Bioresource Technology 101 (2010) 7523–7528

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Bioresource Technology

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Enhanced solid-state anaerobic digestion of corn stover by alkaline pretreatment

Jiying Zhu 1, Caixia Wan 1, 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, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 February 2010Received in revised form 19 April 2010Accepted 24 April 2010Available online 21 May 2010

Keywords:Anaerobic digestionSolid-state fermentationCorn stoverAlkaline pretreatmentBiogasification

0960-8524/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.biortech.2010.04.060

* Corresponding author. Tel.: +1 330 263 3855; faxE-mail address: li.851@osu.edu (Y. Li).

1 These authors contributed equally to this work.

Alkaline pretreatment was applied to enhance biogas production from corn stover through solid-stateanaerobic digestion. Different NaOH loadings (1%, 2.5%, 5.0% and 7.5% (w/w)) were tested for solid-statepretreatment of corn stover. Lignin degradation during pretreatment increased from 9.1% to 46.2% whenNaOH concentration increased from 1.0% to 7.5%. The NaOH-pretreated corn stover was digested usingeffluent of liquid anaerobic digestion as inoculum and nitrogen source. NaOH loading of 1% did not causesignificant improvement on biogas yield. The highest biogas yield of 372.4 L/kg VS was obtained with 5%NaOH-pretreated corn stover, which was 37.0% higher than that of the untreated corn stover. However, ahigher NaOH loading of 7.5% caused faster production of volatile fatty acids during the hydrolysis and aci-dogenesis stages, which inhibited the methanogenesis. Simultaneous NaOH treatment and anaerobicdigestion did not significantly improve the biogas production (P > 0.05).

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Lignocellulosic biomass, such as crop residues and forest waste,is mainly composed of cellulose, hemicelluloses and lignin. It hasbeen well studied for ethanol production in recent years. The com-plex structure of native lignocellulosic biomass creates recalci-trance to enzymatic hydrolysis by cellulolytic microbes duringanaerobic digestion. In addition, due to the low cellulolytic activityand low specific growth rate of cellulolytic microbes in anaerobicdigestion, hydrolysis of native lignocellulosic biomass is usuallyslow (Noike et al., 1985). As a consequence, hydrolysis appears tobe the most rate-limiting stage among the four stages (hydrolysis,acidogenesis, acetogenesis, and methanogenesis) of anaerobicdigestion (Lu et al., 2007). Pretreatment of lignocellulosic biomassmay represent a feasible solution to improve digestion efficiencyand biogas production.

Alkaline pretreatment is one of the current leading pretreat-ment methods as it has shown several positive effects, includingsolubilization of lignin and neutralization of various acidic prod-ucts degraded from the lignocellulosic complex (Hendriks and Zee-man, 2009; Pavlostathis and Gossett, 1985). Additionally, thepresence of a small amount of residual alkali in the treated solidsmay be helpful for preventing the drop of pH during subsequentacidogenesis process (Pavlostathis and Gossett, 1985). Therefore,alkaline pretreatment is more effective and compatible with subse-

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: +1 330 263 3670.

quent anaerobic digestion when compared to other pretreatmentmethods such as thermochemical pretreatment. Among the threekinds of alkali (NaOH, KOH, lime) tested for pretreatment of ricestraw, NaOH was found to be most effective for lignin removaland biogas production (Yang et al., 2009). Zheng et al. (2009) re-ported that a 72.9% increase in biogas production was obtainedfrom corn stover pretreated with 2% NaOH for 3 days. Althoughalkaline pretreatment, especially with NaOH, can lead to significantimprovement on biogas production, toxicity of cations on metha-nogens was also reported (Feijoo et al., 1995). Compared to theconventional alkaline pretreatment, solid-state alkaline pretreat-ment only uses a limited amount of water to saturate the feedstockand there is no wastewater generated. Solid-state alkaline pre-treatment of crop residues has been shown to be effective forimproving biodegradability and biogas production. Pang et al.(2008) reported that corn stover pretreated with 6% NaOH at 80%moisture content and ambient temperature (20 ± 2 �C) for 3 weeksresulted in 48.5% more biogas yield over the untreated.

