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Low-heat, mild alkaline pretreatment of switchgrassfor anaerobic digestionGuang Jin a , Tom Bierma a & Paul M. Walker ba Environmental Health Program, Department of Health Sciences , Illinois State University ,Normal , Illinois , USAb Department of Agriculture , Illinois State University , Normal , Illinois , USAPublished online: 10 Jan 2014.

To cite this article: Guang Jin , Tom Bierma & Paul M. Walker (2014) Low-heat, mild alkaline pretreatment of switchgrass foranaerobic digestion, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and EnvironmentalEngineering, 49:5, 565-574, DOI: 10.1080/10934529.2014.859453

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Journal of Environmental Science and Health, Part A (2014) 49, 565–574Copyright C© Taylor & Francis Group, LLCISSN: 1093-4529 (Print); 1532-4117 (Online)DOI: 10.1080/10934529.2014.859453

Low-heat, mild alkaline pretreatment of switchgrassfor anaerobic digestion

GUANG JIN1, TOM BIERMA1 and PAUL M. WALKER2

1Environmental Health Program, Department of Health Sciences, Illinois State University, Normal, Illinois, USA2Department of Agriculture, Illinois State University, Normal, Illinois, USA

This study examines the effectiveness of alkaline pretreatment under mild heat conditions (100◦C or 212◦F) on the anaerobic co-digestion of switchgrass. The effects of alkaline concentration, types of alkaline, heating time and rinsing were evaluated. In additionto batch studies, continuous-feed studies were performed in triplicate to identify potential digester operational problems caused byswitchgrass co-digestion while accounting for uncertainty due to digester variability. Few studies have examined anaerobic digestionof switchgrass or the effects of mild heating to enhance alkaline pretreatment prior to biomass digestion. Results indicate thatpretreatment can significantly enhance digestion of coarse-ground (≤ 0.78 cm particle size) switchgrass. Energy conversion efficiencyas high as 63% was observed, and was comparable or superior to fine-grinding as a pretreatment method. The optimal NaOHconcentration was found to be 5.5% (wt/wt alkaline/biomass) with a 91.7% moisture level. No evidence of operational problemssuch as solids build-up, poor mixing, or floating materials were observed. These results suggest the use of waste heat from a generatorcould reduce the concentration of alkaline required to adequately pretreat lignocellulosic feedstock prior to anaerobic digestion.

Keywords: Anaerobic digestion, alkaline pretreatment, biogas, switchgrass, NaOH.

Introduction

Anaerobic digestion (AD) utilizes a mixed population ofmicroorganisms to degrade organic matter and producebiogas, which is predominantly methane, in the absenceof oxygen. AD is a valuable biotechnology for both wastetreatment and renewable energy production. It is most val-ued for treatment of human sewage biosolids as well asanimal manure due to its ability to dramatically reducepathogens and odors as well as degrade the organic contentof these wastes.[1] As a renewable energy technology, ADhas the advantages of working with a variety of biomasssubstrates, whether wet or dry, and preserving most of thenutrient content of the biomass.[2]

It has been demonstrated that anaerobic digestion of ma-nure or sewage biosolids can be enhanced by co-digestionwith high-carbon biomass because the combination resultsin a carbon:nitrogen ratio closer to optimal for the anaer-obic bacteria.[3] Co-digestion can increase biogas output,

Address correspondence to Guang Jin, Environmental HealthProgram, Department of Health Sciences, Campus Box 5220,Illinois State University, Normal, IL 61790-5220, USA; E-mail:gjin@ilstu.eduReceived July 2, 2013.Color versions of one or more of the figures in the article can befound online at www.tandfonline.com/lesa.

providing additional energy and revenue for a given ADinvestment. While some AD operations may have ready ac-cess to waste biomass that is high in sugars, starches, orfats, the most plentiful biomass materials are likely to belignocellulosic – such as agricultural residue or vegetativewastes. Unfortunately, lignocellulosic material often resistsanaerobic digestion.

Lignocellulose, the primary building block of plant cellwalls, is composed of cellulose, hemicelluloses, and ligninpolymers.[4] This material has significant energy content,with most lignocellulosic biomass having a dry weight en-ergy content of 15–19 GJ/tonne (6450–8200 Btu/lb).[5] Cel-lulose and hemicellulose are carbohydrate polymers thatare tightly bound with lignin and resist both physical andbiological degradation.

