Feasibility of biogas production from anaerobic co-digestion of herbal-extraction residues with swine manure

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<ul><li><p>erm</p><p>ua</p><p>ing,330</p><p>Anaerobic co-digestion</p><p>k wacteso94 molidCH4</p><p>respectively. These results suggested that biogas production could be enhanced efciently by the anaer-</p><p>ewablateriastrialagricu2009).</p><p>is little information in the literature about biogas digested fromHER or co-digested from the mixture of HER with other biomasses.</p><p>Anaerobic co-digestion has been dened as the treatment of amixture of at least two different biomasses with the aim of improv-ing process efciencies (lvarez et al., 2010). Nowadays, there is anincreasing interest in using anaerobic co-digestion process treating</p><p>digestion of agro-residues with swine manures would also aid inovercoming ammonia inhibition related to the digestion of pureswine manure. However, it is unclear whether some agro-residues,e.g., HER, might have adverse effects when added into a stabledigester for swine manure.</p><p>The objective of the present work was to investigate the feasi-bility of developing anaerobic co-digestion of HER with swinemanure for efcient biogas production. Batch experiments werecarried out based on a Central Composite Design. The inuenceof the percentage of HER in the substrate and the substrate/inocula</p><p> Corresponding authors.E-mail addresses: envzjh@ncu.edu.cn (J.-H. Zhu), wbzhou@ncu.edu.cn (W.-B.</p><p>Bioresource Technology 102 (2011) 64586463</p><p>Contents lists availab</p><p>T</p><p>elsZhou).extraction residues (HER) is a specic kind of agricultural wastedue to the major importance of the Chinese herbal medicine indus-try in China (Wang et al., 2010). With the rapid development of theChinese herbal medicine industry, about 1.5 million tons of solidwastes, according to statistics, were produced annually after theextraction of medical active components from natural plants(Cheng and Liu, 2010). Because of its abundance in cellulose, hemi-celluloses, lignin as well as proteins, HER could be employed as arenewable energy source. Its environmentally friendly exploitationbecomes an important topic due to the increase in environmentalpollution and the shortage of energy resource. In addition, there</p><p>by co-digestion with other agro-residues (Chae et al., 2008;Lansing et al., 2010; Riao et al., 2011). Moreover, the anaerobicdigestion of agro-residues is often associated with poor bufferingcapacity during anaerobic treatment and its inherent high C/N ra-tio. Anaerobic co-digestion of themwith swine manure could over-come these problems by maintaining a stable pH within themethanogenesis range due to the inherent high buffering capacityof swine manure (Banks and Humphreys, 1998). Additionally,swine manure could present high ammonia content and a widevariety of nutrients needed by the methanogens during the anaer-obic process (Cheng, 2009). From another point of view, the co-Biogas</p><p>1. Introduction</p><p>Biomass is an abundant and rencould be derived from all organic mand natural activities, including induwastes, forestry wastes as well aset al., 2003; Wang and Keshwani,0960-8524/$ - see front matter 2011 Elsevier Ltd. Adoi:10.1016/j.biortech.2011.03.093obic co-digestion of herbal-extraction residues with swine manure. 2011 Elsevier Ltd. All rights reserved.