Applied Energy 103 (2013) 61–72

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Applied Energy

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Anaerobic treatment of apple waste with swine manure for biogas production:Batch and continuous operation

Gopi Krishna Kafle 1, Sang Hun Kim ⇑Department of Biosystems Engineering, Kangwon National University, Chuncheon, Kangwon-do, Republic of Korea

h i g h l i g h t s

" Apple waste (AW) was co-digested with swine manure (SM)." Mixture of AW and SM produced a higher biogas yield than SM only." Mixture of AW and SM produced a higher biogas yield at 55 �C than at 36.5 �C." Modified Gompertz model best fitted to the substrates used." Positive synergetic effect up to 33% AW during continuous digestion.

a r t i c l e i n f o

Article history:Received 13 July 2012Accepted 4 October 2012Available online 12 December 2012

Keywords:Anaerobic treatmentApple wasteBatch and continuous testBiogasKinetic studySwine manure

0306-2619/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.apenergy.2012.10.018

⇑ Corresponding author. Address: Department of Biwon National University, 192-1 Hyoja 2-dong, ChuncRepublic of Korea. Tel.: +82 33 250 6492; fax: +82 33

E-mail addresses: gopikafle@yahoo.com (G.K. K(S.H. Kim).

1 Tel.: +82 33 250 6490; fax: +82 33 255 6406.

a b s t r a c t

This study evaluated the performance of anaerobic digesters using a mixture of apple waste (AW) andswine manure (SM). Tests were performed using both batch and continuous digesters. The batch testevaluated the gas potential, gas production rate of the AW and SM (Experiment I), and the effect ofAW co-digestion with SM (33:67,% volatile solids (VSs) basis) (Experiment II) at mesophilic and thermo-philic temperatures. The first-order kinetic model and modified Gompertz model were also evaluated formethane yield. The continuous test evaluated the performance of a single stage completely stirred tankreactor (CSTR) with different mixture ratios of AW and SM at mesophilic temperature. The ultimate bio-gas and methane productivity of AW in terms of total chemical oxygen demand (TCOD) was determinedto be 510 and 252 mL/g TCOD added, respectively. The mixture of AW and SM improved the biogas yieldby approximately 16% and 48% at mesophilic and thermophilic temperatures, respectively, compared tothe use of SM only, but no significant difference was found in the methane yield. The difference betweenthe predicted and measured methane yield was higher with a first order kinetic model (4.6–18.1%) thanwith a modified Gompertz model (1.2–3.4%). When testing continuous digestion, the methane yieldincreased from 146 to 190 mL/g TCOD added when the AW content in the feed was increased from25% to 33% (VS basis) at a constant organic loading rate (OLR) of 1.6 g VS/L/d and a hydraulic retentiontime (HRT) of 30 days. However, the total volatile fatty acids (TVFA) accumulation increased rapidlyand the pH, methane content, and biogas production decreased continuously when the AW content inthe feed was increased to 50%.

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1. Introduction

Approximately 150,000 tons/day (wet basis or w.b.) of waste isproduced on swine farms in Korea, and approximately 5.7% of thatwaste is disposed of by ocean dumping [1,2]. Swine waste is amajor source of odour production, vermin attraction, toxic gas

ll rights reserved.

osystems Engineering, Kang-heon, Kangwon-do 200-701,255 6406.afle), shkim@kangwon.ac.kr

emissions and groundwater contamination. Meanwhile, approxi-mately 14,452 tons/day (w.b.) of food waste is produced in Korea[3], including approximately 55% fruit and vegetable waste(FVW). The moisture content in FVW is very high (75–95%), andit decomposes readily, leading to many unpleasant environmentalconsequences when it is abandoned in fields or near factories.

