4635r 2009 American Chemical Society pubs.acs.org/EF

Energy Fuels 2009, 23, 4635–4639 : DOI:10.1021/ef900384pPublished on Web 08/19/2009

Anaerobic Co-Digestion of Cattle Manure with Corn Stover Pretreated by Sodium

Hydroxide for Efficient Biogas Production

Xiujin Li,*,† Laiqing Li,† Mingxia Zheng,‡ Guozhi Fu,† and Jam Saifullah Lar†

†Department of Environmental Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P.R. China,and ‡Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, P.R. China

Received April 30, 2009. Revised Manuscript Received August 4, 2009

Anaerobic co-digestionof cattlemanurewithNaOH-treated corn stover for biogasproductionwas investigated.Four cattlemanure to corn stover ratios (CM/CS) of 1:1, 1:2, 1:3, and1:4, and three feeding concentrations (FC)of 50, 65, and 80 gL-1were used. The results showed that the co-digestionwithCM/CS ratio of 1:3 obtained thehighest total biogas production of 20.5 L, methane yield of 194 mL CH4 g

-1 VS, and TS and VS reductions of45.0% and 53.0% at the FC of 65 g L-1. Therefore, the FC of 65 g L-1 and CM/CS ratio of 1:3 was selected asthe optimal one. Compared to the single digestion, 4.9-7.4% more biogas productions were obtained at thesame FC of 65 g L-1 due to the synergistic effect. The synergistic effect is mainly attributed to more balancednutrients and increased buffering capacity. It indicated that co-digestion of cattle manure with NaOH-treatedcorn stover could be one of the options for efficient biogas production and waste treatment.

1. Introduction

Anaerobic digestion technology can be applied to convertvarious plant biomass, crop residues, and animal manure intomethane-rich biogas, a carbon-neutral source of domesticrenewable energy.1 The technology has been given greatattention recently for its important role in clean energyproduction and pollution reduction. So far, over 30 millionhousehold-scale small digesters and 3000 large-scale digestershave been built in China. Chinese government has set the goalof building 60 million household-scale digesters in rural areasby 2020.2 Currently, most of digesters use animal manure asfeedstock to produce biogas in China. However, manure haslimited availability. The shortage of feedstock has becomemajor bottleneck for achieving the goals; therefore, feedstockalternatives need to be developed.

On the other hand, China is one of the largest agriculturalcountries in the world. Approximately, 0.6 billion tons ofvarious crop residues are generated annually.3 Although thereare a few methods available for crop residues reutilization,over 50%remainsunused.2Corn is one of the important cropsthat is widely planted in the northern part of China. It is quitecommon to see wide fires from open-field burning of cornstover across the corn planting areas during harvest season.4

Serious environmental and safety problems are caused due tothe open-field burning of corn stover. On the other hand,Corn stover is one kind of organic material; it has potential tobe used as alternative feedstock to produce biogas. If so, itwould bring two benefits: (1) to replace partial animalmanureand solve the feedstock shortage problem; (2) to provide one

alternative to reuse corn stover and mitigate the pollutionassociated. However, the biodegradability of corn stoverneeds to be significantly improved before corn stover can beefficiently used for anaerobic digestion because corn stovercontains a high percentage of lignocelluloses, which cannot beeffectively digested by anaerobic bacteria and lead to lowbiogas yield and long digestion time. Pretreatment proved tobe one simple but effective method for improving the biode-gradability of crop residues. Various pretreatment methodshave been investigated by researchers.5-7 Our previous re-search showed that NaOH pretreatment could significantlyimprove biodegradability and enhance biogas production ofcorn stover.2 Our idea was to co-digest animal manure withthe NaOH-pretreated corn stover. This could develop newfeedstock without affecting biogas production efficiency.

Co-digestion of various organic wastes for energy produc-tion has attracted increasing interests recently.8 The benefits ofco-digestion includes improved biogas yield, economic advan-tages derived from the sharingof equipment, easier handlingofmixed wastes, and synergistic effect.9 According to Mata-Alvarez et al., digestion of more than one kind of substratein the same digester could establish positive synergism in thedigester; the added nutrients would support microbial growth.Co-digestion of animal manure with various agro-industrialresidues has been reported previously,10,11 with particular

*To whom correspondence should be addressed. E-mail: xjli@mail.buct.edu.cn. Telephone: þ86-10-6443-2281.(1) Lehtomaki, A.; Huttunen, S.; Rintala, J. A. Resour., Conserv.

