Applied Energy 93 (2012) 229–236
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Applied Energy
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Mechanisms of microwave irradiation pretreatment for enhancing anaerobicdigestion of cattail by rumen microorganisms
Zhen-Hu Hu a,b, Zhen-Bo Yue a, Han-Qing Yu a,⇑, Shao-Yang Liu a, Hideki Harada c, Yu-You Li c
a Department of Chemistry, University of Science & Technology of China, Hefei 230026, Chinab School of Civil Engineering, Hefei University of Technology, Hefei 230092, Chinac Department of Civil Engineering, Tohoku University, Sendai 980-8579, Japan
a r t i c l e i n f o
Article history:Received 12 September 2011Received in revised form 4 December 2011Accepted 5 December 2011Available online 27 December 2011
Keywords:Anaerobic digestionCattailTypha latifoliaMicrowave irradiationPretreatmentRumen microorganisms
0306-2619/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.apenergy.2011.12.015
⇑ Corresponding author. Fax: +86 551 3601592.E-mail address: [email protected] (H.-Q. Yu).
a b s t r a c t
In this study, the mechanisms of anaerobic digestibility of cattail improved by microwave irradiation pre-treatment were explored. Both Box–Behnken design and response surface methodology (RSM) were usedto optimize the microwave irradiation pretreatment with regard to chemical oxygen demand equivalentof the products formed in the anaerobic digestion. The optimum products obtained was as follows: sub-strate loading of 17.0 g volatile solids/L, microwave intensity of 500 W, and irradiation time of 14.0 min.The product formation rate increased by 32% and product yield by 19% after microwave irradiation pre-treatment at 100 �C, as compared with the conventional water-heating pretreatment under the sameconditions. The X-ray diffraction (XRD) analysis showed that the crystallinity index of cattail was reducedby 9.4% with the microwave irradiation, compared with the conventional heating pretreatment. Theatomic force microscope analysis further confirmed that the recalcitrant structure of wax and lignin cov-ering on the cattail surface was disrupted by microwave irradiation. These results demonstrate that thebreakdown of the crystalline and physical structure of cattail caused by microwave irradiation pretreat-ment improved its anaerobic digestibility.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Lignocellulosic biomass will play an important role in the futurerenewable and sustainable energy system because of its huge pro-duction in the world [1]. Cattail (Typha latifolia), a typical lignocel-lulosic aquatic plant with a high yield, has been widely used forconstructing wetlands to remove nitrate, ammonia and heavy met-als [2,3], treatment of landfill leachate [4], and remediation of aqui-fers containing trichloroethylene [5]. Re-utilization of the biomassproduced will realize the green circulation from contaminants toresources and improve the application of cattail in phytoremedica-tion [6].
Lignocellulosic biomass has demonstrated the advantages inreducing greenhouse gas emission and increasing energy security[7]. Recovering energy from renewable biomass to partly replacefossil fuel has attracted great interests in recent years [8,9]. A varietyof chemical and biological processes have been proposed andexplored to produce bioenergy from lignocellulosic biomass[10,11]. In comparison with chemical processes, biological pro-cesses are more environmentally friendly and less energy intensive.However, because of the recalcitrant structure of lignocellulosic bio-mass, cost-effective pretreatment is necessary prior to enzymatic
ll rights reserved.
hydrolysis or microbial digestion [12]. Various physical and chemi-cal pretreatments have been attempted to improve their conversion[9]. Microwave irradiation pretreatment has shown the effective-ness in improving the enzymatic hydrolysis of switchgrass [13]and in enhancing the anaerobic digestion of sewage sludge [14].
Anaerobic digestion is an attractive treatment method because ofits low processing costs and environmental benefits, but the lowdigestibility of fiber has limited its application for the treatment oflignocellulosic wastes. Rumen microorganisms with high cellulo-lytic activities have shown the advantage for the anaerobic digestionof lignocellulosic wastes. Our previous studies showed that therumen microorganisms enhanced the in vitro digestibility of cornstover and wheat straw [15,16]. Compared with conventional anaer-obic digestion system, the system inoculated rumen microorgan-isms showed a higher degradation efficiency and conversion ratefor both lignocellulosic and high-cell-soluble wastes. Therefore,anaerobic conversion of aquatic plants by rumen microorganismsmight be a promising way.