Solid-state anaerobic digestion (SS-AD) is generally used to pro-cess organic waste with high solid content (20–35%). It has beensuccessfully implemented in Europe for processing the organicfraction of municipal solid waste (OF-MSW) (Bolzonella et al.,2003; Mata-Alvarez et al., 2000). Compared to liquid phase anaer-obic digestion (0.5–15% solid loading), SS-AD is relatively stableand requires less energy input. However, the disadvantages ofSS-AD include the requirements of large amounts of inoculum(up to 50%), much longer retention time (three times longer thanliquid AD), and nitrogen supplementation when lignocellulosicbiomass is used. In this study, an integrated anaerobic digestion

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system was tested to evaluate the effectiveness of biogas produc-tion from lignocellulosic biomass in SS-AD using effluent of liquidAD as inoculums and nitrogen source. This process can overcomethe major disadvantages for liquid anaerobic digestion (treatmentof effluent) and conventional SS-AD (large volume recycling of dig-estate and/or leachate for inoculation and costly nitrogen sourcesupplementation). It can also potentially reduce the productioncost and increase the energy efficiency of biogas production. Themain objective of this study was to investigate the effect of NaOHpretreatment on the biogas production from corn stover via SS-AD.Lignin and holocellulose (cellulose and hemicellulose) degradationof corn stover during NaOH pretreatment were studied. The reduc-tion of total solids (TS) and volatile solids (VS) of NaOH-pretreatedcorn stover and the variation of volatile fatty acids, pH and alkalin-ity during digestion were also investigated.

2. Methods

2.1. Feedstock and inoculum

Corn stover was collected from the farm of Ohio Agricultural Re-search and Development Center (OARDC) in Wooster, OH and air-dried to a moisture content less than 10%. The dried corn stoverwas than ground to pass through a 5 mm sieve and stored in air-tight containers prior to use. Effluent of liquid anaerobic digestionwas supplied by Akron facility of quasar energy group and used asinoculum. The characteristics of corn stover and effluent are pre-sented in Table 1.

2.2. Solid-state NaOH pretreatment

Two hundred grams of corn stover (dry basis) were mixedevenly with 200 ml NaOH solution (0.25, 0.625, 1.25, and1.85 M). The corresponding NaOH loadings over the substrate sol-ids were 1.0%, 2.5%, 5.0% and 7.5% (w/w), respectively. The NaOHsoaked corn stover was kept at ambient temperature (20 ± 0.5 �C)in sealed zipper plastic bags for 24 h. Samples were taken for com-positional analysis before the anaerobic digestion tests.

2.3. Solid-state anaerobic digestion

The pretreated corn stover was mixed with 800 g effluent to ob-tain a C/N ratio of 18 and a TS content of 22% which are optimal formethane production (Zhu et al., 2009). The inoculated corn stoverwas placed into 2 L reactors and digested at 37 �C for 40 days. Bio-gas was collected daily with 5 L Tedlar gas bags throughout thedigestion process for gas composition and production analysis.TS, VS, pH, alkalinity, and volatile fatty acids (VFA) were analyzed

Table 1Characteristics of corn stover and effluent.

Parameter Corn stover Effluent

Total solids (%) 94.7 ± 0.3 9.8 ± 0.1Volatile solids (%) 88.0 ± 0.8 6.4 ± 0.1Total carbon (%) 43.6 ± 0.4 4.1 ± 0.2Total nitrogen (%) 0.6 ± 0.0 0.7 ± 0.0C/N ratio 67.6 ± 0.0 5.8 ± 0.3pH value ND 8.3 ± 0.1VFA (g/kg) ND 6.8 ± 0.2Alkalinity (g CaCO3/kg) ND 19.9 ± 0.1Cellulose (%) 40.7 ± 1.6 NDHemi-cellulose (%) 22.5 ± 0.3 NDAcid-insoluble lignin (%) 20.2 ± 0.5 NDAcid-soluble lignin (%) 1.5 ± 0.3 NDLignin (%) 21.7 ± 0.5 ND

ND = not determined.

at the end of the digestion tests. To analyze the pH, alkalinity andVFA during digestion, nine identical reactors with 5% NaOH-pre-treated corn stover were set up at the same time and terminatedon day 3, 6, 9, 12, 15, 18, 25, 32 and 40 for sampling and subse-quent analysis.