Lignocellulosic biomass can be recalcitrant to micro-bial and enzymatic activity not only in the production ofbiogas but also in the production of ethanol through fer-mentation.[4] The recalcitrance of lignocellulosic biomasshas been attributed to several factors, particularly cellulosecrystalinity, hemicellulose composition and cross-linking,lignin content and bonding, and overall porosity.[4,6] Cel-lulose is a linear glucose polymer with chains of cellulosebound together in alternating sections of crystalline andamorphous structures. Crystalline sections of cellulose havegenerally been found to be more resistant to hydrolysis thanamorphous cellulose.[4]

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Pretreatment of lignocellulosic biomass can reduce re-calcitrance by making cellulose and hemicellulose moreaccessible to enzymatic activity.[7–9] Although evidencefrom ethanol production provides valuable insights intosuccessful pretreatments, optimal pretreatment conditionsfor production of ethanol and biogas may be quite dif-ferent. This is because fermentation of ethanol requiressugars, whereas biogas fermentation can occur from a widevariety of biodegradable compounds.[6,10] Thus, some pre-treatments rejected for ethanol production, due to loss ofsugars, may be excellent pretreatments for biogas produc-tion. In addition, there is some evidence that microorgan-isms involved in biogas production may be more tolerantthan ethanol-fermenting organisms of inhibiting chemicalsproduced during pretreatment.[10]

A number of methods have been explored for pretreat-ing lignocellulosic material to enhance enzyme accessibility.However, these methods are generally capital- or energy-intensive. The most common pretreatment methods includephysical (including size reduction and thermal treatment),chemical, and a combination of these two (such as ther-mochemical).[11] It is well established that size reductioncan enhance hydrolysis of biomass. Particle size can reducerecalcitrance by increasing the surface area available to hy-drolyzing enzyme, and also by initiating breakup of thechemical structure of the lignocellulose complex, includ-ing cellulose crystallinity.[10] The influence of particle sizeon biogas yields has been investigated for a wide range offeedstocks.

Many studies have shown increasing biogas yields withreduced particle size of fibers in manure, hay, maple leaves,wheat straw, rice straw, Bermudagrass, grass silage, andmaize silage.[12] However, the energy required for size re-duction can be economically prohibitive. It has been es-timated that the energy required to produce particle sizesunder 6 mm could exceed the theoretical energy content ofmost biomass.[12]

Heat can also be used to increase enzyme accessibilityof biomass. Steam explosion is one of the most commonlyused biomass pretreatment methods.[13] Biomass is treatedwith steam under high pressure and then allowed to rapidlydecompress. Under high pressure, the steam acts as anacid, hydrolyzing hemicellulose and lignin.[4] The explosivedecompression dramatically increases porosity, exposingthe biomass for subsequent enzyme attack. In addition tothe specialized equipment required for steam explosion, adisadvantage can be production of inhibitory compoundsfrom the high heat and pressure.[4]

A variety of chemicals can also enhance hydrolysis oflignocellulosic biomass. Both acids and bases have beenshown to be effective in pretreatment. Dilute acids—mostcommonly sulfuric acid—appear to work primarily by pref-erentially hydrolyzing and removing hemicellulose, leavingcellulose more accessible in the residual solids.[13] This is ad-vantageous for ethanol production but not necessarily forbiogas production. Moreover, acid treatment may enhance

production of inhibitory compounds as well as recalcitrantcellulose/lignin crystals.[14]

Alkaline pretreatment is considered one of the mostpromising options for both ethanol and biogas produc-tion.[10] Alkaline pretreatment appears to act primarilyfrom degradation of the hemicellulose-lignin bonds withpartial removal of the lignin from the biomass, increasingporosity and exposing the cellulose and hemicellulose tosubsequent enzyme attack.[4] Compared to acid pretreat-ment, alkaline pretreatment can generally be performedunder milder conditions (even room temperature), thoughpretreatment times are often hours or days compared tominutes for acid pretreatment.[4] A number of chemicalshave been used for alkaline pretreatment, including sodiumand potassium hydroxide, ammonia, and calcium hydrox-ide (lime).[6] The cost of these chemical inputs can be signif-icant and the economic feasibility of alkaline pretreatmentdepends on finding operating parameters that minimizechemical expenses and overall costs.[13]

Factors that appear to influence the effectiveness of al-kaline pretreatment include choice of alkaline chemistry(NaOH, KOH, ammonia, lime, etc.), alkali concentration,temperature and reaction time.[15] These factors appear tobe somewhat interchangeable. In general, increasing con-centration, time, and temperature produce more effectivepretreatment, but increase cost and the production of in-hibitory compounds, limiting the extent to which these fac-tors can be increased.[15]

Research on the effects of temperature in alkaline pre-treatment are limited and somewhat conflicting, but over-all evidence suggests that temperatures around 100◦C(212◦F) could significantly increase pretreatment ef-fectiveness.[16–20] Waste heat from biogas-powered elec-tric generating equipment—often associated with ADoperations—offers an opportunity to reduce the cost ofpretreatment if it can be used to increase reaction tempera-ture and allow alkali concentration to be reduced withoutcompromising pretreatment effectiveness.