Microwave irradiation pretreatment has shown its effectivenessin improving the anaerobic digestion of lignocellulosic wastes.However, the mechanisms of anaerobic digestion of lignocelluloseto be improved by microwave irradiation pretreatment are notclear yet. The main objective of this work was to optimize themicrowave irradiation pretreatment conditions with response sur-face methodology (RSM) analysis, and to elucidate the mechanisms
230 Z.-H. Hu et al. / Applied Energy 93 (2012) 229–236
for such an improvement of anaerobic digestibility by microwaveirradiation pretreatment.
2. Materials and methods
2.1. Substrate
Fresh cattail with 60–70% moisture was collected from a pondnear the campus (Hefei, China). Then, the cattail was sun-dried,ground and passed through 40-mesh sieve, and stored in desicca-tor at room temperature for use. The composition of cattail is listedin Table 1.
2.2. Microwave pretreatment
Microwave irradiation pretreatments were carried out in a2.45 GHz commercial microwave oven (Gelanshi Co., China). In thisoven, microwaves irradiation intensity can be adjusted from 0% to100% with a maximum electrical power of 800 W in eight equivalentlevels. Serum bottles with 250-mL capacity were used as thepretreatment reactor and the working volume was 50 mL. The pre-treatments were realized by keeping the reaction temperature at100 �C with a temperature sensor, which controlled the switch ofmicrowave oven on or off. The pretreatment time was the real work-ing time of the microwave oven. For the optimization, the pretreat-ments were carried out at various irradiation intensities, substrateloadings and irradiation times. In the tests of confirmation experi-ments, kinetic parameter determination, and atomic force micro-scope (AFM) imaging and X-ray diffraction (XRD) analysis, thepretreatments were carried out under the optimized conditionsobtained from the above experiments. The conventional heatingpretreatment was used as the control with water bath heating underthe above optimized pretreatment temperature and time.
2.3. Microorganisms, media and anaerobic digestion
Rumen microorganisms, obtained from a fistulated goat, wereused as seed microorganisms and the inoculums’ concentrationwas 1.0 g/L. The media used in the experiments contained thefollowing ingredients (in g/L): NaHCO3 4; KH2PO4 0.5; K2HPO4 1.5;CaCl2�2H2O 0.03; MgCl2�6H2O 0.08; NH4Cl 0.18; and 1 mL micro-mineral solution, which had the following composition (in g/L):ZnSO4�7H2O 0.1; MnCl2�4H2O 0.03; H3BO3 0.3; CoCl2�6H2O 0.2;CuCl2�2H2O 0.01; NiCl2�6H2O 0.02; NaMoO4�2H2O 0.03; FeCl2�4H2O1.5.
Anaerobic digestion was carried out at substrate loading 10 g VS/L and 40 ± 1 �C in an air-shaking incubator at 150 rpm as describedby Hu and Yu [16]. Each trial was replicated three times and the aver-aged results were presented in this paper. After 120-h digestion, thevolume and composition of biogas were measured. The liquid sam-ples were collected and centrifuged at 12,000g for 10 min. The
Table 1Compositions of cattail.
Item Percentagea
TS (%) 90.2 ± 1.3VS (% TS) 91.2 ± 2.3Ash (% TS) 8.8 ± 0.3Neutral detergent fiber (% TS) 64.1 ± 6.2Acid detergent fiber (% TS) 41.5 ± 5.3Hemicellulose (% TS) 22.6 ± 2.5Cellulose (% TS) 31.0 ± 2.8Lignin (% TS) 10.5 ± 1.4
a Standard deviations were calculated with three measurements.
supernatant was used for the analysis of volatile fatty acids (VFAs)and soluble total organic carbon (STOC).
2.4. Experimental design
RSM and Box–Behnken design were used for experimental designand statistical model development in order to optimize the micro-wave irradiation pretreatment conditions with a minimum numberof experiments [17]. The independent variables considered weresubstrate loading, microwave irradiation intensity and irradiationtime. Table 2 lists the experimental design matrixes. The variableswere coded according to the following equation:
xi ¼Xi � X�i
DXið1Þ
where xi is the coded value of the ith test variable; Xi is the real valueof the ith test variable, X�i is the real value of Xi at the center point ofthe investigated area, and DXi is the real step size.
