enzymatic digestibility of different feedstocks in L-AD and SS-AD. Feedstock Methane yield L/kg VSteedstock Enzymatic digestibility Lignin content ( SS-AD L-AD Of theoretical g/100g biomass Corn stover 131.8 124.0 8.8 14.4 17.1 Switchgrass 116.9 111.0 3.5 6.4 17.8 Wheat straw 123.9 139.1 8.5 15.8 15.2 Leaves 753 81.0 2.5 14.3 22.8 Yard waste 49.3 59.7 2.6 65 22.1 Maple 46.9 57.2 5.2 102 22.0 Paper 15.3 3124 59.5 775 2.1 Pine 17.0 54.3 5.2 15.3 28.3 lower cellulose and hemicellulose degradation.Waste paper had napier grass,wood grass,newspaper and white fir.at a TS content been processed and thus,most of the lignin had been removed.Lig- of less than 1%(Tong et al.,1990). nin content,crystallinity of cellulose,and particle size limit the Since lignin content is regarded as a key factor affecting the digestibility of cellulose and hemicellulose(Hendriks and Zeeman. enzymatic digestibility of lignocellulosic biomass (Mosier et al.. 2009).Lignin retards cellulose accessibility to enzymes and micro- 2005),the digestibility of lignocellulosic biomass was tested at a bial attacks due to its protective sheathing and hydrophobic nature cellulase loading of 10 FPU/g TS and correlated with methane yield (Chang and Holtzapple,2000:Himmel et al.,2007).The cellulose (Table 4).A fair linear relationship between methane yield and and hemicellulose degradation results combined with the observa- enzymatic digestibility of tested feedstocks was seen for L-AD tion that lignin content negatively relates to methane yield indi- (2=0.8641);however,during SS-AD,no linear relationship be- cate that lignin is one of the key factors controlling the SS-AD of tween methane yield and enzymatic digestibility of tested feed- lignocellulosic biomass;however,further study is required to clar- stocks was observed.It is well documented that hydrolysis of ify how the compositional and structural differences in lignocellu- lignocellulosic biomass is the rate limiting step in anaerobic diges- losic feedstocks affect methane production during SS-AD due to the tion and pretreatment improves digestion efficiency and biogas complexity of the reaction system. production.For example,alkaline pretreatment is effective in lig- Extractive degradation of the lignocellulosic biomass was not nin removal and subsequently improves enzymatic digestibility measured in this study;however,a previous study indicated that and biogas production during subsequent anaerobic digestion extractive degradation in lignocellulosic biomass with higher lig- (Zhu et al.,2010).Bochmann et al.(2007)found that enzymatic nin content was higher than that with lower lignin content (Liew pretreatment promoted the hydrolysis of lignocellulosic biomass et al.,2011).It was seen in other studies that higher degradation and increased biogas production owing to the breakdown of cellu- of extractives corresponded with high daily methane yields ob- lose and hemicelluloses to lower molecular weight substances.Cui served during the first couple of days,which is supported by find- et al.(2011)revealed that the spent wheat straw was more digest- ings that extractives are easily degradable(Tong et al.,1990:Chen ible by enzymes than wheat straw which is in accordance with the et al.,2007).It is likely that degradation of extractives was the high methane yield from SS-AD of spent wheat straw.Liew et al. main contributor to the cumulative biogas generation from yard (2012)reported a strong positive linear relationship(r2=0.99)be- waste,leaves,maple,and pine tween the methane yield and the enzymatic digestibility of four types of lignocellulosic biomass including corn stover,wheat 3.6.Relationship between methane yield and composition and straw,yard waste and leaves.Although in the present study a lin- enzymatic digestibility of lignocellulosic biomass ear relationship between methane yield and enzymatic digestibil- ity was observed for L-AD,in agreement with results reported There is general agreement that the presence of lignin increases elsewhere,it should be noted that methane yield is associated with the resistance of lignocellulosic biomass to biodegradation,result- both lignin content and enzymatic digestibility and the availability ing in lower methane yields;however,the quantitative relation- of cellulose and hemicellulose.Organic compounds such as extrac- ship between biodegradability and lignin content is not well tives also contribute to methane production and thus,it is not yet understood as data are inconclusive.In the present