Anaerobic digestion can be used to convert organic matter intobiogas for energy recovery and achieve waste stabilisation andodour reduction. Although there are many successful anaerobicdigesters currently treating swine manure, the high ammonia con-tent in this type of manure is a major limitation that has plagueddigesters for many years [4–6]. Similarly, FVW has major limita-tions to its usefulness in anaerobic digestion because of how

62 G.K. Kafle, S.H. Kim / Applied Energy 103 (2013) 61–72

rapidly it acidifies, stressing and inhibiting the activity of methano-gens [7,8]. Apples are one of the most popular fruits in Korea, and alarge amount of AW is generated everyday from markets, agro-industries and kitchens, making them a possible candidate foruse in anaerobic digestion. However, the experimental data onthe biomethanisation of AW are limited. Lane [9] and Contreras Lo-pez and Lopez Bobo [10] conducted anaerobic digestion of AW incontinuous digesters. Similarly, Llaneza Coalla et al. [11] investi-gated the anaerobic digestion of apple pulp, in which AW wasco-digested with slaughterhouse waste in a laboratory scale 10 LCSTR.

Although biodegradable organic matter can be used as the solefeedstock in anaerobic digestion, the digestion process tends to failwithout the addition of external nutrients and buffering agents[12]. Co-digestion with substrates that have high buffering capac-ity (alkalinity) such as manure can be good alternatives for theeffective treatment of highly biodegradable materials. During theco-digestion of plant materials and animal manure, the manureprovides buffering capacity and various nutrients, while the plantmaterial provides high carbon content. The result is a more bal-anced C/N ratio, and the co-digestion of manure and plant materi-als decreases the risk of ammonia inhibition [13,14] andacidification. Additionally, the input of readily biodegradable or-ganic matter into animal manure digesters could significantly in-crease biogas production [15,16]. Utilising apple waste withswine manure in biogas production could be an appropriate solu-tion for the manure’s effective treatment and energy recovery.

In co-digestion, the digester performance is influenced by themixing ratio or influent composition. Kaparaju and Rintala [5] sug-gested that 15–20% (on a VS basis) potato waste can be includedwhen treating swine manure for successful digester operation. Cal-laghan et al. [17] improved the methane yield from 0.23 to0.45 m3/kg VS added by increasing the proportion of FVW from20% to 50% during co-digestion with cattle manure, but it was alsoobserved that a high concentration of VFA was produced when theproportion of FVW reached 30% or more. Similarly, Li et al. [16]recommended 75% (VS basis) kitchen waste as optimal for co-digestion with cow manure. The above studies showed that,depending on the characteristics of the substrates used, the opti-mal mixing ratio will be different for different waste productsbeing co-digested. To the best of our knowledge, no previous studyhas examined the anaerobic co-digestion of AW with SM; there-fore, this study co-digested apple waste with swine manure, firstin batch digesters and then in continuous co-digestion tests basedon the batch test results.

Table 1Characteristics of materials used in batch and continuous tests.

Particular Units Batch test

AW SM Inoculum

Experiment I Expe

Mesophilic Meso

TS % 13.7–14.2 5.9 1.39 2.0–VS % 13–13.1 3.85 0.56 0.91VS/TS ratio 0.92–0.95 0.65 0.40 0.46pH 4.05–4.15 7.95 8.1 8.60TCOD mg/L 139,200–156,800 85,000 4373 –SCOD mg/L 112,000–119,200 37,760 3413 –TVFA mg/L 1900 14,168 1014 1183TA mg/L 0 17,096 11,160 14,1TVFA/TA ratio – 0.829 0.091 0.06NH3–N mg/L 370–450 5160 2540 3480TKN mg/L 2521–2661 8054 2647 –TOC mg/L 72,222–72,778 21,390 2139 –C/N ratio 27.3–28.9 2.7 0.80 –

C/N ratio: TOC to TKN ratio.

The objective of this study was to evaluate the performance ofanaerobic digesters for the treatment of AW with SM. The specificobjectives were (1) to determine the biochemical methane poten-tial (BMP) and gas production rate of AW and SM, (2) to investigatethe performance of batch digesters with the mixture of AW and SM(AW:SM = 33:67) under mesophilic and thermophilic tempera-tures, (3) to predict methane production using a first-order kineticmodel and modified Gompertz model, and (4) to investigate theperformance of a continuous digester with different AW:SM ratiosat mesophilic temperature.

