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Journal of Environmental Management 133 (2014) 293e301

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Effect of feed to microbe ratios on anaerobic digestion of Chinesecabbage waste under mesophilic and thermophilic conditions: Biogaspotential and kinetic study

Gopi Krishna Kafle a,*, Sujala Bhattarai b, Sang Hun Kim c,**, Lide Chen a

aDepartment of Biological and Agricultural Engineering, University of Idaho, ID, USAbDepartment of Biological Systems Engineering, Washington State University, Pullman, WA 99164, USAcDepartment of Biosystems Engineering, Kangwon National University, Chuncheon, Kangwon-do, Republic of Korea

a r t i c l e i n f o

Article history:Received 15 June 2012Received in revised form3 December 2013Accepted 9 December 2013Available online 8 January 2014

Keywords:Anaerobic digestionChinese cabbage waste (CCW)Feed to microbe ratio (F/M)Kinetic studyMesophilic and thermophilic temperature

* Corresponding author. Tel.: þ1 208 736 3604; fax** Corresponding author. Tel.: þ82 33 250 6492; fax

E-mail addresses: gopikafle1982@gmail.com, gouidaho.edu (G.K. Kafle), shkim@kangwon.ac.kr (S.H. K

0301-4797/$ e see front matter Published by Elseviehttp://dx.doi.org/10.1016/j.jenvman.2013.12.006

a b s t r a c t

The objective of this study was to investigate the effect of the feed-to-microbe (F/M) ratios on anaerobicdigestion of Chinese cabbage waste (CCW) generated from a kimchi factory. The batch test was con-ducted for 96 days under mesophilic (36.5 �C) (Experiment I) and thermophilic (55 �C) conditions(Experiment II) at F/M ratios of 0.5, 1.0 and 2.0. The first-order kinetic model was evaluated for methaneyield. The biogas yield in terms of volatile solids (VS) added increased from 591 to 677 mL/g VS undermesophilic conditions and 434 to 639 mL/g VS under thermophilic conditions when the F/M ratioincreased from 0.5 to 2.0. Similarly, the volumetric biogas production increased from 1.479 to 6.771 L/Lunder mesophilic conditions and from 1.086 to 6.384 L/L under thermophilic conditions when F/M ratioincreased from 0.5 to 2.0. The VS removal increased from 59.4 to 75.6% under mesophilic conditions andfrom 63.5 to 78.3% under thermophilic conditions when the F/M ratio increased from 0.5 to 2.0. The first-order kinetic constant (k, 1/day) decreased under the mesophilic temperature conditions and increasedunder thermophilic conditions when the F/M ratio increased from 0.5 to 2.0. The difference between theexperimental and predicted methane yield was in the range of 3.4e14.5% under mesophilic conditionsand in the range of 1.1e3.0% under thermophilic conditions. The predicted methane yield derived fromthe first-order kinetic model was in good agreement with the experimental results.

Published by Elsevier Ltd.

1. Introduction

Energy production from biomass (biogas production) provides arenewable alternative to fossil fuels, considering the huge amountof organic residues such as agro-industrial residues and municipalsolid wastes produced around the world. Many of these residuesare still unexploited and contribute to environmental pollution inboth urban and rural areas. The total annual production of Chinesecabbage (CC) is approximately threemillion tons in Korea, and up to30% of the total production is discarded as waste (Choi and Park,2003). Cabbage waste is produced during harvest, transport andat the wholesale markets. A large amount of cabbage waste is alsogenerated from kimchi (fermented cabbage) factories during thetrimming process. Because the moisture content in cabbage wasteis generally more than 95% and this waste decomposes readily,

: þ1 208 736 0843.: þ82 33 255 6406.pikafle@yahoo.com, gopik@im).

r Ltd.

many unpleasant environmental consequences arise when cabbagewaste is abandoned in fields or near factories. Anaerobic digestionof this biodegradable waste will provide a solution for reducingboth this environmental problem and the consumption of fossilfuels. An additional advantage of anaerobic digestion is that, inaddition to the produced biogas, a mineralized effluent that can beutilized as a biofertiliser with high NPK concentrations is obtained(Díaz et al., 2011).

Anaerobic digestion treatment has been practiced in both batchand continuous digesters. Batch digesters are simpler in bothconstruction and operation than continuous digesters. In the batchtest, the selected substrate is incubated in closed vials or flasks at aspecific temperature with a certain amount of methanogenicinoculum. After incubation, the degree of degradation of the sub-strate is evaluated at pre-set time intervals to determine the rateand ultimate extent of biodegradation (Raposo et al., 2009). Thesebatch digesters are applied in large-scale installations and in lab-oratories when assessing the biochemical methane potential(BMP). The F/M ratio is important when operating a large-scalebatch digester and when estimating the BMP of the feed stock

Table 1Characteristics of substrates used.