study,a good possible to predict the performance of the AD process solely based inverse linear relationship was obtained between the methane on the composition or enzymatic digestibility due to the hetero- yield during L-AD and lignin content of the feedstocks with an r genic nature of lignocellulosic biomass and complicated nature of of 0.9296 (Table 4).Nevertheless,a fair inverse linear relationship AD reactions.Therefore,future studies should be performed to (r of 0.8734)was obtained between the methane yield during SS. understand the controlling factors that determine the methane AD and lignin content of tested feedstocks,excluding waste paper. yield of SS-AD of lignocellulosic biomass. These results agree with those from a few previous studies that re- ported a strong negative linear correlation between lignin content 4.Conclusions and biodegradability of lignocellulosic biomass during L-AD. Chandler et al.(1980)found an inverse linear relationship between The present study compared liquid(L-AD)and solid state(SS- lignin content and VS destruction with a correlation coefficient(r2) AD)on eight lignocellulosic feedtocks using a single source of inoc- of 0.94 of AD of herbaceous materials,animal manures,and ulum.Methane yields of agricultural residues and perennial crops newspaper.During short-term(48 h)digestion by rumen microor- were higher than those obtained from woody biomass and yard ganisms,Bjorndal and Moore (1985)found a weak inverse linear wastes during both L-AD and SS-AD.The methane yield from waste relationship(r2=0.75)between lignin content and VS destruction paper was high during L-AD,but low during SS-AD because of acid- of over 100 different lignocellulosic biomass samples.A poor linear ification.Methane production was in an inverse linear relationship correlation (r=0.59-0.69)was observed between methane with the lignin content for both L-AD and SS-AD.Pretreatment of conversion efficiency and lignin content of seven kinds of lignocel- woody biomass is suggested to increase the biogas yield for both lulosic biomass,including corn stover,two batches of wheat straw, L-AD and SS-AD
lower cellulose and hemicellulose degradation. Waste paper had been processed and thus, most of the lignin had been removed. Lignin content, crystallinity of cellulose, and particle size limit the digestibility of cellulose and hemicellulose (Hendriks and Zeeman, 2009). Lignin retards cellulose accessibility to enzymes and microbial attacks due to its protective sheathing and hydrophobic nature (Chang and Holtzapple, 2000; Himmel et al., 2007). The cellulose and hemicellulose degradation results combined with the observation that lignin content negatively relates to methane yield indicate that lignin is one of the key factors controlling the SS-AD of lignocellulosic biomass; however, further study is required to clarify how the compositional and structural differences in lignocellulosic feedstocks affect methane production during SS-AD due to the complexity of the reaction system. Extractive degradation of the lignocellulosic biomass was not measured in this study; however, a previous study indicated that extractive degradation in lignocellulosic biomass with higher lignin content was higher than that with lower lignin content (Liew et al., 2011). It was seen in other studies that higher degradation of extractives corresponded with high daily methane yields observed during the first couple of days, which is supported by findings that extractives are easily degradable (Tong et al., 1990; Chen et al., 2007). It is likely that degradation of extractives was the main contributor to the cumulative biogas generation from yard waste, leaves, maple, and pine. 3.6. Relationship between methane yield and composition and enzymatic digestibility of lignocellulosic biomass There is general agreement that the presence of lignin increases the resistance of lignocellulosic biomass to biodegradation, resulting in lower methane yields; however, the quantitative relationship between biodegradability and lignin content is not well understood as data are inconclusive. In the present study, a good inverse linear relationship was obtained between the methane yield during L-AD and lignin content of the feedstocks with an r 2 of 0.9296 (Table 4). Nevertheless, a fair inverse linear relationship (r 2 of 0.8734) was obtained between the methane yield during SSAD and lignin content of tested feedstocks, excluding waste paper. These results agree with those from a few previous