2. Materials and methods

2.1. Collection and characterisation of test materials

Swine manure was obtained from the Anseong swine farm lo-cated in Gyeonggi Province, Korea, and stored at 4 �C. Fresh appleswere obtained from a market in Korea. The apples were crushed ina blender and stored at 4 �C. The mesophilic anaerobic digestedsludge (inoculum) was obtained from a 13 L CSTR installed at theDepartment of Biosystems Engineering, Kangwon National Univer-sity, Korea. Thermophilic inoculum was obtained from a biogasplant at the Anseong swine farm. Swine manure was used as a sub-strate in both the mesophilic and thermophilic reactors fromwhich the inoculum was obtained. The characteristics of the feed-stock and inoculum used for the batch and continuous tests areshown in Table 1.

2.2. Batch digester start-up and experimental design

The batch test was divided into two experiments labelled I andII. The experimental design for each portion is shown in Table 2.Experiment I used 1.2 L glass bottles (liquid volume of 0.8 L) andExperiment II used 2.3 L glass bottles (liquid volume of 1.8 L) tocarry out the anaerobic digestion tests. The biogas potential andproduction rate of apple waste and swine manure were studiedin Experiment I. Tests were then performed in Experiment II witha feed mixture ratio of AW:SM = 0:100 (SM-100%) andAW:SM = 33:67 (AW-33%) on a VS basis at an OLR of 5.0 g VS/Land feed-to-microbe (F/M) ratio of 1.0 under mesophilic and ther-mophilic conditions (Table 2). The F/M ratio was calculated basedon the initial VS of the substrate and inoculum.

F=M ¼ Substrate added ðg VSÞInoculum added ðg VSÞ ð1Þ

Continuous test

SM

riment II Days 1–74 Days 75–98 Days 99–146

philic Thermophilic

2.5 2.6 3.41 5.57 4.33–4.8–1.2 1.4 2.42 3.9 2.91–3.50–0.48 0.54 0.70 0.71 0.67–0.73–8.75 8.2 6.33 6.74 7.80–7.85

– 63,947 73,067 68,200–72,100– – –

–1910 2060 13,371 16,329 14,000–15,03192–17,300 20,320 3600 5887 11,713–12,1008–0.135 0.101 3.71 2.77 1.15–1.28–3860 – 3220 3360 5500–5830

– – –– – –– – –

Table 2Experimental design for batch and continuous tests.

Particular Experimental designa

F/M ratio Substrate loading (g VS/Lb or g VS/L/dc) Feed composition (% VS basis) Temperature (�C) Number of replications

Batch test Experiment I 0.5 2.5b AW: SM = 100:0(AW-100%) 36.5 30.5 2.5b AW: SM = 0:100(SM-100%) 36.5 3

Experiment II 1.0 5.0b AW: SM = 0:100(SM-100%) 36.5 21.0 5.0b AW: SM = 33:67(AW-33%) 36.5 21.0 5.0b AW: SM = 33:67(AW-33%) 55.0 2

Continuous test Days 1–90 – 1.0, 1.4,1.6c AW: SM = 25:75(AW-25%) 36–38 1Days 91–123 – 1.6c AW: SM = 33:67(AW-33%) 36–38 1Days 124–146 – 1.7c AW: SM = 50:50(AW-50%) 36–38 1

–: Not determined.SM: Swine manure; AW: Apple waste.

a Based on oven dried VS.b Gram volatile solids per litre.c Gram volatile solids per litre per day.