The response variables (CODpro) were fitted using a predictivepolynomial quadratic regression equation, in order to correlatethe response variables with the independent variables. The generalequation form is:
Y ¼ A0 þXk
i¼1
Aiyi þXk
i¼1
Aiiy2i þ
Xk
i
Xk
j
Aijyiyj ð2Þ
where yi are input variables, real values converted from coded values,which influence the response variable Y; A0 is the offset term; Ai is theith linear coefficient; Aii is the quadratic coefficient and Aij is the ijthinteraction coefficient. Regression equation and three-dimensionalresponse surfaces were performed using the software MATLAB 6.1(MathWorks Inc., USA).
2.5. Analysis and calculation
The concentration of STOC was measured using a TOC analyzer(TOC-VCPN, Shimadzu Co., Japan), whereas VFAs were determinedwith a gas chromatography (GC-6890N, Agilent Inc., USA). X-raydiffraction was carried out using an 18-kW rotating anode X-raydiffractometer (MAP18AHF, MAC Sci. Co., Japan). AFM imaginganalysis was carried out with a NanoScope IIIa Multimode scan-ning probe microscope (Digital Instruments, Inc. USA) as describedby Hu et al. [15], in which various locations were scanned for eachsample and only the representative images were presented in thispaper. Neutral detergent fiber, acid detergent fiber, cellulose, hemi-cellulose, lignin and ash contents were measured as described byHu and Yu [16]. TS and VS were analyzed according to the StandardMethods [18], whereas biogas volume was measured using waterdisplacement method. The biogas composition was determinedusing another gas chromatograph (SP-6800, Lulan Co., China).
Crystallinity index (CI) was calculated using the following equa-tion [19]:
Crystallinity index ¼ Ac
Ac þ Aað3Þ
where Ac represents the crystalline portion corresponding to theupper area, Aa is the amorphous portion corresponding to the lowerarea as shown in Fig 4.
Accessibility was measured according to the method describedby Goto and Yokoe [19]. After pretreatment, samples were centri-fuged at 3000 g for 15 min and the residues were weighed (W1)and dried at 105 �C to a constant weight (W2). Accessibility wascalculated using the following equation:
Accessibility ¼W1 �W2
W2� 100% ð4Þ
Table 2Design matrixes and measured and predicted values of the experiment.
Run Coded values Real values Product formed (mg COD/g VS of cattail)
x1 x2 x3 X1 X2 X3 Experimental Predicted
1 1 1 1 600 50 16 320.8 ± 13.8 343.32 1 1 �1 600 50 6 340.5 ± 11.2 335.23 1 �1 1 600 20 16 499.1 ± 17.5 502.14 1 �1 �1 600 20 6 432.4 ± 12.6 432.45 �1 1 �1 200 50 6 422.8 ± 14.6 435.56 �1 1 1 200 50 16 452.9 ± 14.2 467.77 �1 �1 1 200 20 16 398.4 ± 12.9 422.58 �1 �1 �1 200 20 6 331.6 ± 16.3 328.89 1.682 0 0 700 35 11 322.6 ± 17.5 340.4
10 �1.682 0 0 100 35 11 376.8 ± 14.5 356.011 0 1.682 0 400 60 11 459.5 ± 18.5 461.412 0 �1.682 0 400 10 11 506.7 ± 17.1 504.813 0 0 1.682 400 35 20 469.0 ± 12.5 454.914 0 0 �1.682 400 35 2 347.6 ± 13.1 363.115 0 0 0 400 35 11 407.8 ± 11.9 411.216 0 0 0 400 35 11 413.9 ± 21.5 411.217 0 0 0 400 35 11 400.6 ± 19.8 411.218 0 0 0 400 35 11 397.9 ± 14.6 411.219 0 0 0 400 35 11 424.7 ± 14.5 411.220 0 0 0 400 35 11 388.1 ± 11.7 411.2
Note: X1 = microwave power (W), X2 = cattail concentration (g/L), X3 = irradiation time (min).
Z.-H. Hu et al. / Applied Energy 93 (2012) 229–236 231
Solubilization was calculated with the following equation:
Solubilization ¼Wa �Wb
Wa� 100% ð5Þ
where Wa is the dry weight of the sample or components beforepretreatment, Wb is the dry weight of the sample or componentsafter pretreatment.