Switchgrass is a prairie grass native to the U.S. thatproduces a high yield of cellulose. It is a perennial crop thatusually needs less fertilization and pest control, and is ableto retain soil and nutrients better than traditional row crops.Grown on marginal land, switchgrass could supplementfarm income and contribute to rural development. Switch-grass has been extensively studied as a biomass sourcein ethanol production. Pretreatments that have proveneffective in improving ethanol yield from switchgrassinclude dilute acid, lime, liquid hot water, sulfur dioxide,aqueous ammonia, ammonia fiber explosion (AFEX), andsteam explosion.[21–23] However, few studies have examinedthe effectiveness of switchgrass pretreatment for biogasproduction, and little is known about how switchgrass willbe behave in an anaerobic digester following pretreatmentin terms of possible operational problems such as solidsaccumulation, poor mixing, or floating layers. Moreover,evidence suggests that switchgrass is more resistant to

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Alkaline pretreatment of switchgrass for anaerobic digestion 567

anaerobic digestion than many other types of biomass.[24]

Success with pretreatment of switchgrass would suggestthat the pretreatment could be successful with many formsof biomass.

The study reported here focuses on the effectiveness ofmild heat (100◦C or 212 ◦F) and alkaline pretreatment un-der varying concentrations, types of alkaline, heating time,and rinsing following pretreatment on the anaerobic co-digestion of switchgrass with sewage biosolids. Althoughthere are many laboratory investigations of co-digestionusing a variety of lignocellulosic materials, the vast major-ity used batch digesters that are unable to test potentialoperational problems—such as solids accumulation, poormixing, or floating layers—associated with a continuous-feed digester.[25–27] A few studies that used continuous-feeddigesters [28,29] used only one digester for each conditiontested, as opposed to duplicate or triplicate digesters to as-sess the effects of digester variation. The research reportedhere utilizes not only batch digesters to screen pretreatmentconditions, but also continuous-feed digesters to examinepotential operational problems and methane productionunder steady-state conditions. All reactions are run in trip-licate to assess uncertainty due to digester variation.

Materials and methods

Origin of materials

Cave-in-Rock switchgrass (Panicum virgatum), used as thelignocellulosic feedstock in this study, was obtained fromthe Horticulture Center at Illinois State University (Nor-mal, IL, USA). Switchgrass was harvested in June (at theearly flowering stage). It was air dried to an average mois-ture content of 4.74% and a VS% of 92.62%. Switchgrasswas cut into 2-in pieces using a paper cutter and thencoarsely ground in a Ronson Model HB-2 Blender (RonsonConsumer Products Corporation, Somerset, NJ, USA) un-til particles were small enough to fall through the bottomscreen of a two-sieve Forage Particle Separator (NASCO,Modesto, CA, USA), resulting in particles generally lessthan 0.78 cm.

Seed sludge used to inoculate the bench-top digesters waseffluent from a local sewage treatment district’s mesophilicdigester. Feed sludge used for co-digestion with switchgrasswas raw feed sludge used in the local district’s digester andwas composed of 72% primary sludge and 28% secondarysludge.

Pretreatment conditions

A variety of pretreatment conditions were evaluated inbatch studies (Table 1). Various concentrations of NaOHwith mild heat (100◦C or 212◦F for 6 h) were examined(conditions A–C in Table 1). The effects of not rinsing pre-

Table 1. Summary of pretreatment conditions.

PretreatmentCode

Alkaline Conc.(wt/wt%)

(NaOH exceptwhere noted) Heating Conditions

Rinse AfterPretreatment

Control 0% Room temp., 24 h YesA 0% 100◦C (212◦F), 6 h YesB 2.2% 100◦C (212◦F), 6 h YesC 11% 100◦C (212◦F), 6 h YesD 11% 100◦C (212◦F), 6 h NoE 22% Room temp., 6 h YesF 11% NaOH +

6.6% H2O2

Room temp., 6 h Yes

G 5.5% 100◦C (212◦F), 6 h NoH 11% 100◦C (212◦F), 3 h NoI 11% Ca(OH)2 100◦C (212◦F), 6 h No

treated switchgrass prior to digestion (condition D), notheating (condition E), and adding hydrogen peroxide (con-dition F) also were evaluated. Once the superior pretreat-ment condition was selected from among these options, anumber of variations were explored: using half the NaOH(condition G), using half the heating time (condition H),and substituting lime (Ca(OH)2) for NaOH (condition I).The most promising conditions from batch studies were fur-ther evaluated in continuously-fed digesters. Note that allalkaline concentrations are on a wt/wt (alkaline/biomass)basis.