</p><p>e energy source whichls produced by humanwastes, municipal solidltural wastes (BerndesAmong them, herbal-</p><p>industrial and agricultural organic wastes and swine manure forbiogas production (Kaparaju and Rintala, 2005; Riao et al.,2011; Wu et al., 2010). The advantages of anaerobic co-digestionprocess often lie in balancing the carbon/nitrogen (C/N) ratio inthe mixture, macro and micronutrients, pH, inhibitors/toxic com-pounds as well as dry matter (Hartmann et al., 2003). Thus, themanure specic methane production potential could be improvedKeywords:Herbal-extraction residuesSwine manure</p><p>pended solids. Furthermore, specic methane yields were 162, 180 and 220 mL CH4 g1 volatile solidsadded d1 for the reactors co-digesting mixtures with 10%, 25% and 50% herbal-extraction residues,Feasibility of biogas production from anaof herbal-extraction residues with swine</p><p>Yan Li a,b, Xi-Luan Yan a, Jie-Ping Fan a, Jian-Hang ZhaCentre for Low-Carbon Biotechnology, School of Environmental and Chemical Engineerb State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang</p><p>a r t i c l e i n f o</p><p>Article history:Received 16 February 2011Received in revised form 27 March 2011Accepted 29 March 2011Available online 2 April 2011</p><p>a b s t r a c t</p><p>The objective of this worco-digestion of herbal-extrwere carried out under mspecic biogas yield was 2idues and 3.50 g volatile smanure alone was 207 mL</p><p>Bioresource</p><p>journal homepage: www.ll rights reserved.obic co-digestionanure</p><p>,, Wen-Bin Zhou a,Nanchang University, Nanchang 330031, China047, China</p><p>as to examine the feasibility of biogas production from the anaerobicion residues with swine manure. Batch and semi-continuous experimentsphilic anaerobic conditions. Batch experiments revealed that the highestL CH4 g1 volatile solids added, obtained at 50% of herbal-extraction res-s g1 mixed liquor suspended solids. Specic methane yield from swineg1 volatile solid added d1 at 3.50 g volatile solids g1 mixed liquor sus-</p><p>le at ScienceDirect</p><p>echnology</p><p>evier .com/locate /bior tech</p></li><li><p>ratio were evaluated in terms of methane yield. Finally, the effectof the feed component ratio of HER to swine manure on processperformance was investigated in a semi-continuously fed stirredtank reactor (CSTR) under mesophilic anaerobic conditions.</p><p>2. Methods</p><p>2.1. Origin of HER, swine manure and inocula</p><p>The HER investigated in this study was obtained from Jiangz-hong herbal medicine group (Jiangxi, China). Swine manure wasobtained from a pig farm (Jiangxi Evergreen Biotechnology Co.,Ltd.) located in Fengcheng (Jiangxi, China). The anaerobic sludgeused as inocula was collected from the anaerobic biogas digesterin Jiangxi Evergreen Biotechnology Co., Ltd., the mixed liquid sus-pended solids (MLSS) and pH value of which were 21 1.3 g L1</p><p>and 7.19 0.6, respectively. The substrates and inocula were indi-vidually homogenized and subsequently stored at 4 C for furtheruse. The characterization of each waste employed was shown inTable 1. All samples were collected in triplicate, and the averageddata of the measurements were presented.</p><p>2.2. Batch experiments</p><p>Batch experiments were carried out at 35 2 C for 30 daysbased on a Central Composite Design, which is a second order fac-torial design used when the number of runs for a full factorial de-sign is too large to be practical (Box andWilson, 1951). This type offactorial design usually consists of a 2k factorial nucleus, ve repli-</p><p>terms of grams of volatile solid (VS) (g VS)/g of MLSS (g MLSS) ra-tio, and the percentage of HER in the substrate (% HER), measuredin terms of VS of HER in relation to the organic carbon of the feed.The selected range for factor one (SC) was 1.15.9 g VS g1 MLSS.The selected range for factor two (% HER) was 0100%. Factorial de-sign levels were codied from 1 to +1. The central point was rep-licated ve times in order to estimate experimental error. Axialpoints ensure design rotatability and their distance to the centralpoint (a) was calculated according to Eq. (1).