In anaerobic digestion, the pretreated cattail was decomposedinto two parts: one was sugars and proteins that were further di-gested into VFAs by rumen microorganisms; another was the prod-ucts such as lignin that cannot be easily utilized by microorganisms.Based on the mass balance of the digested products [20], the prod-ucts in the reactor were composed of four parts: undigested sugarsand proteins, VFAs, biogas, and the mass for growth and mainte-nance of rumen microorganisms. Because the limited step foranaerobic digestion of lignocellulosic materials is their hydrolysis,the concentrations of sugars and proteins in the liquid could beignored [20]. While all the fermentations were controlled underthe same conditions, the differences for the mass used for growthand maintain of rumen microorganisms in all the tests could alsobe ignored. Thus, for the simplification, the products formed couldbe expressed by the sum of VFAs and biogas produced in anaerobicdigestion.
For the calculation, both VFAs and biogas were expressed aschemical oxygen demand equivalent of products (CODpro). Sinceno hydrogen was produced, the value of CODpro produced per gramof volatile solid of cattail was simplified to equal the sum of theoret-ical COD of both VFAs and methane produced. One mol of methane,acetic acid, propionic acid, butyric acid, valeric acid consumes 2, 2,3.5, 5.0 and 6.5 mol of oxygen, respectively, when they are com-pletely oxidized into carbon dioxide and water. The molecularweight of methane, acetic acid, propionic acid, butyric acid, valericacid and oxygen is 16, 60, 74, 88, 102 and 32, respectively. Therefore,the conversion coefficient for per gram of methane, acetic acid, pro-pionic acid, butyric acid, valeric acid to gram of COD is 4, 1.067,1.514, 1.818, and 2.039, respectively. The CODpro was calculated onthe basis of measured VFAs and biogas.
The energy consumption (Ec) for microwave pretreatment wasevaluated with the following equation:
Ec ¼PM � t
VSð6Þ
where PM is the microwave irradiation power (W), t is the irradia-tion time, and VS is the volatile solid weight of the pretreated cattailsample.
The energy consumption (Es) for the conventional heating pre-treatment was evaluated with the following equation:
Ec ¼ ½mwaterðgÞ � CpðJ=gKÞ � DTðKÞ � 10�3�=ðmVS � 0:4Þ ð7Þ
where mwater is the quality of water (g), Cp is the specific heat, DT isthe temperature variation, and 0.4 is the heating coefficient. In thisstudy, the initial temperature of water is 293 K, and the final is373 K.
The energy efficiency was evaluated with the ratio of energyconsumption (Ec) to energy production from the produced methane(Ep), which was based on the assumption that all VFAs producedwere completely converted methane without any loss.
3. Results
3.1. Optimization of microwave irradiation pretreatment
The effect of substrate loading, irradiation intensity and irradia-tion time on the formation of CODpro during the anaerobic digestionof the microwave-pretreated cattail by rumen microorganisms wasinvestigated. The corresponding CODpro values, as shown in Table 2,were further subjected to regression analysis, generating the follow-ing quadratic regression equation:
CODpro ¼ 78:217þ 1:195x1 þ 0:136x2 þ 15:268x3 � 0:0007x21
þ 0:115x22 � 0:027x2
3 � 0:017x1x2 � 0:006x1x3
� 0:205x2x3 ð8Þ
where x1, x2 and x3 are the real values of substrate loading, micro-wave irradiation intensity, and irradiation time.
Prob(p) values were used as a tool to check the significance ofeach of the coefficients which, in turn, are necessary to understandthe pattern of the mutual interactions between the test variables.The smaller the magnitude of the Prob(p), the more significant isthe correlation corresponding coefficient. Table 3 shows that themain effects of substrate loading and irradiation time; the second-order effect of substrate loading, microwave irradiation intensityand irradiation time; and the two-level interactions of substrate
Table 3Estimated regression coefficients for the model.
Term Regression analysis
Coefficient SE-coe t-Value p
Intercept 78.217 65.238 1.199 0.258A1 1.195 0.150 7.945 0.000**
A2 0.136 1.936 0.070 0.945A3 15.268 5.348 2.855 0.017**
A11 �0.0007 0.0001 �5.072 0.000**
A22 0.115 0.021 5.519 0.000**
A33 �0.0267 0.165 �0.162 0.875A12 �0.017 0.002 �8.368 0.000**
A13 �0.006 0.006 �1.004 0.339A23 �0.205 0.083 �2.477 0.033**
** Highly significant.
Table 4ANOVA analysis of the calculated model.