To pretreat the switchgrass, 3.6 g of dry coarse-groundswitchgrass was soaked in 42 mL NaOH solution (or re-verse osmosis (RO) water in the case of control) in a beaker.The beaker was placed in a 100◦C water bath (or left atroom temperature, 25◦C) for the desired amount of time.When pretreatment was complete, switchgrass was drainedof liquid using a strainer for about 5 min (no drying). Ifrinsing was specified, a spray bottle of RO water was usedto administer a 5-sec rinse for a total of three times andallowed to drain. The wet, pretreated switchgrass was thentransferred to the digester.

Batch studies

Short-term batch studies were used to screen pretreatmentconditions. One-liter flasks were filled with 470 mL seedsludge plus about 30 mL feed mix while nitrogen was used topurge the flasks. The feed mix was composed of pretreatedswitchgrass plus 30 mL feed sludge.

The bench-scale digester setup is pictured in Figure 1.Water baths were controlled at 35◦C ± 1◦C (95◦F ± 2◦F)and digesters were stirred using magnetic stir bars. Biogasgenerated from each digester was collected in a graduatedcylinder and displaced the water inside the cylinder to awater reservoir. The reservoir was placed lower than thecylinder to maintain a siphon and produce slight nega-tive pressure in the airspace of the cylinder and digester.

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Digester

Graduatedcylinder

Waterreservoir

To methane analyzer

Water bath

Magne�cs�rrer

Fig. 1. Bench-top digester and gas collection/measurement setup.

Biogas production volume was measured daily by thechange in cylinder water level (after placing the reservoirand cylinder at the same level to eliminate pressure differ-ences). To analyze for methane concentration in the biogasthe water reservoir was raised above the cylinder to forcethe biogas out through a take-off valve connected to themethane analyzer.

Cumulative methane production was then measured overthe next nine days without additional feeding. All condi-tions were run in triplicate. Each time a new pretreatmentcondition was evaluated a new set of control digesters wasestablished to adjust for variations in seed culture activity.For conditions G–I, condition D was rerun for comparisonpurposes instead of the control conditions.

Continuous-feed studies

The most promising pretreatment conditions from thebatch studies were evaluated using longer-term (approxi-mately seven weeks) continuously-fed, bench-top digestersin order to assess potential long-term operational concernssuch as solids accumulation, poor mixing, or floating lay-ers. One-liter digesters were operated to simulate a typicalU.S. anaerobic digester used at a local municipal wastewa-ter treatment plant.

The digester and gas collection set-up was essentially thesame as in batch studies (Fig. 1) but digesters were fedevery-other day, following withdrawal of a 30-mL sludgesample. The mixing was performed using the magnetic stir-rer as shown in Fig. 1. To withdraw a sludge sample thedigester was disconnected from the graduated cylinder andconnected to a nitrogen cylinder through the side arm tub-ing (Fig. 2). Nitrogen gas was then introduced and the flaskstopper was replaced with a stopper/tube assembly. Nitro-gen pressure created inside the digester pushed the sludgethrough the sampling tube into a collection flask. Sludgewas stirred during withdrawal process to assure represen-tativeness of the sample. The stopper/tube assembly wasthen removed and 30 mL of feed sludge/switchgrass mix

DigesterN

2

Sample collec�on

flask

Fig. 2. Anaerobic sludge sampling setup.

was poured into the flask while the nitrogen purge con-tinued. Controls received feed sludge without switchgrass.All conditions were run in triplicate. Digester performancewas monitored as methane production, total solids, volatilesolids, and pH. pH was self-sustained; no adjustment wasneeded. Pretreatment effect was evaluated using data col-lected only after digesters reached equilibrium (750 h).

Analysis and calculations

Methane analysis. Methane analysis was conducted usingHegmyer Model MX6 -0000N201 iBridTM methane ana-lyzer from Industrial Scientific Corporation (Oakdale, PA,USA) with a detection limit of 1% and a sensitivity of ±1%.Calibration of the instrument was conducted twice everyday to a known concentration of methane.