</p><p>a 2k=4 1The selected response for analysis was the methane yield</p><p>(ml CH4 ml g1ml VS added), calculated from the methane contentin biogas and the volume of biogas produced from per unit of VSadded. The variables, Xi, were coded as xi according to Eq. (2), suchthat X0 corresponded to the central value:</p><p>xi Xi Xi =DXi; where i 1;2;3; . . . ; k; 2where xi is the dimensionless coded value of an independent vari-able, Xi is the actual value of an independent variable for the ith test,Xi is the actual value of an independent variable at the centre pointand DXi is the step change (Chong et al., 2009; Riao et al., 2011).All the evaluated levels were combined in 13 different treatments(T1T13). Codied and real values for both factors are presentedin Table 2.</p><p>For predicting the optimal point, a second order polynomialfunction (linear regression) was performed Eq. (3):</p><p>Y b0 b1X1 b2X2 b11X21 b22X22 b12X1X2; 3</p><p>erba</p><p>SM</p><p>81.2VS (%) 32.2 1.6 23.5</p><p>1 5.6414.47.7870.8</p><p>exp</p><p>es</p><p>g1</p><p>T7 1 1 1.80</p><p>Y. Li et al. / Bioresource Technology 102 (2011) 64586463 6459T8 0 0 3.50T9 0 0 3.50T10 1 1 1.80T11 0 0 3.50T12 1.4142 0 1.10T13 0 0 3.50TKN (g kg ) 4.50 0.17C/N ratio* 36.6pH value 5.69 0.14Moisture content (%) 58.30 1.20</p><p>* C/N ratio was dened as the ratio of TOC to TKN.</p><p>Table 2Codied, real values and responses for swine manure co-digestion with HER in batch</p><p>Treatmentsa Codied values Real valu</p><p>SC (g VS g1 MLSS) %HER SC (g VS</p><p>T1 0 1.4142 3.50T2 0 0 3.50T3 1 1 5.20T4 1 1 5.20T5 0 1.4142 3.50T6 1.4142 0 5.90cates of the central point and 2k axial points, where k is the num-ber of factors evaluated. More specically, in the present study thetwo factors selected were the solid concentration (SC), measured in</p><p>Table 1Composition of the substrates in batch experiments and semi-continuous digestion: h</p><p>Parameters Batch experiments</p><p>HER</p><p>TOC (g kg1) 164.8 5.2a Data are means of three replicates, except T2, T8, T9, T11 and T13 were performedb The unit is ml CH4 g1 VS added.where Y represents the predicted response, b0, b1, b2, b11, b22, andb12, are the regression coefcients. X1 and X2 are the evaluated fac-tors (SC and %HER). The coefcient of determination (R2) was calcu-</p><p>l-extraction residue (HER) and swine manure.</p><p>Semi-continuous digestion</p><p>HER SM</p><p>9 3.3 156.8 7.2 71.24 4.2 1.2 31.9 1.4 20.7 + 1.7 0.22 4.65 0.23 4.86 0.19</p><p>33.7 14.7 0.21 5.90 0.25 7.59 0.191 1.90 59.20 2.10 72.90 1.70</p><p>eriments.</p><p>Real responseb Predicted responseb</p><p>MLSS) %HER</p><p>0.00 207 9 19850.00 294 29414.64 216 5 22985.36 254 11 260</p><p>100.00 243 10 24450.00 279 9 28385.36 275 6 27050.00 301 29450.00 297 29414.46 235 12 23750.00 283 29450.00 282 10 27050.00 298 294once, respectively.</p></li><li><p>lated to achieve the proportion of data variability that is explainedby the model, thus the quality of t to the model. The p-values ofthe parameter estimation were used to validate the model and p-values less than 0.05 indicate the signicant model terms. Multipleregression analysis for the data sets collected was performed usingDesign-Expert 7.1.6 Trial software.</p><p>The anaerobic assays were conducted in 29 bottles (500 mL)with an inocula volume of 150 mL and a total liquid volume of</p><p>bottles were closed with a septum and the headspace ushed with</p><p>which was monitored daily. The results from the analysis of each</p><p>(1992). Samples from the beginning and the end of the experimentwere analyzed. All measurements were conducted in triplicate andthe averaged data were presented.</p><p>The volume of biogas produced was measured using water dis-placement method. The fraction of CH4 was periodically analyzedby a gas chromatograph (SP6890) provided by Lunan RuihongCompany (Shandong, China) equipped with a thermal conductivitydetector (Ruihong, Shandong, China) and a 2 m stainless steel col-</p><p>erim</p><p>1</p><p>381</p><p>6460 Y. Li et al. / Bioresource Technology 102 (2011) 64586463mixture at steady state were used for evaluating the effect of co-digestion on biogas production efciency.