2.4. Concurrent NaOH pretreatment and anaerobic digestion

Two hundred grams of corn stover (dry basis) were mixedevenly with 200 ml NaOH solution (0.25, 0.625, and 1.25 M) and800 g effluent. The mixture was immediately fed into the SS-ADreactor for digestion tests. The digestion conditions and samplingprocedures were the same as those described in Section 2.3.

2.5. Analytical methods

The cellulose, hemicellulose, and lignin contents of the un-treated and NaOH-pretreated corn stover samples were deter-mined following NREL Laboratory Analytical Procedure (Sluiteret al., 2008). A two-stage acid hydrolysis was used for fractionatingthe corn stover samples. HPLC (Agilent 1200 series, MN, USA)equipped with a Biorad Aminex HPX-87P column and a refractiveindex detector (RID) was used for analysis of monomeric sugarsin the acid hydrolysate. The temperatures of the column and theRID were maintained at 80 and 55 �C, respectively. Water was usedas the mobile phase with a flow rate of 0.6 ml/min. Cellulose andhemicellulose contents were calculated from the correspondingmonomers. The acid insoluble lignin was measured by gravimetricanalysis and the acid soluble lignin was measured by UV–vis spec-troscopy (Biomate™3, Thermo-scientific, MA, USA). Lignin, cellu-lose and hemicellulose degradation was defined as thepercentage of lignin, cellulose and hemicellulose reduction,respectively.

TS, VS, pH and alkalinity were measured according to the Stan-dard Methods for the Examination of Water and Wastewater(APHA, 2005). Total carbon and total nitrogen were determinedwith an elemental analyzer (Elementar Vario Max CNS, ElementarAmericans, Mt. Laurel, NJ, USA). The samples for pH measurementwere prepared by suspending 5 g wet digestate into 50 ml distilledwater. The samples for VFA measurement were prepared by sus-pending 10 g wet digestate in 30 ml distilled water followed bycentrifugation (8000 rpm, 5 min). One fraction of supernatantwas acidified with three fractions of formic acid and then filteredthrough 0.2 lm nylon syringe filter for VFA analysis. VFA analysiswas conducted using a gas chromatograph (GC) (HP5890, AgilentTechnologies, Wilmington, DE, USA) equipped with a flame ioniza-tion detector (FID) and a 30 m � 0.32 mm � 0.5 lm Stabilwax-DAFused Silica column. The temperatures of the injection port anddetector were 230 and 230 �C, respectively. Helium was used asa carrier gas with a flow rate of 20 ml/min. The daily biogas pro-duction was measured using a wet drum gas meter (TG 5, Cali-brated Instruments Company, NY, USA). Gas composition wasdetermined using a GC (HP6890, Agilent Technologies, Wilming-ton, DE) equipped with a 30 m � 0.53 mm � 10 lm alumina/KCldeactivation column and a thermal conductivity detector (TCD).Helium was used as the carrier gas at a flow rate of 5.2 ml/min.The temperatures of the injector and detector were 150 and200 �C, respectively. The biogas yield was expressed as the volumeof biogas produced based on the initial total VS in corn stover. Thealkalinity and VFA concentration expressed in were based on thedigestate (wet basis).

2.6. Statistical analysis

All the NaOH pretreatment and digestion tests were conductedin duplicates and the average values were reported. The software

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Fig. 1. Biogas production of 24-h alkaline pretreated corn stover: (a) daily biogasyield, (b) cumulative biogas yield, and (c) methane content of biogas.