Degree offreedom
Sum ofsquare
Meansquare
Fratio
p
Model 9 49553.1 5505.90 19.76 0.000**
Linear 3 21161.2 7053.7 22.84 0.000**
Square 3 18267.2 6089.1 19.72 0.000**
Interaction 3 23827.1 7942.4 25.75 0.000**
ResidualError
10 3087.8 308.8
Lack of fit 5 2258.7 451.7 2.72 0.148Pure error 5 829 165.8R2 = 94.7%
** Highly significant.
100300
500700
10.0 22.5
35.0 47.5
60.0
100
200
300
400
500
Microwave intensity (W)Substrate (g VS/L)
CO
Dpr
o (m
g /g
VS
)
(a)
100300
500700
2
8
14
20100
200
300
400
500
Microwave intensity (W)Irradiation time (min)
CO
Dpr
o (m
g /g
VS
)
(b)
10.0 22.5
35.0 47.5
60.0
2
8
14
20200
300
400
500
Substrate (g VS/L)
Irradiation time (min)
CO
Dpr
o (m
g/g
VS
)
(c)
Fig. 1. Response surface of COD : (a) effect of cattail concentration and microwave
232 Z.-H. Hu et al. / Applied Energy 93 (2012) 229–236
loading and microwave irradiation intensity, and microwaveirradiation intensity and irradiation time were the significant modelterms.
Standard analysis of variance (ANOVA) and model coefficientsare given in Table 4. The quadratic regression model demonstratesthat the model was highly significant as the Fisher F-test 19.76 witha very low probability value [(Pmodel > F) < 0.05], suggesting that theregression model was highly significant [17]. The high correlationcoefficient value (R2) of 0.951 indicates a good agreement betweenthe measured and predicted values. Fig. 1 shows the responsesurface profiles of the calculated model for the CODpro, obtained bykeeping one variable constant at its optimum level and varying theother two variables within the experimental ranges. The optimumconditions of microwave irradiation intensity, substrate loadingand irradiation time obtained from Eq. (1) were 500 W, 17.0 g VS/Land 14 min, respectively. The corresponding optimum responsevalue for CODpro was estimated as 509 mg COD/g VS of cattail.
pro
power; (b) effect of irradiation time and microwave power; and (c) effect of cattailconcentration and irradiation time.
3.2. Confirmation experimental results
To confirm the validity of the statistical experimental strategiesand to gain a better understanding to the microwave pretreatmenton the enhancement of anaerobic digestion of cattail by rumenmicroorganisms, four additional confirmation experiments wererespectively carried out with microwave irradiation and conven-tional heating pretreatments. Table 5 shows the solubilization ofcomponents of cattail as the results of microwave irradiation andconventional heating pretreatments at the optimum pretreatmentconditions. Microwave irradiation pretreatment enhanced the sol-ubilization of hemicelluloses in comparison with conventionalheating, but had no obvious impact on lignin and cellulose.
The confirmation tests with the inoculated rumen microorgan-isms were carried out after cattail pretreatments. The results of
CODpro and TOC as shown in Fig. 2 reveal that the CODpro measuredwas close to the optimum CODpro estimated. The anaerobic digestionof the microwave-pretreated cattail has a steep CODpro formationcurve as compared with the conventional heating pretreated cattail.For example, 88.4% of CODpro was produced in the initial 48 h diges-tion for the microwave-pretreated cattail, whereas the correspond-ing value was only 40.0% for the conventional heating pretreatedcattail. After 120-h anaerobic digestion, 466 mg COD/g VS for themicrowave-pretreated cattail was produced, which was 1.19 timesmore than the conventional heating pretreated cattail (Fig. 2). Theformation of STOC was similar to which of the CODpro. The improve-ment of the anaerobic digestibility might be attributed to the decon-struction of lignocellulose caused by microwave pretreatment.
Table 5Solubilization of hemicellulose, cellulose and lignin in pretreatment.
Item Microwave irradiationpretreatment (%)a
Conventional heatingpretreatment (%)a
Volatile solid 24.5 ± 1.5 15.4 ± 1.2Hemicellulose 23.5 ± 1.7 7.4 ± 1.4Cellulose 0 0Lignin 2 ± 0.2 1.8 ± 0.2
a Standard deviations were calculated with three measurements.