</p><p>2.4. Analyses</p><p>Volatile solid (VS), total organic carbon (TOC), total Kjeldahlnitrogen (TKN), and pH (Sartorius basic pH meter PB-10, Germany)were performed in accordance with APHA Standard Methods</p><p>Table 3Characteristics of mixed feedstocks at varied different mixture ratio used in CSTR exp</p><p>Parameters HER (%)</p><p>0</p><p>Total organic carbon (g L1) 30.20 2.10VS (%) 8.9 0.4Total nitrogen (g L1) 2.10 0.14pure N2 to remove the oxygen. The volume of biogas produced wasmeasured using water displacement method.</p><p>2.3. Semi-continuous digestion of different mixtures</p><p>Semi-continuous co-digestion of HER with swine manure wascarried out in two identical continuously stirred tank reactors(CSTRs) with a total volume of 10 L and a working volume of 7 L,namely R1 and R2. A water bath was used to maintain the temper-ature of the digesters at 35 2 C. Digesters were mounted sepa-rately on a mechanical stirrer, stirring continuously at 100 rpm.The outlets provided on the top of each digester were used forfeeding inuent, withdrawing efuent and for collecting biogas.Biogas was daily measured by displacement of water.</p><p>Feeding VS was maintained constant during the whole experi-ment resulting in a VS loading rate of about 2.90 g VS L1 d1 underhydraulic retention time (HRT) of 30 days. Varied feeding VS ratiosof HER and swine manure were evaluated. After inoculating the di-gester with 7 L of digested anaerobic sludge, R1 was used to co-di-gest HER with swine manure in a feeding VS ratio of 75% swinemanure and 25% HER, whereas R2 was performed with swine man-ure alone. After 30 days, R1 was fed with swine manure and HER,in a feeding VS ratio of 50% swine manure and 50% HER, whereasthe feed in R2 was made up 90% swine manure and 10% HER.The digesters were fed once a day every weekday. Prior to eachfeeding, a volume equal to the feeding volume was removed tomaintain a constant digester volume. The characteristics of sub-strates are shown in Table 3.</p><p>The semi-continuous digestion experiments on R1 and R2 wereperformed two batches, respectively. And the composition of inu-ent and efuent were determined twice a week except the pH375 mL, respectively. All treatments were performed in three rep-licates, except Treatment 2 (T2), T8, T9, T11 and T13 was per-formed in one bottle, respectively. Two blanks containing 150 mLof inocula and 225 mL of distilled water were also run to determinethe endogenous biogas production of the anaerobic sludge. TheC/N ratio 14.38 1pH value 7.48 0.18 7umn (Ruihong, Shandong, China) packed with 5 molecular sieves(its pore size is about 5 ). High purity nitrogen gas (99.99%) wasused as the carrier gas with a ow rate of 30 mL min1. The tem-peratures of the injection port, oven and detector were 120, 50and 150 C, respectively. Gas analyses were carried out two tothree times a week during the rst 2 weeks of the experiments. A250 ll pressure-locked gas tight syringe (SP-6800A) (Ruihong,Shandong, China) was used for the gas sampling. All measurementswere conducted in triplicate and the averaged data werepresented.</p><p>3. Results and discussion</p><p>3.1. Chemical characteristics of swine manure and HER</p><p>There were signicant differences in the composition of the twobiomass wastes (Table 1). HER had higher C/N ratios in the range of33.736.6, as compared to swine manure, which presented C/N ra-tios in the range of 14.414.7. HER, as shown for pH values, wassignicantly acidic, whereas swine manure had a pH value ofaround 7.6. The characterization of the two wastes indicated thatco-digestion of HER with swine manure could be a good solutionto balance C/N ratio, overcoming the problems of digesting bothsubstrates separately.</p><p>3.2. Batch experiments</p><p>The experimental design data, real responses and predicted re-sponses were presented in Table 2. Regression analyses wereshown in Table 4 and resulted in the following second order poly-nomial Eq. (4):</p><p>YCH4 294:20 4:47 SC 16:11 %HER 8:...</p></li></ul>