Parameters Units CCW Inoculum

Mesophilic Thermophilic

TS % 12.8 1.7 2.17VS % 8.0 0.76 1.13VS/TS ratio 0.63 0.45 0.52pH 5.8 8.6 8.2TKN %w.b. 0.445 e e

C/N ratio 10.1 e e

e: Not determined; CCW: Chinese cabbage waste.

Table 2Experimental design for the test.

Particular Temperature (�C) Substrateloading (g VS/L)

F/M ratio No. ofreplications

Experiment I 36.5 2.5 0.5 25.0 1.0 210 2.0 2

Experiment II 55.0 2.5 0.5 25.0 1.0 210 2.0 2

F/M ratio: Feed to microbe ratio (g VS substrate added/g VS inoculums added).g VS/L: Gram volatile solid per liter.

G.K. Kafle et al. / Journal of Environmental Management 133 (2014) 293e301294

(Hashimoto, 1989; Neves et al., 2004). Contradictory results arereported in the literature concerning the influence of the F/M ratioon methane yield coefficients (Maya-Altamira et al., 2008). Previ-ous studies have shown that decreasing the F/M ratio may have anegative effect on the ultimate practical methane potential (Maya-Altamira et al., 2008). However, other investigations have observedno significant influence on the ultimate practical methane potential(Raposo et al., 2006). Each substrate has its optimum F/M ratio,considering the potential amount of volatile fatty acids producedand the capacity to buffer the medium due to the ammoniumproduced by the hydrolysis of proteins (Lesteur et al., 2010). A smallamount of inoculum is preferred because of endogenous biogasproduction, which can bias the results (Lesteur et al., 2010).Moreover, the increase in the F/M ratio can lead to overload due toaccumulation of volatile fatty acids (Neves et al., 2004). The inoc-ulum concentration should always be high compared to the con-centration of the substrate (VS basis), and the F/M should berecognized as one of the major parameters affecting the results ofanaerobic assays (Neves et al., 2004).

The experimental data on the biomethanisation of cabbagewaste are limited. Some of the literature has reported the BMP teston cabbage (Labatut et al., 2011; Gunaseelan, 2004; Cho et al., 1995).Kalia et al. (1992) studied anaerobic digestion of rotten cabbage in alaboratory scale digester. Liu et al. (2009a) performed anaerobicdigestion testing with Chinese cabbage under both mesophilic andthermophilic conditions. Liu et al. (2009b) investigated the effect offeed on inoculum ratios for biogas yields of food and green wastes.Raposo et al. (2009) investigated the influence of inoculum-substrate ratio on the anaerobic digestion of sunflower oil cake.Neves et al. (2004) studied the influence of inoculum activity on thebio-mechanization of kitchen waste using different waste/inoc-ulum ratios. To the best of our knowledge, no previous study hasexamined the effect of the F/M ratio on the anaerobic digestion ofCCW under both mesophilic and thermophilic conditions. Specificstudy is needed to investigate the influence of the F/M ratios onanaerobic digestion of CCW in batch digesters.

The main goal of this study was to determine the effect of F/Mratios (0.5e2.0) on biogas production and organic matter removalfrom CCWundermesophilic and thermophilic conditions. The first-order kinetic model was evaluated for determining kinetic con-stants and predicting the methane production.

2. Materials and methods

2.1. Feed stock and inoculum

The CCW was obtained from a kimchi factory in Korea andstored at �4 �C for 3e4 months before being used. In the kimchifactory, the CCW was screw-pressed, and its juice (green liquid)was separated. The moisture content of CCW thus decreased frommore than 95% to approximately 87%. This CCW with juiceextractedwas used in this study for testing anaerobic digestion. Themesophilic and thermophilic anaerobic digested sludge (inoculum)was obtained from laboratory-scale anaerobic reactors. Swinemanure was used as the substrate in both the mesophilic andthermophilic reactors from which the inoculum was obtained. Thecharacteristics of CCW and the inoculum used for the tests areshown in Table 1.

2.2. Batch digester startup and experimental design

The batch test was divided into two experiments: Experiments Iand II. The experimental design for each experiment is shown inTable 2. Both Experiments I and II were carried out in 2.3 L glassbottles (liquid volume 1.5 L). The F/M ratio was increased from 0.5

to 2.0 by increasing organic loading rate (OLR) from 2.5 to10.0 g VS/L at a constant inoculum (microbe) loading rate of5.0 g VS/L. The F/M ratio was calculated based on the initial VS ofthe substrate and inoculums (Kafle and Kim, 2013a).

