Mechanisms of microwave irradiation pretreatment for enhancing anaerobic digestion of cattail by rumen microorganisms
Post on 04-Sep-2016
Embed Size (px)
Available online 27 December 2011
strate loading of 17.0 g volatile solids/L, microwave intensity of 500 W, and irradiation time of 14.0 min.
. 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
isms showed a higher degradation efciency 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
Corresponding author. Fax: +86 551 3601592.
Applied Energy 93 (2012) 229236
Contents lists available at
lseE-mail address: email@example.com (H.-Q. Yu).renewable and sustainable energy system because of its huge pro-duction in the world . 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 landll leachate , and remediation of aqui-fers containing trichloroethylene . Re-utilization of the biomassproduced will realize the green circulation from contaminants toresources and improve the application of cattail in phytoremedica-tion .
Lignocellulosic biomass has demonstrated the advantages inreducing greenhouse gas emission and increasing energy security
ness in improving the enzymatic hydrolysis of switchgrass and in enhancing the anaerobic digestion of sewage sludge .
Anaerobic digestion is an attractive treatmentmethodbecause ofits low processing costs and environmental benets, but the lowdigestibility of ber 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 andwheat straw [15,16]. Comparedwith conventional anaer-obic digestion system, the system inoculated rumen microorgan-Anaerobic digestionCattailTypha latifoliaMicrowave irradiationPretreatmentRumen microorganisms
Lignocellulosic biomass will play0306-2619/$ - see front matter 2011 Elsevier Ltd. Adoi:10.1016/j.apenergy.2011.12.015The 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 conrmed 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.
ortant role in the future
hydrolysis or microbial digestion . Various physical and chemi-cal pretreatments have been attempted to improve their conversion. Microwave irradiation pretreatment has shown the effective-Received in revised form 4 December 2011Accepted 5 December 2011
to 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-Mechanisms of microwave irradiation prdigestion of cattail by rumen microorgan
Zhen-Hu Hu a,b, Zhen-Bo Yue a, Han-Qing Yu a,, ShaoaDepartment of Chemistry, University of Science & Technology of China, Hefei 230026,b School of Civil Engineering, Hefei University of Technology, Hefei 230092, ChinacDepartment 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 2011
a b s t r a c t
In this study, the mechanistreatment were explored. B
journal homepage: www.ell rights reserved.reatment for enhancing anaerobicms
ang Liu a, Hideki Harada c, Yu-You Li c
of anaerobic digestibility of cattail improved by microwave irradiation pre-BoxBehnken design and response surface methodology (RSM) were used
vier .com/locate /apenergy
for such an improvement of anaerobic digestibility by microwaveirradiation pretreatment.
2. Materials and methods
Fresh cattail with 6070% moisture was collected from a pondnear the campus (Hefei, China). Then, the cattail was sun-dried,
byHuandYu . Each trialwas replicated three timesand theaver-
Ash (% TS) 8.8 0.3Neutral detergent ber (% TS) 64.1 6.2
230 Z.-H. Hu et al. / Applied EnerAcid detergent ber (% TS) 41.5 5.3Hemicellulose (% TS) 22.6 2.5Cellulose (% TS) 31.0 2.8aged 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.
TS (%) 90.2 1.3VS (% TS) 91.2 2.3ments, 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 heatingpretreatmentwas used as the controlwithwater bathheating underthe above optimized pretreatment temperature and time.
2.3. Microorganisms, media and anaerobic digestion
Rumen microorganisms, obtained from a stulated 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;CaCl22H2O 0.03; MgCl26H2O 0.08; NH4Cl 0.18; and 1 mL micro-mineral solution, which had the following composition (in g/L):ZnSO47H2O 0.1; MnCl24H2O 0.03; H3BO3 0.3; CoCl26H2O 0.2;CuCl22H2O 0.01; NiCl26H2O 0.02; NaMoO42H2O 0.03; FeCl24H2O1.5.
Anaerobic digestionwas carried out at substrate loading 10 g VS/L and 40 1 C in an air-shaking incubator at 150 rpm as describedground 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 amaximumelectrical 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 timewas the realwork-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 conrmation experi-Lignin (% 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
RSMandBoxBehnkendesignwereused for experimentaldesignand statistical model development in order to optimize the micro-wave irradiation pretreatment conditions with aminimum numberof experiments . 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 Xi Xi
where xi is the coded value of the ith test variable; Xi is the real valueof the ith test variable, Xi 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 tted using a predictivepolynomial quadratic regression equation, in order to correlatethe response variables with the independent variables. The generalequation form is:
Y A0 Xk
where yi are input variables, real values converted fromcoded values,which inuence the response variableY;A0 is the offset term;Ai is theith linear coefcient; Aii is the quadratic coefcient and Aij is the ijthinteraction coefcient. 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. , in which various locations were scanned for eachsample and only the representative images were presented in thispaper. Neutral detergent ber, acid detergent ber, cellulose, hemi-cellulose, lignin and ash contents were measured as described byHu and Yu . TS and VS were analyzed according to the StandardMethods , 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 :
Crystallinity index AcAc Aa 3
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 . 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:
gy 93 (2012) 229236Accessibility W1 W2W2
EnerSolubilization was calculated with the following equation:
Solubilization Wa WbWa
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 bymicroorganisms.Based on the mass balance of the digested products , 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 for
Table 2Design matrixes and measured and predicted values of the experiment.
Run Coded values Real valu
x1 x2 x3 X1
1 1 1 1 6002 1 1 1 6003 1 1 1 6004 1 1 1 6005 1 1 1 2006 1 1 1 2007 1 1 1 2008 1 1 1 2009 1.682 0 0 700
10 1.682 0 0 10011 0 1.682 0 40012 0 1.682 0 40013 0 0 1.682 40014 0 0 1.682 40015 0 0 0 40016 0 0 0 40017 0 0 0 40018 0 0 0 40019 0 0 0 40020 0 0 0 400
Note: X1 = microwave power (W), X2 = cattail concentration (g/L), X3 = irradiation tim
Z.-H. Hu et al. / Appliedanaerobic digestion of lignocellulosic materials is their hydrolysis,the concentrations of sugars and proteins in the liquid could beignored . 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 simplication, 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 simplied to equal the sumof 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 coefcient 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 PM tVS 6where 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 mwaterg CpJ=gK DTK 103=mVS 0:4 7where mwater is the quality of water (g), Cp is the specic heat, DT isthe temperature variation, and 0.4 is the heating coefcient. In thisstudy, the initial temperature of water is 293 K, and the nal is373 K.
The energy efciency was evaluated with the ratio of energyconsumption (Ec) to energy production from the producedmethane(Ep), which was based on the assumption that all VFAs produced
Product formed (mg COD/g VS of cattail)
X2 X3 Experimental Predicted
50 16 320.8 13.8 343.350 6 340.5 11.2 335.220 16 499.1 17.5 502.120 6 432.4 12.6 432.450 6 422.8 14.6 435.550 16 452.9 14.2 467.720 16 398.4 12.9 422.520 6 331.6 16.3 328.835 11 322.6 17.5 340.435 11 376.8 14.5 356.060 11 459.5 18.5 461.410 11 506.7 17.1 504.835 20 469.0 12.5 454.935 2 347.6 13.1 363.135 11 407.8 11.9 411.235 11 413.9 21.5 411.235 11 400.6 19.8 411.235 11 397.9 14.6 411.235 11 424.7 14.5 411.235 11 388.1 11.7 411.2
gy 93 (2012) 229236 231were completely converted methane without any loss.