Sludge analysis. For the continuous-feed study, pH of thesludge was measured using a Thermo Fisher Scientific (Ver-non Hills, IL, USA) pH meter, and TS and VS were ana-lyzed according to Standard Method 2540-B and StandardMethod 2540-E, respectively.[31]

Pretreatment effect in the batch studies. To assess the pre-treatment effect in the batch studies, cumulative methaneproduction under each pretreatment condition was com-pared to the control (i.e., switchgrass soaked in RO waterovernight at room temperature) during the same digestionperiod. Percent methane increase as compared to the con-trol is reported for each pretreatment condition. To evalu-ate if the methane increase is statistically significant, t-testswere conducted and P values are reported. A linear regres-sion analysis of results for conditions A–C was conductedto evaluate effect of increasing NaOH concentration.

Pretreatment effect in continuous-feed studies. To access thepretreatment effect in the continuous-feed studies, specificmethane yield, energy conversion efficiency and other op-erational parameters are reported for each condition. The

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calculation of specific methane yield and energy conversionefficiency is described next.

Gross heat of combustion. The gross heat of combustionof switchgrass used in this study was determined usingan oxygen bomb calorimeter (IKA Model C2000, Parr In-strument, Moline, IL, USA) at initial oxygen pressure of3.0 MPa (30 atm) and final temperature of 20◦–30◦C. Ben-zoic acid was used for calibration. Gelatin caps were usedto hold switchgrass in place for ignition.

Specific methane yield. The methane generated under eachcondition was expressed as m3 CH4/kg SGadded (SG standsfor switchgrass) and m3 CH4/kg VSadded. The latter wascalculated based on the VS of switchgrass added to thedigester. These values were estimated by fitting a regres-sion line to the data (daily switchgrass or VS added, anddaily methane production) for each condition and the con-trols. The slope of the regression line represents the specificmethane yield.

The statistical significance of each slope term was eval-uated using a modified ANOVA process to account forthe lack of independence of observations on a givendigester and that each digester was used in only onecondition. Sums of squares were calculated for within-digester, between-digester (but within-treatment group),and between-treatment groups. An F-statistic was cal-culated for the slope terms from mean squares asMSbetween-treatment/MSwithin-digester and degrees of freedomof 1, n-r, where n is the number of observations and r is thenumber of digesters. All statistical analyses were performedwith PASW Statistics 18.

Energy conversion efficiency (%). Energy conversion effi-ciency is calculated based on gross heat of combustion ofadded switchgrass and heat content of methane generatedfrom switchgrass in the digesters. Energy conversion effi-ciency was therefore expressed as percentage of switchgrassenergy content converted into methane.

Results

Batch studies

Cumulative methane production under the various pre-treatment conditions, as compared to controls (switchgrasssoaked in water at room temperature), is summarized inTable 2. Results suggest a trend of increasing methane pro-duction with increasing NaOH concentration under heatedconditions, reaching a high of over 120% increase for 11%NaOH if the switchgrass is not rinsed prior to digestion.Individually, none of the P-values are significant at the P ≤0.05 level except at the highest percent increase in methane.However, these P values do not reflect the trend observedin the data. A linear regression analysis of results for condi-tions A-C produces a P-value < 0.01 (not shown in Table 2),

Table 2. Cumulative methane production increases from pretreat-ment conditions in batch studies.

Pretreatment conditions

Cumulative methaneincrease compared to

controla P value

A) 0% NaOH, heat 25.9% 0.116B) 2.2% NaOH, heat 40.3% 0.161C) 11% NaOH, heat 87.9% 0.094D) 11% NaOH, heat,

no rinse120.4% 0.008

E) 22% NaOH, no heat 16.4% 0.219F) 11% NaOH + 6.6%

H2O2, no heat45.6% 0.178

aControl was switchgrass soaked in RO water at 25◦C for 24 h.

supporting the conclusion of increasing methane produc-tion with increasing NaOH concentration.

Results in Table 2 also suggest the value of heat in the pre-treatment, with heat alone (no NaOH) demonstrating al-most a 26% increase, yet 22% NaOH without heat producedonly about a 16% increase. The addition of hydrogen per-oxide (H2O2) did not appear to improve pretreatment overNaOH alone. Again, variation between triplicate digestersincreased P values and cautions against over-confidence inthe results.

As previously noted in the results, rinsing after pretreat-ment appears to reduce methane production as comparedto digesting both the switchgrass and pretreatment liquid(condition D). Rinsing biomass after pretreatment can im-pact digestion not only because it removes alkali, but itmay also remove inhibitory compounds formed during pre-treatment.[4] However, rinsing after pretreatment may alsoremove sugars and other biodegradable compounds dis-solved during pretreatment. Results of this study suggestthat cellulose and hemicellulose losses from the rinsing stepwere significant, and that omitting the rinsing step can in-crease methane yield.