After adding the required amounts of inoculum and substrate,each digester was filled with tap water to maintain the designatedvolume. The batch digesters were checked for any leakage andflushed with 100% pure nitrogen for approximately 3 min to ensureanaerobic conditions (Chandra et al., 2012). The mesophilic anaer-obic digesters were maintained at 36.5 �C, and thermophilicanaerobic digesters were maintained at 55 �C in a temperature-controlled incubator. Both Experiment I and Experiment II werecarried out in duplicate, and the results were expressed as a mean.Each digester was mixed manually for 1 min once a day just beforethe gas volume measurement. Assays with inoculums alone werealso used as controls. Biogas and methane produced from in-oculumswere subtracted from the sample assays (Asam et al., 2011;Kafle and Kim, 2013a).

2.3. Biogas volume/mass measurement and composition analysis

The daily biogas production of each digester was determined bythe volume of biogas produced, which was calculated from thevolume and pressure in the headspace of the digester (EI-Mashadand Zhang, 2010). The pressure was measured using a WAL-BMP-Test system pressure gauge (type 3150, Wal, Germany). Dailypressure differences were converted into biogas volume using thefollowing equation (Kafle et al., 2013; Kafle and Kim, 2013a):

VB ¼�Pf � Pi

�$VH$C

R$T(1)

where

VB ¼ Biogas volume (L)Pi ¼ Initial pressure in the reactor head space (mbar)Pf ¼ Final pressure in the reactor headspace after 24 h (mbar)VH ¼ Volume of the headspace (L)C ¼ Molar volume (22.41 L/mol)

G.K. Kafle et al. / Journal of Environmental Management 133 (2014) 293e301 295

R ¼ Universal gas constant (83.14 L mbar/K/mol)T ¼ Temperature (K)

Methane concentration (CH4, %), carbon dioxide concentration(CO2, %) and hydrogen sulphide concentration (H2S, ppm) in thebiogas were analysed by using a gas analyser (BioGas Check e

Geotechnical Instruments (UK), Ltd.) (Kafle and Kim, 2011, 2013b).The CH4 and CO2 were measured by dual wavelength infrared cellwith a reference channel, and H2S was measured using an externalgas pod connected to the gas analyser. The gas analyser was cali-brated using certified gases CH4 (60, 15.01, %) and CO2 (40, 15.01, %).The gas composition (CH4 and CO2, %) was also cross-checked fromtime to time using a GC-2014 gas chromatograph (Shimadzu, Kyoto,Japan) equipped with a thermal conductivity detector. Helium wasused as the carrier gas in the GC. The GC was calibrated usingstandard gases consisting of CH4 (60%) and CO2 (40%) on a volumebasis (v/v). The data obtained from two gas-measuring instrumentsdiffered by <2.0% in gas composition (CH4 and CO2, %) whencompared with the biogas. The measured wet biogas and methanevolumes were adjusted to the volumes at standard temperature(0 �C) and pressure (1 atm) (Kafle and Kim, 2013a). The correctedmethane content (CH4 Corr) in the biogas was calculated usingEquation (2) as proposed in German standard procedure (VDI 4630,2006).

CH4Corr ¼ CCH4� 100

CCH4þ CCO2

(2)

where

CH4 Corr ¼ corrected methane content in the dry gas (% byvolume)CCH4

¼ measured methane content in the gas (% by volume)CCO2

¼ measured carbon dioxide content in the gas (% byvolume).

During batch digestion, the biogas production rates andmethane content change considerably over the digestion time. Themethane content on intermediate days was calculated using linearinterpolation by the INTERP1 function in Matlab software R2011b(7.13.0.564). The weighted average methane content over thedigestion period was calculated (EI-Mashad and Zhang, 2010; Kafleand Kim, 2013a; Kafle et al., 2013). The mass removal in the form ofbiogas at the end of the experiment was calculated using a formulashown in Equation (3). The density of CH4 was taken as 0.000668 g/mL, and the density of CO2 was taken as 0.00184 g/mL (Kafle andKim, 2012a, 2013a).

BR ¼ V0 � rmixm

(3)

where

BR ¼ Mass of biogas removed per gram TS or VS added (g/g VSadded or g/g TSadded)V0 ¼ Volume of biogas produced (ml, at STP)rmix ¼ Mass concentration of CH4 þ CO2 in the biogas (g/mL)m ¼ TS or VS added (g)

2.4. Analytical methods and organic matter removal

Total solids (TS) and volatile solids (VS) were determined in thewell-mixed samples in triplicate according to standard methods(APHA, 1998). pH value was determined using a pHmeter (YK-2001PH, Taiwan). Total Kjeldahl nitrogen (TKN) was analyzed using a

Kjeldahl apparatus (Kjeltec 2100, Foss, Sweden). The total organiccarbon (TOC) was calculated using relation, TOC ¼ VS/1.8 (Haug,1993). The TS and VS removal of feed were calculated using Equa-tion (4) (Kafle et al., 2013).