0 30 60 90 120
0 30 60 90 120
Solu
ble
TO
C (
mg/
g V
S)
Incubation time (h)
(b)
(a)
CO
Dpr
o (m
g C
OD
/g V
S)
0
180
360
540
0
80
160
240
Fig. 2. Concentration of: (a) CODpro; and (b) STOC during the anaerobic digestion ofcattail pretreated with (s) microwave irradiation and (d) conventional heating.
Fig. 3. Kinetic parameters for the anaerobic digestion of the cattail pretreated with(s) microwave irradiation and (d) conventional heating.
Table 6Determination of the kinetic parameters for the anaerobic digestion of cattailpretreated with microwave irradiation and conventional heating.
Kinetic parameter Microwaveirradiation
Conventionalheating
Ks (g VS/L) 150.4 152.2Vmax (mg COD h�1 L�1) 357.1 270.3Vmax/Ks
(mg COD h�1 (g VS)�1)2.37 1.78
Z.-H. Hu et al. / Applied Energy 93 (2012) 229–236 233
3.3. Kinetic analysis
Monod equation has been used to study the degradation ofcomplex organic materials. In this work, Monod equation wasapplied to describe the kinetics of anaerobic digestion of the pre-treated cattail inoculated rumen microorganisms:
V ¼ VmaxCs
Ks þ Csð9Þ
where V is the degradation rate of substrate, Vmax, Cs and Ks are max-imum degradation rate, substrate loading and half-saturation con-stant, respectively.
The Ks and Vmax were determined by the measurement of CODpro
formed during the anaerobic digestion with various concentrationsof cattail pretreated by microwave irradiation and conventionalheating. Fig. 3 shows the Lineweaver–Burk plot, which could beused to calculate the kinetic parameters of Ks, Vmax, and Vmax/Ks,as listed in Table 6. It was found that the Ks values between thetwo types of pretreatments were at a similar level. The higherVmax/Ks value of the microwave-pretreated cattail was solely attrib-uted to the greater Vmax value, which was 32% more than that of theconventional heating pretreated cattail.
3.4. X-ray diffraction
Fig. 4 shows the powder X-ray diffraction patterns of the cattailspretreated by microwave irradiation and conventional heating.There was a decrease in d value (where d is the interplanar spacing)as 2h equal to 16 or 23, and no d value was detected as 2h equals to 24or 30 for the microwave-pretreated cattail. The crystallinity index of0.714 ± 0.021 of the conventional heating pretreated cattail was
higher than that of 0.647 ± 0.017 of the microwave-pretreated cat-tail, implying that microwave irradiation pretreatment acceleratedthe disruption of the crystal structure of cattail.
3.5. AFM images
The changes of surface morphology as the results of the pre-treatments of conventional heating (Fig. 5a and b) and microwaveirradiation (Fig. 5c and d) were analyzed by AFM. Fig. 5a shows theAFM image of external surface of the cattail pretreated with heat-ing and Fig. 5b clearly shows the thin film covering on the surfaceat a higher resolution. Fig. 5c shows the AFM images of samplespretreated with microwave irradiation. Microfiber rows and somefractions were observed on the external surface of the microwave-pretreated cattail. The higher resolution image shows that the frac-tions were composed of granules (Fig. 5d).
3.6. Accessibility measurement
Cellulose accessibility reflects the level of cellulose hydroxylbeing contacted by reagents, which is related with the structureof the cellulose, micropore diameters and micropore distributions.Fig. 6 shows the increase of accessibility with increasing pretreat-ment time. An increase of 18.2% of the accessibility was achievedafter 14 min pretreatment with microwave irradiation, while thecorresponding value with conventional heating pretreatment wasonly 3.1%, meaning that there were more surfaces exposed aftermicrowave pretreatment.
3.7. Energy balance analysis
In this study, the microwave pretreatment could provide betterresults than the conventional heating pretreatment in terms ofimproving the anaerobic digestibility of cattail. It is important toevaluate the economical feasibility of these two pretreatmentmethods. Table 7 presents the results of energy efficiencies for thepretreatment under different pretreatment conditions. For the con-ventional heating treatment under the optimized conditions, the Ec
value is 41.8 kJ/g VS, whereas the ratio of Ec/Ep varies from 4.9 and
10 20 30 40 10 20 30 40
(a)
Ac
Aa
Ac d=2.9607
d=3.6391d=4.0846
d=5.9015
Counts
2θ
(b)
Aa
AcAc
d=3.9414
d=5.3746
2θ
0
700
1400
2100
2800
0
800
1600
2400
3200
Fig. 4. X-ray diffraction patterns of: (a) cattail pretreated with conventional heating; and (b) cattail pretreated with microwave irradiation. Ac and Aa represent crystalline andamorphous portions in the X-ray diffractogram respectively.