Recycl. 2007, 51, 591–609.(2) Pang, Y.; Liu, Y.; Li, X.; Wang, K.; Yuan, H. Energy Fuels 2008,

22 (4), 2761–2766.(3) Wang, J.; Dai, L.; Tian, Y.; Qin, S. Trans. Chin. Soc. Agric. Eng.

2007, 23, 276–282.(4) Bian, Y. S. Treatment of Agro-wastes in Eco-agricultural System;

Chinese Press of Chemical Industry Beijing: Beijing, 2000; pp 135-139.

(5) Pavlostathis, S. G.; Gossett, J. M. Biotechnol. Bioeng. 1985, 27,334–344.

(6) Chen, H.; Liu, L.; Yang, Z; Li, Z. Biomass Bioenergy 2005, 28,411–417.

(7) Curreli,N.; Fadda,M.B;Rescigno,A.;Rinaldi,A.C.; Soddu,G..;Sollai, F.; Vaccargiu, S.; Sanjust, E.; Rinaldi, A. Process Biochem. 1997,32, 665–670.

(8) Li, R.; Chen, S.; Li, X.; Lar, J. S.; He, Y.Energy Fuels 2009, 23 (4),2225–2228.

(9) Mata-Alvarez, J.; Mac�e, S.; Llabr�es, P. Bioresour. Technol. 2000,74 (1), 3–6.

(10) Callaghan, F. J.; Wase, D. A. J.; Thayanity, K.; Forster, C. F.Biomass Bioenergy 2002, 22, 71–77.

(11) Kaparaju, P.; Rintala, J.Resour., Conserv. Recycl. 2005, 43, 175–188.

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Energy Fuels 2009, 23, 4635–4639 : DOI:10.1021/ef900384p Li et al.

interest being shown in the co-digestion of animal manureswith straws.1,12-15 However, the straws that they used wereraw ones without any pretreatment. This would lead to lowbiogas yield, limiting their application in large-scale.

The objective of this study was to investigate the perfor-mance and synergistic effect of co-digestion of cattle manurewithNaOH-pretreated corn stover and determine the optimalCM/CS ratio and feeding concentration for efficient bioe-nergy production.

2. Materials and Methods

2.1. Materials. The cattle manure and corn stover used in thisstudywere collected fromShunyi County of Beijing City, China.The cattle manure was stored in a freezer and kept at thetemperature of-20 �C for later use. The corn stover was groundinto 5-10 mm particles by a hammer mill (FE130, Staida Co.,Tianjing, China) after being air-dried. The ground corn stoverwas pretreated with NaOH. The 2% NaOH, based on drymatter of corn stover, was first dissolved in tap water to makeNaOH solution, and then the solution was added into theground corn stover andmixed completely. Themoisture contentof the corn stover was adjusted to 88%. The corn stover, withNaOH addition, was then placed in the laboratory at ambienttemperature for 3 days. The pretreatment procedures usedfollowed our previous study.16

2.2. Experimental Design. The NaOH-pretreated corn stoverwas anaerobically digested in the laboratory by batch tests. Thetotal volume of the digester was 2 L each, with an effectivevolume of 1.0 L. The feeding concentration (FC) of 50, 65, and80 g L-1 and the CM/CS ratios of 1:0, 1:1, 1:2, 1:3, 1:4, and 0:1were used. FC was defined as the dry weight of corn stover fedper liter effective volume of digester (g TS L-1). Each digesterwas seeded with the activated sludge taken from a mesophilicanaerobic digester in Beilangzhong swine farm in Beijing,China. The digester has been stably operated for biogas produc-tion for several years. The seeding sludge contained 4.92% totalsolid (TS), 3.09% volatile solids (VS), and 47.5 g L-1 mixedliquor suspended solids (MLSSs). Each digester was seeded tomaintain the activated sludge MLSS in the digester at 15 g L-1,which was based on the research result from Zhang andZhang.17 The prepared digesters were then placed in shakers(TaicangDHZ-DA, China) operated at mesophilic temperature(35 ( 1 �C) and 120 rpm shaking speed.