TS or VS removal of feedð%Þ ¼ ðF þ IÞ � X � I � YF

(4)

where

F ¼ Total TS or VS feed added to reactor (g)I ¼ Total TS or VS inoculum added to reactor (g)X¼Calculated TS or VS removal of mixture of feed and inoculumbased on total initial and final gram TS or VS present in thetesting reactors (%)Y ¼ Calculated TS or VS removal of inoculum in blank reactors(%)

2.5. Kinetic model and statistical indicators

Due to the role of microbes in the anaerobic process, kineticmodels (particularly first-order kinetics) were commonlyapplied to simulate anaerobic biodegradation. Like the phase ofbacterial growth, biogas production rate showed a rising curve,and a decreasing curve indicated by exponential and linearequations (De Gioannis et al., 2009; Kumar et al., 2004).Assuming first-order kinetics for the hydrolysis of particulateorganic matter, the cumulative methane production can bedescribed by Equation (5) (Kafle et al., 2012b, Kafle and Kim,2013a).

BðtÞ ¼ Bo ��1� eð�ktÞ

�(5)

where

B (t) ¼ The cumulative methane yield at digestion time t days(mL/g VS added)Bo ¼ Methane potential of the substrate (mL/g VS added)k¼Methane production rate constant (first order disintegrationrate constant) (1/day)t ¼ Time (in days).

A nonlinear least-square regression analysis was performedusing SPSS program (IBM SPSS statistics 19 (2010)) to determinethe methane production rate constant (k) and the predictedmethane yield. At the same time, the standard error and coeffi-cient of determination or correlation coefficient (R2) were alsoobtained. The predicted methane yield obtained from the SPSSprogram was plotted with the measured methane yield usingMatlab software R2011b (7.13.0.564). Root mean square error(RMSE) was calculated using Equation (6) (Bhattarai et al., 2012;Stone, 1993).

RMSE ¼0@1m

Xmj¼1

djYj

!21A1=2

(6)

where

m ¼ Number of data pairsj ¼ jth valuesY ¼ Measured methane yield (mL/g VS)d ¼ Deviations between experimental and predicted methaneyield

Fig. 1. Cumulative biogas yield (mL/g VS) and biogas production rate (mL/g VS d) from Chinese cabbage waste at different F/M ratios under mesophilic (Experiment I) andthermophilic conditions (Experiment II). Mean value of two replications are plotted.

G.K. Kafle et al. / Journal of Environmental Management 133 (2014) 293e301296

2.6. Least significant difference analysis

The significance of differences in the average biogas yield,methane yields, methane content, pH and TS, VS removal wasdetermined by using single factor ANOVA (Analysis of Variance) inExcel software 2007. If the calculated F value is higher than thetabulated F value, Least Significant Difference (LSD)was calculated tojudge whether two or more averages were significantly different ornot. LSDwas calculated at a¼ 0.05 (LSD0.05) and at a¼ 0.01 (LSD0.01)as follows (Gomez and Gomez, 1984; Little and Hills, 1978):

LSDa ¼ ta

ffiffiffiffiffiffiffiffi2s2

r

s(7)

where

Table 3Gas potential, pH and TS, VS and biogas removal at different F/M ratios.

Parameters Units Experiment I

Temperature �C 36.5F/M ratio 0.5 1.0Biogas potential mL/g VS 591 (16) 520 (28)Methane potential mL/g VS 372 (16) 309 (20)Methane content % 62.8 (1.0) 60.3 (0.8)TS removal % 44.2 (0.2) 44.6 (0.3)VS removal % 59.4 (1.0) 66.0 (1.2)Biogas removed g/g VS added 0.652 (0.011) 0.553 (0.066)

g/g TS added 0.404 (0.007) 0.348 (0.042)Initial pH 8.28 (0.01) 8.22 (0.02)Final pH 7.77 (0.01) 7.77 (0.01)

d: Digestion period (in days).Values are expressed as mean (standard deviation, n ¼ 2).

ta ¼ Tabulated value chosen for the degree of freedom for errorand level of significance (a) desireds2 ¼ Mean square for error (MSE)r ¼ Number of replications on which the means to be separatedare based

3. Results and discussion

3.1. Biogas yield and biogas production rate

The cumulative biogas yield (mL/g VS) and biogas productionrate (mL/g VS d) from CCWat different F/M ratios under mesophilicand thermophilic conditions are shown in Fig. 1. Biogas productionstarted immediately on the first day of digestion in all the digesters.