Fig. 5. AFM images of the cattail surfaces: (a) and (b) pretreated with conventional heating; (c) and (d) pretreated with microwave irradiation.
234 Z.-H. Hu et al. / Applied Energy 93 (2012) 229–236
130.4, indicating that the input energy is higher than the producedenergy in this study.
4. Discussion
In this work, RSM was used to optimize microwave pretreat-ment conditions. The substrate loading, irradiation intensity andirradiation time had individually significant impact on the CODpro
formation. The highest CODpro was predicted at substrate loadingof 17 g VS/L, microwave irradiation intensity of 500 W, and irradi-ation time of 14 min. The good fit of the observed values with thepredicted ones indicates the adequacy of the quadratic models.
Fig. 2 illustrates the comparison of the two types of pretreat-ments at the optimum conditions with regard to the products pro-duced during anaerobic digestion. The formation rate and productyield with microwave irradiation pretreatment increased by 32%
0 5 10 15 200
5
10
15
20In
crea
sing
rate
of a
cces
sbilit
y (%
)
Pretreatment time (min)
Fig. 6. Accessibility profiles of the cattail pretreated with: (s) microwave irradi-ation and (d) conventional heating.
Z.-H. Hu et al. / Applied Energy 93 (2012) 229–236 235
and 19% respectively as compared with the conventional heatingpretreatment. This suggests that microwave irradiation pretreat-ment was superior to conventional heating pretreatment inimproving the anaerobic digestion of cattail. This could be becausemicrowave irradiation heating is rapid and the material is heatedsimultaneously, whereas conventional heating is slow and the heattransfer is from the outer to inner [21].
The pretreatment of cellulosic biomass at high temperatureresulted in more solubilization of hemicellulose [22]. In our study,microwave irradiation pretreatment resulted in more solubilizationof hemicellulose (Table 5) than the conventional heating pretreat-ment at the same temperature. This can be attributed to the fact thatmicrowave heating is a selective heating of polar molecules, whichgenerates rapid heating to polar substances and no heating to apolarsubstances [21]. In this case, water is a strong polar substance,whereas cellulosic materials are low polar substances. This leadsto the intense vibration of water molecule, the inhomogeneity heat-ing and the temperature in certain zones within the sample beinghigher than the temperature around zones [21], which resulted inthe deconstruction of lignocellulose and the solubilization of hemi-cellulose. This was further certified by the X-ray diffraction and AFManalysis.
The X-ray diffraction patterns of the cattail pretreated withmicrowave irradiation and conventional heating show similar spec-tra (Fig. 4). The main peak reflected the highly organized ‘crystal-line’ cellulose, while the broader peak reflected the less organizedpolysaccharide structure, which was mainly composed of ‘amor-phous’ cellulose, xylans and other non-cellulosic polysaccharides[19]. The decrease by 9.4% in crystallinity index of the micro-wave-pretreated cattail indicates that the highly organized crystal-line cellulose had been partially broken down. Any reduction incrystallinity of cellulose would be associated with the increases ofcell wall fragility and of susceptibility to be attacked by cellulolyticmicroorganisms [23]. Therefore, the increase in anaerobic digest-ibility was attributed partially to the change of the crystalline struc-ture of cattail.
Table 7Energy balance of anaerobic digestion of cattail under different pretrea
Pretreatmenttime (min)
VS treated(g VS)
Microwave power (W) 200 6 2.5200 16 2.5400 11 3.0600 20 0.8500 14 0.9
Conventional heating 14 0.9
The disruption of lignocellulosic structure further confirmed theadvantage of microwave irradiation pretreatment (Fig. 5). The thinfilm covering on the external surface was a wax layer, which pro-tects cell wall against the outside attack of enzyme and microbes[1]. The removal of the thin film would be beneficial to the anaer-obic digestibility. The lignin layer is the aggregates of granuleswith size of 20–100 nm, and under which are microfibrils. Thegranules with sizes of 30–100 nm and the microfibrils, appearedon the external surface in the microwave-pretreated cattail, asshown in Fig. 5d, imply that the lignin layer was broken down bymicrowave irradiation. However, the breakdown was not observedin the samples pretreated by conventional heating. The removal ofthe thin film and the broken lignin layer will improve the contactof fibers with microorganisms and anaerobic digestibility. Theanalysis of the accessibility gave a further support, as the cattailpretreated by microwave irradiation had an approx. 15% moreaccessibility than that of the conventional heating pretreated ones,suggesting that the microwave irradiation pretreatment loosed thecell wall structure of cattail.