Overall, the greatest methane production increase oc-curred using 11% NaOH with heat and without rinsingprior to digestion. Using this as the base treatment, Table 3presents results for reducing the NaOH concentration byhalf, reducing the heating time by half, and substitutinglime for NaOH. It appears that a NaOH concentration of5.5% has approximately the same effect as an 11% concen-tration; a very high P value of 0.842 indicates the differencein methane production is of little statistical significance.Reduced heating time may reduce effectiveness, but the dif-ference appears relatively small and again a very high Pvalue of 0.839 indicates the difference is of little statisticalsignificance. Similarly, substitution of lime for NaOH mayreduce effectiveness somewhat although the difference wasof little statistical significance (P value of 0.508). A rela-tively small loss of effectiveness may be acceptable if limecan be purchased and managed at significantly lower cost.

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Table 3. Comparison of half-strength NaOH, half-heating time,and use of Ca(OH)2 pretreatment compared to 1% NaOH,heated, without rinse.

Pretreatment conditionsCumulative

methane increase P value

G) 5.5% NaOH, heat, no rinse 1.2% 0.842H) 11% NaOH, 3 h heat, no rinse −5.7% 0.839I) 11% Ca(OH)2, heat, no rinse −8.1% 0.508

Continuous-feed studies

In order to assess potential long-term operational con-cerns typically associated with digestion of lignocellulosicfeedstocks such as solids accumulation, poor mixing, orfloating layers, the two most promising conditions from thebatch studies (11% and 5.5% NaOH, heated, no rinse) weretested in 7-week continuously-fed bench-top digesters.Switchgrass was heated for 6 h, even though batch results(Table 3) indicated that 3-h heating might decrease effec-tiveness only slightly. Unless waste heat from a generatorhas an alternative, high-value use, the savings from shorterheating time is not likely to offset any decrease in effective-ness. The control condition for these studies was raw feedsludge only, so that methane resulting from feed and seedsludge could be factored into calculation of switchgrass-specific methane production in the test digesters.

The methane production over time for every conditionincluding controls is presented in Figs. 3a–3d. Controls for5.5% NaOH and 11% NaOH were the same digesters. Forcomparison, prior results for untreated coarse-grind andfine-grind (<2 mm particle size) switchgrass are also pre-sented.[21] Coarse-grind switchgrass digesters were discon-tinued at around 1100 h, since mixing became extremelydifficult and no additional methane production was ob-served as compared to controls.

The trend over time for total solids in the digestersdemonstrated a slow increase from 2.0–2.5% up to2.5%–3.0% within about one retention period (30 days)followed by a leveling off of the solids concentration (datanot shown). This suggests that the digesters had reachedequilibrium by about 30 days, and that solid levels are notlikely to be high enough to cause operational problemsfor typical U.S. digesters (operating at 2.5–3% solids). Inaddition, problems with mixing and/or floating materialwere not observed in any of the tested NaOH pretreatmentconditions.

Specific methane yield, energy conversion efficiency, andother operational and performance parameters for thetwo pretreatment conditions are presented in Table 4.Again, prior results for fine-grind and coarse-grind switch-grass are included for comparison. Both 5.5% and 11%NaOH pretreatment demonstrated methane yields and en-ergy conversion rates comparable to fine-grinding. In con-trast, coarsely-ground switchgrass without pretreatment

has demonstrated very poor digestion.[21] Total VS removalwas comparable between both NaOH pretreatment condi-tions and fine-grind switchgrass.

Discussion

Little research has been reported on anaerobic digestionof switchgrass. Labatut and Scott [24] found that non-pretreated switchgrass actually reduced methane produc-tion when mixed in a 1:3 ratio with manure, as compared tomanure alone. They noted that the switchgrass was “mixedand blended to reduce particle size” but did not note thefinal particle size. Himmelsbach et al. [32] pretreated switch-grass in 29.5% aqueous ammonium (5 mL g−1 of switch-grass, equivalent to 148% wt/wt alkaline/biomass) for5 days at room temperature. In 21-day batch digestion stud-ies, ammonia pretreatment resulted in a methane yield ofabout 0.16 m3/kg VS, an increase of about 65% comparedto untreated switchgrass. However, this methane produc-tion represents only about 30% of theoretical maximum,in contrast to the 63% yield observed in the current studyusing 5.5% NaOH. Ahn et al.[33] pretreated switchgrass byfine-grinding to approximately 2 mm and co-digesting withswine manure, dairy manure, or poultry manure at 15%solids in a batch-digestion process. Both dairy and poul-try manure failed to sustain anaerobic digestion due toexcess acid production. However, swine manure had suffi-cient buffering capacity to sustain digestion, resulting in amethane yield of 0.337 m3 kg−1 VS, roughly comparable toour findings using 5.5% NaOH.