Experiment II

552.0 0.5 1.0 2.0677 (37) 434 (9) 613 (28) 639 (4)389 (29) 169 (4) 312 (30) 328 (3)57.3 (1.1) 38.9 (0.2) 52.0 (0.3) 51.3 (0.1)48.7 (0.1) 35.3 (1.6) 41.6 (3.7) 44.7 (1.2)75.6 (2.3) 63.5 (2.2) 67.2 (5.9) 78.3 (1.7)0.791 (0.034) 0.601 (0.011) 0.751 (0.016) 0.791 (0.004)0.490 (0.021) 0.372 (0.007) 0.473 (0.010) 0.491 (0.002)8.17 (0.01) 8.05 (0.01) 7.86 (0.01) 7.74 (0.03)7.76 (0.01) 7.80 (0.00) 7.76 (0.01) 7.74 (0.03)

Table 4Data obtained from ANOVA analysis and least significant difference (LSD) calculation at the significance level (a) of 5% and 1%.

Parameter Units Time period LSD F Value MSE p-value

a ¼ 0.05 a ¼ 0.01

Biogas yield mL/g VS 96d 51 72 28.27274 544.041667 0.000422824Methane yield mL/g VS 96d 44 62 30.22 398.45 3.5012E-04Methane content % 96d 1.5 2.2 289.0383 0.50538333 4.61664E-07pH 96d e e 0.86 6.75 0.556229791TS removal % 96d 3.8 5.3 13.63 2.93 0.003168074VS removal % 96d 6.3 9.0 12.62 8.31 0.003886563k value 1/d 60d 0.018 0.025 12.37398 0.00006725 0.00409124

d: Digestion period (in days).

G.K. Kafle et al. / Journal of Environmental Management 133 (2014) 293e301 297

The peak values of daily biogas production rates were calculated tobe 43, 46 and 70 mL/g VS d after two, two and three days ofdigestion for mesophilic digestion and 74, 54 and 65 mL/g VS d onthe first, fourth and third days of digestion for thermophilicdigestion at F/M ratios of 0.5, 1.0 and 2.0, respectively.

The specific biogas yield increased until about days 35, 48 and55, respectively, for the mesophilic digesters operated at F/M ratiosof 0.5, 1.0 and 2.0, and gradually leveled off thereafter (Fig. 1).

Table 5Results of least significant difference (LSD) analysis.

Parameters M-0.5 M-1.0 M-2.0 T-0.5 T-1.0 T-2.0

Biogas yield (mL/gVS added)M-0.5 e SD VSD VSD NSD NSDM-1.0 SD e VSD VSD VSD VSDM-2.0 VSD VSD e VSD SD NSDT-0.5 VSD VSD VSD e VSD VSDT-1.0 NSD VSD SD VSD e NSDT-2.0 NSD VSD NSD VSD NSD e

Methane yield (mL/gVS added)M-0.5 e VSD NSD VSD SD SDM-1.0 VSD e VSD VSD NSD NSDM-2.0 NSD VSD e VSD VSD SDT-0.5 VSD VSD VSD e VSD VSDT-1.0 SD NSD VSD VSD e NSDT-2.0 SD NSD SD VSD NSD e

Methane content (%)M-0.5 e VSD VSD VSD VSD VSDM-1.0 VSD e VSD VSD VSD VSDM-2.0 VSD VSD e VSD VSD VSDT-0.5 VSD VSD VSD e VSD VSDT-1.0 VSD VSD VSD VSD e NSDTS removal (%)M-0.5 e NSD SD VSD NSD NSDM-1.0 NSD e SD VSD NSD NSDM-2.0 SD SD e VSD VSD VSDT-0.5 VSD VSD VSD e VSD VSDT-1.0 NSD NSD VSD VSD e NSDT-2.0 NSD NSD SD VSD NSD e

VS removal (%)M-0.5 e SD VSD NSD SD VSDM-1.0 SD e VSD NSD NSD VSDM-2.0 VSD VSD e VSD SD NSDT-0.5 NSD NSD VSD e NSD VSDT-1.0 SD NSD SD NSD e VSDT-2.0 VSD VSD NSD VSD VSD e

k valueM-0.5 e NSD NSD NSD VSD NSDM-1.0 NSD e NSD NSD VSD SDM-2.0 NSD NSD e NSD VSD SDT-0.5 NSD NSD NSD e VSD SDT-1.0 VSD VSD VSD VSD e SDT-2.0 NSD SD SD SD SD e

NSD: No significant difference, SD: Significant difference, VSD: Very significantdifference.M-Mesophilic conditions (M-0.5 represents, under mesophilic conditions at F/Mratio of 0.5).T-Thermophilic conditions (T-0.5 represents, under thermophilic conditions at F/Mratio of 0.5).

Similarly, biogas yield increased until about days 40, 21 and 28,respectively, for the thermophilic digesters operated at the F/Mratios of 0.5, 1.0 and 2.0 and gradually leveled off thereafter.Approximately 33, 43 and 46 days were required for mesophilicdigesters to produce 90% of the biogas production at F/M ratios of0.5, 1.0 and 2.0, respectively, and approximately 58, 18 and 25 dayswere required for thermophilic digesters, respectively.