2.3. Analytical Methods. Biogas volume was monitored everyday by the water displacement method, and the correspondingcumulative biogas volume was calculated. The measured

volume was then converted to a volume of gas at standardtemperature and pressure (STP) using the ideal gas law, and thisvolume was then used to calculate the mass of CH4 based on themethane content. The methane content was analyzed every dayby a gas chromatograph (GC) (SP-2100, BeiFenRuiLi Co.,Beijing) equipped with a molecular sieve (TDX-01) packed2 m � 3 mm stainless-steel column and a thermal conductivitydetector (TCD). The temperatures of oven, injector port, andTCD were 120, 150, and 150 �C, respectively. Hydrogen wasused as the carrier gas at a flow rate of 30mLmin-1. A standardgas (by BeiFenRuiLi Co., Beijing), consisting of 5.0% N2,60.1% CH4, and 34.9% CO2, was used for the calibration ofthe system. TS, VS, MLSS, pH, and alkalinity were measuredaccording to the APHA standard methods.18 The contents oflignin, cellulose, and hemicellulose were determined accordingto the procedures proposed by Van Soest et al.19 The totalcarbon (TC) and total nitrogen (TN)were determined by the TCanalyzer (Skalar Primacsslc, The Netherlands) and the totalKjeldahl nitrogen analyzer (Model KDN-2C, Shanghai).

3. Results and Discussion

3.1. FeedstockCharacteristics.The characteristics of cattlemanure and corn stover are shown in Table 1. Compared tothe untreated one, the lignin content of NaOH-treated cornstover was decreased from 8.4% to 7.5%, which was bene-ficial for the release of cellulose and hemicellulose andincreasing the accessibility of the microorganism. The cellu-lose and hemicellulose content of corn stover was about61.3%, which was 35.3% higher than the cattle manure. Thecellulose and hemicellulose were the main carbon source foranaerobic microorganism. The C/N ratio of cattle manurewas 6, which was too low to meet the nutrients balancerequirement of anaerobic microorganism, while the cornstover had a higher C/N ratio (59). The proper C/N ratiofor anaerobic digestion is 25-35.17 The co-digestion of cattlemanure with corn stover could provide a more appropriateC/N ratio and more balanced nutrients. In addition, thealkalinity of cattle manure was 4.2 g CaCO3 L-1; it couldincrease the buffering capacity of the digesters and preventthe acidification occurrence during digestion.

3.2. Co-Digestion Performances. Four co-digestion CM/CS ratios of 1:1, 1:2, 1:3, and 1:4 were used, and each CM/SM ratio was tested at three feeding concentrations of 50, 65,and 80 g L-1. The daily biogas production for each co-digestion test is shown in Figure 1. Similar trends of dailybiogas production were observed for all co-digestion tests.The biogas production started after seeding, kept increasing

Table 1. Characteristics of Cattle Manure and Corn Stover Used in the Experimentsa

characteristic cattle manure raw corn stover NaOH-treated corn stover

total solid (TS), wt % 17.1 ( 0.6 92.6 ( 1.7 90.4 ( 1.4volated solid (VS), % dry basis 84.3 ( 1.2 89.7 ( 1.3 87.6 ( 1.1ash, % dry basis 15.7 ( 1.3 10.3 ( 1.3 12.4 ( 1.2C, % dry basis 28 ( 2.0 35.2 ( 2.4 33.5 ( 2.2N, % dry basis 4.8 ( 0.5 0.6 ( 0.2 0.7 ( 0.2C/N 6 59 49alkalinity (g CaCO3 L

-1) 4.2 ( 0.4Cellulose, % dry basis 28.1 ( 1.7 37.5 ( 0.9 36.2 ( 1.6hemicellulose, % dry basis 17.2 ( 0.6 30 ( 2.8 25.1 ( 1.6lignin, % dry basis 7.3 ( 0.1 8.4 ( 0.7 7.5 ( 0.1

aValues are means ( SD (n = 3).

(12) Hills, D. J. Trans. Am. Soc. Agric. Eng. 1980, 23, 1500–1504.(13) Hashimoto, A. G. Biotechnol. Bioeng. 1983, 25, 185–200.(14) Fischer, J. R.; Iannotti, E. L.; Fulhage, C. D. Trans. Am. Soc.

Agric. Eng. 1983, 26, 546–548.(15) Somayaji, D.; Khanna, S. World J. Microbiol. Biotechnol. 1994,

10, 521–523.(16) Zheng, M.; Li, X.; Li, L.; Yang, X.; He, Y. Bioresour. Technol.

2009, 100, 5140–5145.(17) Zhang, R.; Zhang, Z. Bioresour. Technol. 1999, 68, 235–245.

(18) APHA. Standard Methods for the Examination of Water andWastewater, 20th ed.; American Public Health Association: Washington,DC, 1998.

(19) Van Soest, P. J.; Robertson, J. B.; Lewis, B. A. J. Dairy Sci. 1991,74, 3583–3597.