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SAS 9.1 (SAS Inc., Cary, NC, USA) was used for analysis of variance(ANOVA).

3. Results and discussion

3.1. Degradation of corn stover during pretreatment

NaOH pretreatment of corn stover for 24 h caused significantdegradation of corn stover (Table 2). An increase in NaOH concen-tration resulted in increased lignin removal. When the NaOH con-centration increased from 1% to 7.5%, the lignin degradationincreased from 9.1% to 46.2%, respectively. The highest hemicellu-lose loss of 13.8% was obtained with 7.5% NaOH addition, whichwas about two times of that obtained with 1.0–5.0% NaOH addi-tion. The cellulose loss caused by pretreatment was less than 7%for all testes. The low cellulose degradation was partially attrib-uted to the physical protection from lignin and hemicellulose.Alkaline treatment generally caused the swelling of cellulose fibersand modification of cellulose crystallinity rather than direct degra-dation (Hendriks and Zeeman, 2009). Meanwhile, the hetero-struc-tures of hemicellulose branched with short lateral chains(Hendriks and Zeeman, 2009) contributed to the higher hemicellu-lose degradation at a higher NaOH concentration. Our results indi-cated that NaOH pretreatment can preserve most of thecarbohydrates but caused substantial lignin degradation during24-h pretreatment at room temperature. In addition to significantlignin degradation by alkaline pretreatment, an increase in waterextractives was also reported (Pang et al., 2008; Zheng et al.,2009). Therefore, substantial lignin degradation, increased sub-strate porosity, and exposed remaining carbohydrate make theNaOH-pretreated corn stover more accessible to cellulolytic micro-organism than the untreated.

3.2. Solid-state anaerobic digestion

3.2.1. Biogas productionNaOH-pretreated corn stover was directly digested for biogas

production without any washing or detoxification. As shown inFig. 1a, the daily biogas production of 1.0% and 2.5% NaOH-pre-treated corn stover reached 4.6 and 4.1 L/kg VS, respectively, dur-ing 24 h of digestion, which is similar to that of the untreatedcorn stover. The rapid initial biogas production is due to readilybiodegradable organic matter in the substrate irrespective of thepretreatment. A temporary decline after the first day might becaused by dissipation of substrate readily available for microbialdecomposition (Ahn et al., 2009). The daily biogas yield of 1.0%and 2.5% NaOH-pretreated corn stover reached their peak valuesof 18.2 and 22.5 L/kg VS, respectively, on day 7. Thereafter, the dai-ly biogas yield decreased rapidly to 8.3 and 9.4 L/kg VS on day 12and finally decreased to 2.1 and 2.6 L/kg VS on day 40. Biogas yieldof the untreated corn stover recovered slowly and reached the peakvalue (20.7 L/kg VS) on day 9. This result indicates that the 1% and2.5% NaOH pretreated corn stover were more easily accessible tohydrolytic bacteria at the early stage of digestion. For 5% NaOH-pretreated corn stover, the daily biogas yield reached 7.1 L/kg VSon day 3 and then decreased slightly followed by a gradual in-

Table 2Effect of 24-h alkaline pretreatment on corn stover degradation.

NaOH concentration (%) Degradation (%)

Cellulose Hemicellulose Lignin

1.0 6.5 ± 1.4 6.1 ± 1.4 9.1 ± 0.42.5 5.8 ± 0.4 4.0 ± 0.5 11.4 ± 0.35.0 5.1 ± 0.8 6.9 ± 1.1 31.4 ± 0.67.5 6.8 ± 0.1 13.8 ± 3.1 46.2 ± 0.7

crease. The daily biogas yield reached a peak value of 19.9 L/kg VSon day 11 and decreased slowly to 5.9 L/kg VS on day 28. Duringthis period, the biogas production of 5% NaOH-pretreated corn sto-ver was substantially higher than that of all other treatments(Fig. 1a and b). Since 5% NaOH pretreatment removed 31.4% ligninfrom corn stover, the hydrolysis may not be a rate-limiting stepduring the digestion process. However, sodium ions and aromaticcompounds derived from lignin degradation are known to beinhibitory to methanogens (Gossett et al., 1982), which might con-tribute to the lower daily biogas yield in the first 10 days whencompared to 1.0% and 2.5% NaOH-pretreated corn stover (Fig. 1a).