Many studies have examined pretreatment of switchgrassand other biomass for ethanol production. While thesestudies can provide important insights for biogas produc-tion, results must be interpreted carefully. The purpose ofpretreatment in ethanol production is to “improve enzy-matic hydrolysis, minimize carbohydrate losses, and preventformation of by-products that might inhibit subsequent hy-drolysis and fermentation steps.”[23] These objectives areless important in biogas production because the anaerobicbacteria are able to produce methane from a wide array ofcompounds, not just sugars, and appear to be far more tol-erant of high pH as well as inhibitory compounds.[10] Nev-ertheless, such pretreatment studies may provide insightsfor optimal pretreatment for biogas production.

Keshwani and Cheng [34] tested NaOH (2% wt/wt) andlime (2% wt/wt) combined with microwave heating forswitchgrass pretreatment. They observed about a 2-foldincrease in sugar yield with lime, and about a 3-fold in-crease for NaOH. They attribute the superior performanceof NaOH to a greater disruption of the biomass struc-ture, allowing greater enzymatic access to the celluloseand hemicellulose. Chang et al.[35] found that lime pre-treatment using 10% wt/wt for 2 hours at 100◦C (212◦F)produced a 5-fold increase in switchgrass sugar yield.

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Fig. 3. Cumulative methane production over time for continuous-feed studies: (a) 5.5% NaOH with controls; (b) 11% NaOH withcontrols; (c) fine-grind switchgrass with controls;[21] and (d) coarse-grind switchgrass with controls.[21]

Soaking in 25–28% aqueous ammonia (2 mL g−1, equiv-alent to 50%–56% wt/wt) at 120◦C (248◦F) for 20 minuproduced a 3–5-fold increase in sugar production.[36] Am-monia fiber explosion (AFEX) has been found to be amore effective switchgrass pretreatment than steam explo-

sion in ethanol production because less hemicellulose ishydrolyzed.[23] Optimum AFEX conditions have been re-ported to be 100◦C and a residence time of 5 min, resultingin a 6-fold improvement in sugar production.[37] These re-sults confirm that alkaline pretreatment at temperatures in

Table 4. Specific methane yield, energy conversion efficiency, and VS removal for continuous-feed digesters.a

Pretreatment conditions

Switchgrass-specificmethane yield

(m3 CH4/kg SGadded)

Switchgrass-specificmethane yield

(m3 CH4/kg VSadded)bSwitchgrass energy

conversion efficiencyTotal VSremovalc

5.5% NaOH, heat, no rinse 0.293d 0.332d 63% 69.7%11% NaOH, heat, no rinse 0.235d 0.267d 50% 55.9%Fine-grind, no pretreatment 0.261d 0.296d 56% 62.1%Coarse-grind, no pretreatment 0.012 0.014 2% 38.2%

aDigesters operated at 35◦C, feed sludge TS was 6.58% solids, and the organic loading rate was 1.70 kg VS m−3d−1 (of which 31% was from switchgrassin test digesters).bOnly volatile solids from switchgrass are counted as VSadded, not those from feed sludge.cTotal VS removal is calculated based on total volatile solids reduction from feed sludge and switchgrass.dStatistically significantly different from control (feed sludge only) at P < 0.01.

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the range of 100◦C–120◦C (212◦F–248◦F) can be effectivein disrupting the switchgrass lignin-hemicellulose-cellulosestructure making the carbohydrates more accessible to en-zyme hydrolysis.

Removal of lignin, or delignification, during pretreat-ment has been used by many researchers as an indicatorof increased carbohydrate availability. Sharma [19] foundthat 20% KOH (wt/wt) at 121◦C (250◦F) for 24 hoursproduced the greatest delignification of switchgrass. Isciet al.[38] found the greatest delignification of switchgrassafter soaking switchgrass in 29.5% aqueous ammonium(10 mL g−1 switchgrass, equivalent to 295% wt/wt) for10 days. Interestingly, while delignification has been takenas an indicator of enzyme accessibility, Xu et al.[20] foundhigh sugar yields even with relatively low levels of deligni-fication when lime (CaO) was used for pretreatment. Thisseemed to be due to cross-linking of lignin molecules withCa, reducing their solubility, yet opening the carbohydratestructure to enzyme attack. Thus, while delignification canbe an indicator of biogas production potential, some pre-treatments may make biomass more digestible without re-moving the lignin.