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Energy Fuels 2009, 23, 4635–4639 : DOI:10.1021/ef900384p Li et al.

until reaching the first peak, and then began to decline.Several peaks appeared as the digestion process proceeded.The main differences were the peak values of daily biogasproduction and lasting time of biogas production. Generallyspeaking, the biogas started generating earlier but shortertime for the lower FC, while generated later but lasted for alonger time for the higher FC. This could be explained by thefact that higher FC means more substrate applied, thustaking a longer time for the anaerobic microorganisms todigest. The co-digestions with CM/CS ratios of 1:3 and 1:4 atthe FC of 80 g L-1 showed apparent acidification pheno-mena at an early stage but recovered two weeks later. Thisshould be attributed to the improved buffering capacity ofco-digestion.

The total biogas productions (TBPs) for different co-digestion tests are shown in Figure 2. For four CM/CS ratiosof 1:1, 1:2, 1:3, and 1:4, the co-digestions with the FC of65 g L-1 obtained TBPs of 19.8-20.5 L, which were34.8-42.3% and 9.0-113.5% higher than the co-digestionswith the FC of 50 and 80 g L-1, respectively. The lower TBPswith the FC of 50 g L-1 should be attributed to less substrateavailable to anaerobic bacteria. However, the lower TBPswith the FC of 80 g L-1 were mainly due to the acidificationthat occurred at the early stage of anaerobic digestion.Among the four ratios, the co-digestion with CM/CS ratioof 1:3 achieved highest TBP of 20.5 L at the FC of 65 g L-1.

To further compare the biodegradability improvement,biogas yield, whichwas defined as biogas production per unitvolatile solids feed (B/VS),was advanced (Figure 3). For four

CM/CS ratios, the co-digestions with the FC of 50 g L-1 and65 g L-1 achieved 306-340 mL g-1VS and 325-356 mL g-1

VS biogas yields, respectively, which were obviously higherthan the FC of 80 g L-1. The co-digestion with a CM/CSratio of 1:3 and FC of 65 g L-1 obtained the highest biogasyield of 356mL g-1 VS. This indicated that this CM/CS ratiomight provide more balanced nutrients and buffering capa-city and thus enhance the anaerobic process and bioenergyproduction.

The energy contained in biogas is determined by bothbiogas volume and methane content. The methane contentsfor all corn stoversweremeasured during thewhole digestionperiod of time. The total methane productions (TMP) foreach feedstock was calculated by summing daily methaneproduction, which was calculated by timing daily biogas

Figure 1. Daily biogas production of co-digestion.

Figure 2. Total biogas production of co-digestion of cattle manurewith corn stover.

Figure 3. Comparisons of biogas yields.

Figure 4. Total methane production of co-digestion of cattle man-ure with corn stover.

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Energy Fuels 2009, 23, 4635–4639 : DOI:10.1021/ef900384p Li et al.

productionwith correspondingmethane content. The resultsare presented in Figure 4. It can be seen that the TMP of thereactors fed with 65 g L-1 was significantly higher than theothers. At the FC of 65 g L-1, the TMPs were 9.6-11.1 L forthe four reactors with the manure to corn stover ratio of 1:1,1:2, 1:3, and 1:4, respectively. The methane yield was definedas methane production per unit VS feed (CH4/VS). It wasobserved that the co-digestion with the FC of 65 g L-1 andCM/CS ratio of 1:3 achieved the highest methane yield of194 mL CH4 g

-1 VS (Figure 5). The result conformed to theTBP result as mentioned above. Therefore, the FC of65 g L-1 and a CM/CS ratio of 1:3 were selected as theoptimal ones. Hashimoto13 reported that the methane yield

of co-digestion of cow manure and wheat straw at the ratioof 1:3 was 70 mL CH4 g

-1 VS, which was much lower thanthe results in this study. The difference is that the researcheremployed raw stalks without NaOH pretreatment as donein this study; therefore, the synergistic effect scale wasdifferent.

Biogas is generated from the biological conversion ofsubstrate. The amount of drymatter of substrate and organiccompounds, which are represented by TS and VS, will bereduced. There are close relationships between biogas yieldand TS and VS reductions. Therefore, TS and VS reductionswere calculated. The results are shown in Figure 6. It can beseen that with the FCs of 50 and 65 g L-1, the reactors withthemanure to corn stover ratio of 1:3 and 1:4 obtained higherTS andVS reduction as compared to the other two ratios. TSreductions for the reactors with the manure to corn stoverratio of 1:3 and 1:4 at 50 g L-1 were 53.1% and 52.1%,respectively; the corresponding VS reductions were 57.8%and 58.3%, respectively. TS reductions for the reactors withthemanure to corn stover ratio of 1:3 and 1:4 at 65 gL-1 were45.0% and 49.7%, respectively; the corresponding VS re-ductions were 53.0% and 56.2%, respectively. The resultsimply that the biodegradability was improved by addingcorn stover into cattle manure from the manure to cornstover ratio 1:1 to 1:4. The increased TS and VS reductionscould explain why biogas yields of the reactors with themanure to corn stover ratios at 50 and 65 g L-1 were morethan others.