Table 3Change of VFA, pH and alkalinity during anaerobic digestion.

NaOHconcentration(%)

pHia pHf

b Alkalinityi Alkalinityf VFAf

(g CaCO3/kg)

(g CaCO3/kg)

(g/kg)

Untreated 8.5 ± 0.0 8.5 ± 0.0 13.6 ± 0.7 11.9 ± 0.2 2.0 ± 0.11.0 8.5 ± 0.0 8.3 ± 0.0 14.3 ± 0.8 14.0 ± 0.1 2.6 ± 0.22.5 8.6 ± 0.0 8.5 ± 0.0 17.6 ± 0.9 15.3 ± 0.7 2.0 ± 0.75.0 8.9 ± 0.1 8.5 ± 0.1 21.8 ± 0.8 20.2 ± 1.3 2.4 ± 0.67.5c 9.5 ± 0.4 6.0 ± 0.3 26.4 ± 1.3 10.7 ± 0.5 22.1 ± 1.1

a i – initial values at the beginning of the digestion.b f – final value after 40 days of digestion.c f – final value after 20 days of digestion.

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Corn stover pretreated with 7.5% NaOH caused failure of meth-ane production. It was observed that corn stover pretreated with7.5% NaOH resulted in a low daily biogas yield with low methanecontent and the digestion ceased on day 13 (Fig. 1a and c). A highlyacidic environment (pH 6.0) caused by the accumulation of VFAinhibited the methanogenic bacteria when the 7.5% NaOH-pre-treated corn stover was digested (Table 3). Similar results were re-ported by Penaud et al. (1999) who observed that methaneproduction decreased when 5 g/L or higher NaOH loading wasused. Although sodium ions were not found to inhibit the methaneproduction in the study of Penaud et al. (1999), sodium concentra-tions of 0.25–0.4 M (10–16 g/L) were reported to impose sodiumion toxicity on methanogenic bacteria by Rinzema et al. (1988).

The cumulative biogas yields of corn stover pretreated with dif-ferent NaOH loadings are presented in Fig. 1b. The highest cumula-tive biogas yield (372.4 L/kg VS) for 40-day digestion was obtainedwith 5.0% NaOH-pretreated corn stover, which was 37.0% higherthan that of the untreated samples. The cumulative biogas yieldsfor 40 days reached 266.8 and 275.9 L/kg VS for 1.0% and 2.5%

Table 4Degradation of substrates after 40-day anaerobic digestion.

NaOH concentration(%)

Degradation (%)

TS VS Cellulose Hemicellulose

Untreated 21.8 ± 0.4 35.3 ± 2.3 44.5 ± 3.2 45.7 ± 3.81.0 21.2 ± 0.4 35.8 ± 0.3 43.2 ± 1.9 43.6 ± 3.22.5 22.2 ± 0.2 38.4 ± 2.0 44.1 ± 6.2 46.0 ± 3.35.0 25.4 ± 1.3 44.4 ± 0.8 65.1 ± 0.2 63.6 ± 2.57.5a 7.7 ± 0.4 13.7 ± 0.7 27.6 ± 2.5 61.3 ± 3.7

a Twenty days of digestion.

Fig. 2. Variation of VFA, pH and alkalinity during anaero

NaOH-pretreated corn stover, respectively, which were not signif-icantly different from the control (untreated corn stover). These re-sults indicated that NaOH concentrations lower than 2.5% were noteffective in improving the biogas production. On the other hand,7.5% NaOH-pretreated corn stover inhibited the methaneproduction.