The average biogas yields from the digesters operated at F/Mratios of 0.5, 1.0 and 2.0 were calculated to be 591, 520 and 677 mL/g VS, respectively, for mesophilic conditions and 434, 613 and639mL/g VS, respectively, for thermophilic digestions (Table 3). TheLSD values for biogas yield were calculated to be 51 and 72 mL/g VSadded for at significance levels of 5% and 1%, respectively (Table 4).The biogas yield from CCW increased significantly (P < 0.01) underboth mesophilic and thermophilic conditions when the F/M ratioincreased from 0.5 to 2.0 (Table 5). Zhou et al. (2011) observed alinear increase in biogas yield by increasing the F/M ratio from 0.1to 0.6. From 0.7 to 0.9, the biogas yield was almost constant, butwhen the F/M ratio exceeded 0.9, the biogas yield decreasedsignificantly during 20 days of anaerobic digestion of bean curdrefuse-okra.

The volumetric biogas production under mesophilic conditions(BPVM) increased by 360% (from 1.479 to 6.771 L/L) and the volu-metric biogas production under thermophilic conditions (BPVT)increased by 490% (from 1.086 to 6.384 L/L) when OLR increasedfrom 2.5 to 10 g VS/L (F/M ratio 0.5e2.0) at a constant inoculuminput (5 g VS/L). A good correlation was obtained between the OLR(g VS/L) and BPVM or BPVT (L/L) as shown in Equation (8) andEquation (9).

BPVM ¼ 0:724� OLR � 0:6065�R2 ¼ 0:983

�(8)

BPVT ¼ 0:7003� OLR � 0:5735�R2 ¼ 0:998

�(9)

3.2. Biogas composition and methane yield

Themethane content (%) and H2S concentration (ppm) in biogasfrom Experiment I and II are shown in Fig. 2. Under mesophilicconditions, the methane concentration in the biogas increasedregularly for all of the F/M ratios until approximately days 33e35and thereafter remained almost constant. Under thermophilicconditions, the methane concentration in biogas increased regu-larly till day 60 and remained constant thereafter in digesters withan F/M ratio of 0.5, however, at F/M ratios of 1.0 and 2.0, themethane content rose regularly until days 20e22, reached itsmaximum value and remained almost constant thereafter. Theaverage methane content was calculated to be 62.8%, 60.3% and57.3% at F/M ratios of 0.5, 1.0 and 2.0, respectively for mesophilicdigestion and 38.9%, 52.0% and 51.3%, respectively, for thermophilic

Fig. 2. Methane contents (%) and H2S concentration (ppm) in the biogas produced from Chinese cabbage waste at different F/M ratios under mesophilic (Experiment I) andthermophilic conditions (Experiment II). Mean value of two replications are plotted.

G.K. Kafle et al. / Journal of Environmental Management 133 (2014) 293e301298

digestion (Table 3). The LSD value for methane content was calcu-lated to be 1.5% and 2.2% at significance levels of 5% and 1%,respectively (Table 4). The methane content decreased significantly(P < 0.01) when the F/M ratio increased from 0.5 to 2.0 undermesophilic conditions, but under thermophilic conditions, themethane content increased significantly (P < 0.01) (Table 5). Themethane content was significantly higher (P < 0.01) under meso-philic conditions than under thermophilic conditions (Table 5).

The H2S concentration in biogas increased with an increase inthe F/M ratios (0.5e2.0) under both mesophilic and thermophilicconditions (Fig. 2). Higher H2S concentration was recorded inbiogas produced under mesophilic conditions than thermophilicconditions. Under mesophilic conditions, the H2S concentration inthe biogas increased regularly during days 1e8 at an F/M ratio of

Table 6Kinetic constant and predicted methane yield of CCW at different F/M ratio.

Units Mesophilic conditions

F/M ratio 0.5 1.0k 1/day 0.041 (0.000) 0.033 (0.002)Standard error 0.002 (0.000) 0.002 (0.000)R2 0.887 (0.003) 0.824 (0.007)Predicted methane

yield (for 60d)mL/g VS 340.1 (14.4) 264.0 (10.7)

Experimental methaneyield (for 60d)

mL/g VS 352.5 (14.8) 308.9 (19.9)

d: Digestion period (in days).Values are expressed as mean (standard deviation, n ¼ 2).

0.5, and thereafter decreased rapidly. However, at F/M ratios of 1.0and 2.0, the H2S concentration increased until days 3e5, main-tained a high value (>5000 ppm; the maximum limit of the gasanalyser was 5000 ppm) during days 5e20 and then regularlydeclined as shown in Fig. 2. Under thermophilic conditions, the H2Sconcentration increased regularly until days 6, 8 and 13 anddropped rapidly thereafter at F/M ratios of 0.5, 1.0 and 2.0,respectively (Fig. 2). The maximum H2S concentrations measuredunder thermophilic conditions were 1123, 2565 and 4985 ppm at F/M ratios of 0.5, 1.0 and 2.0, respectively (Fig. 2).