studies that reported a strong negative linear correlation between lignin content and biodegradability of lignocellulosic biomass during L-AD. Chandler et al. (1980) found an inverse linear relationship between lignin content and VS destruction with a correlation coefficient (r 2 ) of 0.94 of AD of herbaceous materials, animal manures, and newspaper. During short-term (48 h) digestion by rumen microorganisms, Bjorndal and Moore (1985) found a weak inverse linear relationship (r 2 = 0.75) between lignin content and VS destruction of over 100 different lignocellulosic biomass samples. A poor linear correlation (r 2 = 0.59–0.69) was observed between methane conversion efficiency and lignin content of seven kinds of lignocellulosic biomass, including corn stover, two batches of wheat straw, napier grass, wood grass, newspaper and white fir, at a TS content of less than 1% (Tong et al., 1990). Since lignin content is regarded as a key factor affecting the enzymatic digestibility of lignocellulosic biomass (Mosier et al., 2005), the digestibility of lignocellulosic biomass was tested at a cellulase loading of 10 FPU/g TS and correlated with methane yield (Table 4). A fair linear relationship between methane yield and enzymatic digestibility of tested feedstocks was seen for L-AD (r 2 = 0.8641); however, during SS-AD, no linear relationship between methane yield and enzymatic digestibility of tested feedstocks was observed. It is well documented that hydrolysis of lignocellulosic biomass is the rate limiting step in anaerobic digestion and pretreatment improves digestion efficiency and biogas production. For example, alkaline pretreatment is effective in lignin removal and subsequently improves enzymatic digestibility and biogas production during subsequent anaerobic digestion (Zhu et al., 2010). Bochmann et al. (2007) found that enzymatic pretreatment promoted the hydrolysis of lignocellulosic biomass and increased biogas production owing to the breakdown of cellulose and hemicelluloses to lower molecular weight substances. Cui et al. (2011) revealed that the spent wheat straw was more digestible by enzymes than wheat straw which is in accordance with the high methane yield from SS-AD of spent wheat straw. Liew et al. (2012) reported a strong positive linear relationship (r 2 = 0.99) between the methane yield and the enzymatic digestibility of four types of lignocellulosic biomass including corn stover, wheat straw, yard waste and leaves. Although in the present study a linear relationship between methane yield and enzymatic digestibility was observed for L-AD, in agreement with results reported elsewhere, it should be noted that methane yield is associated with both lignin content and enzymatic digestibility and the availability of cellulose and hemicellulose. Organic compounds such as extractives also contribute to methane production and thus, it is not yet possible to predict the performance of the AD process solely based on the composition or enzymatic digestibility due to the heterogenic nature of lignocellulosic biomass and complicated nature of AD reactions. Therefore, future studies should be performed to understand the controlling factors that determine the methane yield of SS-AD of lignocellulosic biomass. 4. Conclusions The present study compared liquid (L-AD) and solid state (SSAD) on eight lignocellulosic feedtocks using a single source of inoculum. Methane yields of agricultural residues and perennial crops were higher than those obtained from woody biomass and yard wastes during both L-AD and SS-AD. The methane yield from waste paper was high during L-AD, but low during SS-AD because of acidification. Methane production was in an inverse linear relationship with the lignin content for both L-AD and SS-AD. Pretreatment of woody biomass is suggested to increase the biogas yield for both L-AD and SS-AD. Table 4 Relationship between methane yield and lignin content and enzymatic digestibility of different feedstocks in L-AD and SS-AD. Feedstock Methane yield L/kg VSfeedstock Enzymatic digestibility Lignin content (%) SS-AD L-AD % Of theoretical g/100 g biomass Corn stover 131.8 124.0 8.8 14.4 17.1 Switchgrass 116.9 111.0 3.5 6.4 17.8 Wheat straw 123.9 139.1 8.5 15.8 15.2 Leaves 75.3 81.0 2.5 14.3 22.8 Yard waste 49.3 59.7 2.6 6.5 22.1 Maple 46.9 57.2 5.2 10.2 22.0 Paper 15.3 312.4 59.5 77.5 2.1 Pine 17.0 54.3 5.2 15.3 28.3 D. Brown et al. / Bioresource Technology 124 (2012) 379–386 385
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