Among the functional digesters, the methane contents in-creased gradually and reached about 60% on day 10 (Fig. 1c). Therewere variations of methane content among different treatments,but they were not significantly different. The methane contentsranged between 50% and 60% after day 10 for all the functionaldigesters.

3.2.2. Degradation of corn stoverThe efficiency of solid-state anaerobic digestion was evaluated

in terms of TS and VS reduction as well as carbohydrate reduction.As shown in Table 4, VS reduction increased as NaOH loading in-creased from 1.0% to 5.0%. Pretreatment with 5.0% NaOH causedthe highest VS reduction (44.4%) in SS-AD. No significant improve-ment on VS reduction was observed with 1.0% and 2.5% NaOH pre-treatment. The TS reduction followed a similar pattern as that ofthe VS. Cellulose and hemicellulose reductions of 1.0–2.5%NaOH-pretreated corn stover during SS-AD were close to that ofthe untreated samples. The highest cellulose and hemicellulosereductions of 65.1% and 63.1%, respectively, were observed with5% NaOH-pretreated corn stover, which is in agreement with thehighest biogas production at this condition. The lowest cellulosereduction of 27.6% was observed with 7.5% NaOH-pretreated cornstover. The hemicellulose degradation of 7.5% NaOH-pretreatedcorn stover reached 61.3% which was higher than that of 1.0%and 2.5% NaOH-pretreated corn stover. This may be caused bythe high alkali residue in the 7.5% NaOH-pretreated corn stoverwhich continuously degraded the hemicellulose during the diges-tion process.

3.2.3. Variation of VFA, pH and alkalinityVolatile fatty acids (mainly acetic acid, propionic acid, and buty-

ric acid) produced during the acidogenic stages are vital to theanaerobic digestion process. The degradation of propionate andbutyrate by syntrophic acetogenic bacteria (e.g. Syntropher wolinii,syntrophomonas wolfei) produces acetic acid that is subsequentlydegraded into methane and CO2 by acetoclastic methanogens(Montero et al., 2008). The evolution of VFAs plays an importantrole in maintaining efficient anaerobic digestion as it strongly af-fects the pH value, alkalinity, and the activity of methanogens(Buyukkamaci and Filibeli, 2004; Moller et al., 2004). The irrevers-

bic digestion of 5.0% NaOH-pretreated corn stover.

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Fig. 3. Biogas production of simultaneous alkaline treatment and anaerobicdigestion: (a) daily biogas yield, (b) cumulative biogas yield, and (c) methanecontent of biogas.

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ible acidification of the digestion, resulting from rapid hydrolysisand acidogenesis, is the major challenge of anaerobic digestion,as it can cause inhibition of methanogenesis or failure of the diges-tion (Veeken and Hamelers, 1999; Wang et al., 2009).

Variation of total short chain VFAs during anaerobic digestion of5% NaOH-pretreated corn stover is presented in Fig. 2. Acetic acidwas the dominant volatile fatty acids. There was no significantaccumulation of propionic acid and butyric acid, probably becauseof the sufficient propionate- and butyric-degrading syntrophs inthe inoculum which can rapidly convert propionic acid and butyricacid to acetic acid (Montero et al., 2008). The acetic acid increasedrapidly after starting the test and reached a maximum of 21.5 g/kgon day 12. During this period, the acetic acid production rate wasapparently higher than the acetic acid consumption rate. After 15days, acetic acid concentration rapidly dropped to 1–2 g/kg and re-mained stable until the end of the digestion process. During thisperiod, the digestion reached the stabilization stage (methanogen-esis) with a balance between the production and consumption ofacetic acid and a constant methane content of 50–60% wasobtained.