In Monod equation, the catalytic efficiency of enzyme was mea-sured by kcat/Km. As kcat = Vmax/[total enzyme], Vmax/Km reflects therelative catalytic efficiencies, if the same amount of enzyme is usedin the two cases and this ratio could also be used to compare sub-strate specificities [17]. In this case, it means that which kind ofsubstrate is easily digested by rumen microorganisms. The highervalue of 2.37 mg COD h�1 (g VS)�1 obtained with the microwave-pretreated samples, as compared with 1.80 mg COD h�1 (g VS)�1
of the conventional heating pretreated samples, clearly shows thatthe former was a better substrate for anaerobic digestion. This re-sult clearly shows that microwave pretreatment was an efficientpretreatment mean to improve the anaerobic digestibility of cattail.
The energy efficiency is an important index for the technologyapplication at large scale [24–26]. The energy balance analysis inTable 7 shows that for both the microwave pretreatment and theconventional heating pretreatment, the input energy at laboratoryscale is higher than the benefit recovered from the production ofmethane by anaerobic digestion, which is consistent with theresults obtained by Jackowiak et al. [27]. In their study, the Ec/Ep
varied from 54 to 103. The study by Tang et al. [28] has shown thatwater content was the most important factor affecting energyefficiency for the pretreatment. Another important factor is thetreatment time. The high solid content and short treatment timecan significantly increase the energy efficiency. This implies thatfor large-sale applications a high solid content and a flash micro-wave device could be used to improve the energy efficiency ofpretreatment.
Recently, lignocellulosic biomass is increasingly regarded as avaluable resource for renewable and green energy generation [29].Anaerobic digestion is a more sustainable way for the treatment ofcellulosic wastes as compared with other disposal methods, suchas landfill and composting [30]. Cellulose and hemicellulose arethe principal biodegradable components of biowaste, but they formrigid complexes with lignin to resist biodegradation, especially un-der anaerobic conditions [31,32]. In this work, microwave pretreat-
tment conditions.
Energy produced(Ep) (kJ/g VS)
Energy consumed(Ec) (kJ/g VS)
Ec/Ep ratio
5.9 28.8 4.96.3 76.8 12.26.4 88.0 13.86.9 900.0 130.47.1 467.0 65.87.1 41.8 5.9
236 Z.-H. Hu et al. / Applied Energy 93 (2012) 229–236
ment was demonstrated as a simple and effective means to improvethe anaerobic conversion of cattail. Such a pretreatment resulted in ahigh solubilization of hemicellulose, an obvious decrease in cellu-lose crystallinity, the breakdown of wax and lignin and the increasesin formation rate and product yield. Thus, microwave irradiationpretreatment could be potentially used for enhancing anaerobicdigestibility of lignocellulosic wastes.
5. Conclusions
The mechanisms of microwave irradiation pretreatment improv-ing the anaerobic digestion of cattail by rumens and the optimalpretreatment conditions are explored in this study. The followingoptimum microwave pretreatment conditions obtained are: sub-strate loading of 17.0 g VS/L, microwave intensity of 500 W, andirradiation time of 14.0 min. The kinetic analysis of anaerobic diges-tion confirms that the microwave-pretreated cattail is more readilydigested by rumen microorganisms than the conventional heatingpretreatment. Compared with the conventional heating pretreat-ment, the crystallinity index of cattail with microwave pretreatmentis reduced by 9.4%. The AFM analysis shows that the structure of waxand lignin covering on the cattail surface are broken down by micro-wave irradiation. The results demonstrate that the breakdown of thecrystalline and physical structure of cattail caused by microwaveirradiation improves its anaerobic digestibility.
Acknowledgments
The authors wish to thank the NSFC (51078122), the NSFC-JSTJoint Project (21021140001), and National 863 Program(2008BADC4B18) for the support of this study.
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