Comparison with studies of other biomass must be madecarefully, since there are many factors that can account forvariation in optimal alkaline pretreatment conditions, suchas type of biomass, rinsing prior to digestion, and par-ticle size. In addition, “optimal” conditions are typicallyselected from only a limited number of tested conditions.Some research seems to suggest that pretreatment can besuccessful under relatively mild conditions. For example,Zheng et al.[39] found that optimal pretreatment condi-tions for corn stover using 2% NaOH (wt/wt) was 20◦C(68◦F) for 3 days. Higher temperatures actually resulted indecreasing substrate-specific methane yields. Corn stoverhad been ground to 5–10-mm particles, but it is not clearwhether pretreated biomass was rinsed prior to digestion.Similarly, He et al.[40] found that optimal conditions for ricestraw were 6% NaOH (wt/wt) for 3 weeks at 20◦C (68◦F).They reported a particle size of 5–10 mm and did not rinsebiomass prior to digestion.

For pretreatment at elevated temperature, Mirahmadiet al.[16] found 7% NaOH (wt/wt) at 100◦C (212◦F) for 2 hwas optimal for pretreatment of birch prior to anaerobicdigestion. Particle size was reported to be under 0.8 mmand biomass was washed prior to digestion. Raju et al.[17]

explored a temperature range of 20◦C–225◦C (68◦F–437◦F)and found that 100◦C (212◦F) was an optimal temperaturefor 15-min pretreatment of wheat and rapeseed straw with0.75% lime (wt/wt). Particle size was 4 cm and biomasswas not rinsed prior to digestion. However, only a relativelysmall range of results were found in this study, and no anal-ysis of statistical significance was reported. For example, at100◦C (212◦F), biomass treated with 1.5% Ca(OH)2 pro-duced only about 11% more biogas than biomass receivingno Ca(OH)2.

Chandra et al.[41] treated wheat straw at 37◦C (99◦F) for120 h with NaOH at 4% wt/wt concentration. Methane

production was found to increase over 111% compared tountreated controls. Teghammar [42] found that treatment ofpaper tubes with 2% wt/wt NaOH at 190◦C (374◦F) in-creased methane production 21% compared to untreatedtubes, but treatment at higher temperatures resulted in in-hibition of digestion.

Overall, the body of pretreatment research literatureshowed quite varied results, reflecting not only the diversityof feedstocks tested, but also the diversity of experimen-tal protocols as well as the natural variation in biologicalsystems. Nevertheless, there is a general pattern of the in-terchangeable nature of concentration, time, and tempera-ture for alkaline pretreatment. The results of this study aregenerally consistent with this body of research and demon-strate that moderate alkaline concentrations (5.5% wt/wtNaOH) and mild temperatures (100◦C) can dramaticallyincrease methane production, equivalent to fine grindingthe switchgrass.

In situations where biogas production is followed by elec-tricity generation, the use of waste heat from the generatorcould significantly reduce the cost of alkaline pretreatmentdue to the success of alkaline pretreatment at mild tem-peratures. This appears to be true for a variety of alkalis,including NaOH, KOH, CaO, and NH3.

Conclusion

Mild heat (100◦C or 212◦F for 6 h) with alkaline pre-treatment was an effective pretreatment option for coarse-ground switchgrass prior to anaerobic digestion. Energyconversion efficiency as high as 63% was observed, and wascomparable or superior to fine-grinding as a pretreatmentmethod. An optimal NaOH concentration of 5.5% (wt/wtalkaline/biomass) was found (at 91.7% moisture level). Noevidence of operational problems such as solids build-up,poor mixing, or floating materials were observed. These re-sults suggest the use of waste heat from a generator couldreduce the concentration of alkaline required to adequatelypretreat lignocellulosic feedstock prior to anaerobic diges-tion.

Acknowledgments

Bench-top digester seed culture and feed sludge was sup-plied by Bloomington/Normal Wastewater ReclamationDistrict. We would also like to recognize our under-graduate research assistants: Doug Becker, Alex Choo,Melissa Gawron, Chris Lund, Morgan Martin, and TonySchwegmann.

Funding

This research was supported in part by grants from the USDepartment of Energy (the agency did not play a role instudy design, conduct, or interpretation).

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