The pH is an important parameter for proper operation andmonitoring of the anaerobic digestion process. It strongly dependson VFA and buffering capacity. The pH value decreased with the in-crease in VFA production by acidogenic bacteria. The lowest pH va-lue of 6.6 was observed on day 12, which corresponds to themaximum VFA production of 24.7 g/kg. Subsequently, pH valuesincreased between 8.4 and 8.5 with a decrease in VFA concentra-tion, indicating the further conversion of VFA to methane by meth-anogens. The alkalinity had a similar profile as pH, reaching theminimum level of 17.1 g CaCO3/kg on day 12 and subsequently in-creased to 20.4–21.1 g CaCO3/kg on day 12 (Fig. 2). The increase ofalkalinity during the later stage could be mainly due to the con-sumption of VFA by methanogens, suggesting that the alkali(OH�) remaining in the substrate solids after pretreatment didnot increase buffering capacity of the digestion(Ahn et al., 2009;Kim et al., 2002).

The final VFA concentrations for each experimental conditiontested are shown in Table 3. Acetic acid was the dominant compo-nent of VFA. For all the functional digesters, the VFA concentrationfell between 2.0 and 2.4 g/kg and the final pH values were between8.3 and 8.5, close to the initial values. In contrast, an accumulationof VFA (22.1 g/kg) was observed for 7.5% NaOH-pretreated cornstover at the end of the 20-day digestion, resulting in irreversibleacidification of digestion with a pH decreasing from 9.5 to 6.0.The acetoclastic methanogens were reported to be severely inhib-ited by the accumulation of VFA and a decrease in pH (Powell andArcher, 1986). Separated acidogenic and methanogenic phaseswith leachate recirculation may provide buffering capacity and im-prove digestion performance (Myint and Nirmalakhandan, 2009;Veeken and Hamelers, 1999).

3.3. Simultaneous alkaline treatment and SS-AD

In order to simplify the process and reduce the capital cost,simultaneous alkali treatment and digestion (SATD) was alsotested. Compared to the 24 h-pretreated corn stover, the highestpeak daily biogas production of SATD was delayed from day 7 today 9 for treatments with 1.0% and 2.5% NaOH addition. The24-h pretreated corn stover can provide readily degradable carbo-hydrates to hydrolytic bacteria, which enhanced the biogas in theearly stage. In SATD, corn stover needs to be depolymerized byalkali before hydrolysis and acidogenesis, resulting in no improve-ment on daily biogas production by alkali addition in the firstseveral days. As shown in Fig. 3b, the 40-day accumulative biogasyield of corn stover with 1.0% NaOH addition was 278.9 L/kg VS,which was close to that of the untreated corn stover (275.9 L/kg VS).

However, the biogas production at 2.5% NaOH loading in SATDbetween days 9 and 16 was higher than that of the untreated(Fig. 3a), which caused 9.0% increase in the cumulative biogas yieldover the untreated.

SATD with 5% NaOH addition was inhibited at the early stage,resulting in a lower daily biogas production. The cumulative biogasproduction was 199.4 L/kg VS, which was 27% lower than thatof the untreated. A high initial pH value (pH 9.42) caused by 5%NaOH addition might be the major reason causing the inhibition

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to the anaerobic microbial community, especially hydrolytic andacidogenic bacteria. As shown in Fig. 3c, the methane content ofthe biogas increased to 50–60% on day 7 for all the tests includingcontrol. Only slight changes in methane content were observedafter day 16.

4. Conclusions

Solid-state alkali pretreatment for 24 h can effectively depoly-merize corn stover with significant lignin degradation and limitedcellulose loss. Solid-state anaerobic digestion of 5.0% NaOH-pre-treated corn stover produced 37% more biogas when comparedto that of the untreated. But a higher NaOH concentration (7.5%)caused inhibition of methanogenesis due to rapid hydrolysis andacidogenesis. The simultaneous alkaline treatment and digestionshowed no significant improvement on biogas production.

Acknowledgements

This project was supported by Ohio Agricultural Research andDevelopment Center (OARDC) Seeds Program. The authors wouldlike to thank Mrs. Mary Wicks (Department of Food, Agriculturaland Biological Engineering, OSU) for reading through the manu-script and providing useful suggestions.

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