Based on themethane contents, the averagemethane yields werecalculated to be 372, 309 and 389mL/g VS at F/M ratios of 0.5,1.0 and2.0, respectively, for mesophilic digestions and 169, 312 and 328 mL/g VS, respectively, for thermophilic digestions (Table 3). The LSD

Thermophilic conditions

2.0 0.5 1.0 2.00.031 (0.004) 0.031 (0.008) 0.075 (0.001) 0.051 (0.000)0.002 (0.000) 0.001 (0.000) 0.004 (0.001) 0.002 (0.000)0.864 (0.035) 0.864 (0.083) 0.927 (0.011) 0.934 (0.009)326.0 (11.3) 139.7 (16.7) 314 (16) 312.2 (3.4)

372.5 (24.7) 144.1 (28) 317.4 (16) 317.4 (0.9)

Fig. 3. Measured and predicted (calculated) methane yield with statistical indicators (a) under mesophilic conditions; (b) under thermophilic conditions.

G.K. Kafle et al. / Journal of Environmental Management 133 (2014) 293e301 299

values for methane yield were calculated to be 44 and 62 mL/g VSadded at significance level of 5% and 1%, respectively (Table 4). LSDanalysis showed no any significant difference in methane yield for F/M ratio from 0.5 to 2.0 under mesophilic conditions, however, underthermophilic conditions, the methane yield increased significantlywhen the F/M ratio increased from 0.5 to 2.0 (Table 5). Methane yieldshowed no significant difference at F/M ratios of 1.0 and 2.0 underthermophilic conditions (Table 5). Labatut et al., 2011 reportedmethane yield of 257 mL/g VS at F/M ratio of 1.0 (VS basis) andGunaseelan (2004) reported methane yield in the range of 291e309 mL/g VS at F/M ratio of 4.0 (volume basis) for cabbage undermesophilic conditions (35 �C). Similar to our results, Raposo et al.(2006) found no significant difference in methane yield coefficientswith a decrease in the inoculum to substrate ratio from 3.0 to 1.0during anaerobic digestion of maize. However, the methane yielddecreased with an increase of the F/M ratio from 0.67 to 4.0 in thestudy conducted by Parawira et al. (2004). Similarly, Gunaseelan(1995) also reported a significant increase in methane yield withan increase in the inoculum to substrate ratio (i.e., decrease in the F/M ratio) during anaerobic digestion of Parathenium at room tem-perature (26 �C). In case of bean curd refuse-okra, Zhou et al. (2011)noticed a linear increase inmethane yield for a substrate to inoculumratio between 0.1 and 0.6, but the methane yield decreased signifi-cantly when the ratios exceeded 1.0.

3.3. Organic matter removal and final pH

The TS and VS removal increased significantly with the increasein F/M ratio from 0.5 to 2.0 under bothmesophilic and thermophilic

conditions (Table 5). The VS removal was significantly higher(P < 0.01) under thermophilic temperatures than at mesophilictemperatures (Table 5). The TS removal increased by approximately2.5% and 27% and VS removal increased by approximately 27% and23% under mesophilic and thermophilic conditions, respectively,when the F/M ratio increased from 0.5 to 2.0 (Table 3).

The final pH values under mesophilic conditions weremeasuredto be 7.77, 7.77 and 7.76 and under thermophilic conditions weremeasured to be 7.80, 7.76 and 7.74 at F/M ratios of 0.5, 1.0 and 2.0,respectively (Table 3). The statistical analysis showed no significantdifference in final pH value between different F/M ratios (0.5, 1.0and 2.0) and digestion temperatures (36.5 and 55.0 �C) at an OLR of2.5e10 g VS/L (Table 4). The final pH values >7.70 in all the di-gesters, linear increase in volumetric biogas productions (L/L) andsignificant increase in specific biogas yield (mL/gVS) with increasein F/M ratio from 0.5 to 2.0 under bothmesophilic and thermophilicconditions showed that the CCW can easily be digested anaerobi-cally without disturbing the digester process for F/M ratios up to 2.0under both mesophilic and thermophilic conditions. To determinethe optimum limit of the F/M ratio, further studies need to beperformed at F/M ratios higher than 2.0.