3.3. Synergistic Effect Analysis. One of the importantbenefits of co-digestion is synergistic effect. The co-diges-tions with four CM/CS ratios at the FC of 65 g L-1 achievedhigher biogas yield and methane yield. Thus, they wereselected for synergistic effect analyses. The biogas produc-tions for the co-digestions with CM/CS ratios of 1:1, 1:2, 1:3,and 1:4 were obtained directly from experiments. The biogasproductions for manure fraction and stover fraction in co-digestions were calculated in terms of the biogas productionsof single-digestions withmanure alone (1:0) and stover alone(0:1), respectively. The experimental and calculating resultsare shown in Table 2. It was found that compared to thesingle-digestions at four CM/CS ratios, the co-digestionsachieved 4.9-7.4%more biogas production. The differenceswere significant (p < 0.05). This means that based on thesame amount of manure and stover feedstock, more bio-energy can be generated when the co-digestion process isapplied. The increase in biogas production is considered tobe from the synergetic effect in the co-digestion process.Mata-Alvarez et al.9 stated that digestion of more than onekind of substrate could establish positive synergism in thedigester. Other researchers12-14 also found synergistic effectsin co-digestion of other materials and animal manures.Li et al.8 found that the co-digestion of kitchen waste andcattle manure increased methane yield by 44% as comparedto the single digestion of kitchen waste, and the increase

Figure 5. Comparisons of methane yields.

Figure 6. Comparisons of TS and VS reductions.

Table 2. Synergistic Effect of Co-Digestion of Cattle Manure with Corn Stover at a Feeding Concentration of 65 g L-1

biogas production (mL)

total dry matter (g) CM/CS ratio dry matter of manure (g) dry matter of stover (g) co-dige-stion single manure single stover increase (%)

65 1:0 65 0 14430 ( 135 065 1:1 32.5 32.5 18390 ( 380 7215 ( 65 10275 ( 275 5.1 ( 0.165 1:2 21.7 43.3 19790 ( 750 4817 ( 70 13689 ( 910 6.9 ( 1.665 1:3 16.3 48.8 20450 ( 500 3619 ( 110 15428 ( 130 7.4 ( 1.365 1:4 13 52 20280 ( 330 2886 ( 40 16440 ( 190 4.9 ( 0.565 0:1 0 65 0 20550 ( 225

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Energy Fuels 2009, 23, 4635–4639 : DOI:10.1021/ef900384p Li et al.

could be attributed to the synergistic effect of the increasedalkalinity and buffering capacity and the structural changesof cattle manure fibers in the co-digestion process. Thesynergistic effect is mainly attributed to more balancednutrients and increased buffering capacity in co-digestion.More balanced nutrients in co-digestion would supportmicrobial growth for efficient digestion, while increasedbuffering capacity would help maintain the stability of theanaerobic digestion system.

4. Conclusions

Anaerobic co-digestion of cattle manure with NaOH-trea-ted corn stover could enhance biogas production and obtainstable performances of anaerobic digestion. The co-digestionwith the FC of 65 g L-1 and CM/CS ratio of 1:3 achieved thehighest biogas production, methane yield, and TS and VSreductions, which were 20.5 L and 194 mL CH4 g

-1 VS and

45.0% and 53.0%, respectively. Therefore, the FCof 65 g L-1

and CM/CS ratio of 1:3 were selected as the optimal ones.Compared to single-digestion, 4.9-7.4% more biogas pro-ductions were obtained at the FC of 65 g L-1 due to thesynergistic effect. The synergistic effect is mainly attributed tomore balanced nutrients and increased buffering capacity.The results showed that co-digestion of cattle manure withNaOH-treated corn stover could be one of the options forefficient biogas production and waste treatment. Furtherresearch needs to be conducted on how nutrient balance isimproved in co-digestion and how synergestic effect impactsbiogas production in continuous feeding digesters.

Acknowledgment. The authors are grateful for the financialsupport from National Hi-tech R&D Program of China(2006AA10Z425) and National Key Technologies R&D Pro-gram of China (2008BADC4B13).