3.4. Relationship between TS, VS removal, and BR

The calculated BR, TS, and VS removals at the end of Experi-ments I and II are shown in Table 3. The BR increased withincreasing TS removal and VS removal under both mesophilic andthermophilic conditions. The BR showed better correlation with TSremoval and VS removal under thermophilic conditions than under

G.K. Kafle et al. / Journal of Environmental Management 133 (2014) 293e301300

mesophilic conditions (Table 3). Theoretically, the calculated BRshould be higher than the measured VS destruction (Richards et al.,1991). In our study, the calculated BR was found to be higher thanVS destruction. In contrast to our results, Liu et al. (2009b) reportedlower calculated biogas mass removal than measured VS de-structions. A maximum discrepancy of 10% was observed betweenBR and the mass of VS removed at an F/M ratio of 1.0 under mes-ophilic conditions. Similar to our results, Liu et al. (2009b) alsoreported a maximum difference of approximately 23% duringanaerobic digestion of green waste. Hayward and Pavlicick (1990)reported that the excessive loss of VFAs and other volatile com-pounds during the drying process of VS measurement can causeerrors in determining the actual VS destruction.

3.5. Results of the kinetic model

Table 6 shows the first-order kinetic constants (k values) and thepredicted methane yield for CCW at different F/M ratios undermesophilic and thermophilic conditions. Under mesophilic condi-tions, the k value decreased by approximately 20% when the F/Mratio increased from 0.5 to 1.0 and decreased by approximately 6%when the F/M ratio increased further from 1.0 to 2.0 (Table 6).Statistically no significant difference in k value was found when theF/M ratio increased from 0.5 to 2.0 under mesophilic conditions.The k value under thermophilic conditions increased significantly(approximately 142%) with an increase in F/M ratio up to 1.0, butthe k value decreased significantly (approximately 32%) when theF/M ratio was further increased to 2.0 under thermophilic condi-tions (Table 5). Eskicioglu and Ghorbani (2011) reported a signifi-cant decrease (180%) in k value with decrease in the inoculum tosubstrate ratio from 3.67 to 0.46 during batch anaerobic digestionof bioethanol plant whole stillage under mesophilic conditions.Raposo et al. (2009) also observed a decrease in k value with thedecrease of the inoculum to substrate ratio from 3.0 to 0.5 duringanaerobic digestion of sunflower oil cake under mesophilic condi-tions. Similarly, Sánchez et al. (1996) also reported a decrease in kvalue from 1.76 to 0.05 (1/day) when feed loading was increasedfrom 40 to 140 mL during anaerobic digestion of sugar-mill-mudwaste under mesophilic condition (35 �C). The R2 value was inthe range of 0.824e0.887 under mesophilic conditions and in therange of 0.864e0.934 under thermophilic conditions (Table 6). Thedifference between the experimental and predicted methane yieldwas in the range of 3.4e14.5% under mesophilic conditions and inthe range of 1.1e3.0% (Table 6) under thermophilic conditions. Thepredicted methane yield derived from the first-order kinetic modelwas in good agreement with the experimental results. Raposo et al.(2009) reported an error of 10% or less in predicting methaneproduction from sunflower oil cake when using a first-order kineticmodel.

To evaluate the soundness of the model results, the predictedvalues of methane yield were plotted against the measured valuesas shown in Fig. 3. The statistical indicators (RMSE and R2 value) arealso shown in Fig. 3. The RMSE value was in the range of 0.6402e1.9211, and the R2 value was in the range of 0.865e0.950 undermesophilic conditions. Similarly, the RMSE value was in the rangeof 0.2023e0.5687, and the R2 valuewas in the range of 0.968e0.990under thermophilic conditions. The first-order kinetic model pre-dicted methane yield better under thermophilic conditions thanunder mesophilic conditions.

4. Conclusions

The results of this study showed that the CCW can be anaero-bically treated without disturbing the digester process for F/M ratioup to 2.0 under both mesophilic and thermophilic conditions. The

volumetric biogas production (L/L) from CCW linearly increasedwith increase in F/M ratio from 0.5 to 2.0. Similarly, the specificbiogas yield (mL/g VS) increased significantly under both meso-philic and thermophilic conditions when the F/M ratio increasedfrom 0.5 to 2.0. The TS and VS removal increasedwith an increase inF/M ratio under both mesophilic and thermophilic conditions. TheVS removal was significantly higher (P < 0.01) under thermophilictemperature conditions than under mesophilic temperature con-ditions. The first-order kinetic constant (k) was significantly higherunder thermophilic conditions than under mesophilic conditions.The predicted methane yield derived from the first-order kineticmodel was in good agreement with the experimental results, andthe fit to the model was better under thermophilic conditions thanunder mesophilic conditions. The data obtained from this studycould be useful in designing field scale batch anaerobic digesters fortreatment of CCW. Further study at higher F/M ratios (>2.0) isrecommended to determine the optimum limit of the F/M ratio forbatch anaerobic digestion of CCW.

Acknowledgements

This work was supported by a research grant from the RuralDevelopment Administration (RDA), Republic of Korea. The authorswould like to extend sincere thanks to Kangwon National Univer-sity research grant 2013 (No. C1009636-01-01) and University ofIdaho College of Agricultural and Life Sciences for their support.

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