G.K. Kafle, S.H. Kim / Applied Energy 103 (2013) 61–72 63

After adding the required amounts of inoculum and substrate,each digester was filled with tap water to maintain a designatedvolume. The digester was flushed with 100% N2 for 2–3 min beforesealing. The mesophilic and thermophilic digesters were kept at36.5 �C and 55 �C, respectively, in a temperature-controlled incu-bator. Experiment I was performed in triplicate, and ExperimentII was performed in duplicate; the results were expressed as amean. Each digester was mixed manually for 20–30 s once a day.Assays with the inoculum alone were used as controls. The biogasand methane produced from the inoculum was subtracted fromthe sample assays [18,19].

2.3. Continuous test setup and design

A single-stage continuous process was performed in a 5.5 LCSTR with 4.5 L of working volume. The reactor was installed in-side a temperature-controlled chamber (36–38 �C) and fed once aday using a peristaltic pump. An equivalent volume of digestercontent was discharged prior to feeding. The reactor was stirred(7 min for each hour) by circulating the produced biogas usingthe peristaltic pump. A continuous anaerobic digestion test wasperformed with different mixture ratios of SM and AW. The exper-imental design for the continuous test is shown in Table 2. In thiscase, the reactor was inoculated on day 0 with 4.25 L of digestedsludge (inoculum), and anaerobic conditions were created byflushing the head space with nitrogen gas. Then, different mixtureratios of SM and AW were fed at a constant HRT of 30 days. The ap-ple waste content in the feed was increased from 25% to 50% basedon the VS content (Table 2). Different OLRs were used with differ-ent mixtures of SM and AW to maintain a constant HRT.

2.4. Biogas measurement and analytical methods

The daily biogas production of each digester in the batch testwas determined by the volume of biogas produced, which was cal-culated from the volume and pressure in the headspace of the di-gester. The pressure was measured using a WAL-BMP-testsystem pressure gauge (type 3150, Wal, Germany) [20]. In the con-tinuous test, the biogas was collected in a gas collector by thewater displacement method. The methane concentration and H2Sconcentration in the biogas were analysed using a gas analyser(BioGas Check-Geotechnical Instruments (UK) Ltd.). The gas ana-lyser was calibrated using a certified gas containing methane(50.13, 15.01, 5.01, %) and carbon dioxide (49.87, 15.01, 5.00, %).The measured wet biogas and methane volumes were then ad-justed to the volumes at standard temperature (0 �C) and pressure(1 atm) [20–22].

VSTP ¼VT � 273� ð760� pwÞð273þ TÞ � 760

ð2Þ

where VSTP is the volume of gas measured at standard temperatureand pressure (L), VT is volume of gas measured at temperature T (L),T is temperature of the fermentation gas or of the ambient space(�C), pw is the Vapor pressure of the water as a function of temper-ature (mm Hg), The corrected methane content (CH4 Corr) in the bio-gas was calculated using Eq. (3) as proposed in German standardprocedure [22].

CH4 Corr ¼CCH4 � 100CCH4 þ CCO2

ð3Þ

where CH4 Corr is the Corrected methane content in the dry gas (% byvolume), CCH4 is Measured methane content in the gas (% by vol-ume), CCO2 is the Measured carbon-dioxide content in the gas (%by volume).

The total solids (TSs) and VS in the well-mixed samples weredetermined in triplicate according to standard methods [23]. Theclosed reflux titration method was used for TCOD and solublechemical oxygen demand (SCOD) analysis, and the pH value wasdetermined using a pH metre (YK-2001 PH, Taiwan) at digestertemperature (36–38 �C). The total Kjeldahl nitrogen (TKN) wasanalysed using a Kjeldahl apparatus (Kjeltec 2100, Foss, Sweden),and the ammonia nitrogen (NH3–N) was measured using the Ness-ler method and determined using a spectrophotometre (DR 2500,Hach, USA). The TVFA, total alkalinity (TA) and TVFA/TA ratio weredetermined using the Nordmann-titration method [24]. The bicar-bonate alkalinity (BA) was calculated using the relationshipBA = 0.71 � TVFA [25], and the total organic carbon (TOC) was cal-culated using the relationship TOC = VS/1.8 [26].

The mass removal of biogas at the conclusion of the experimentwas calculated using a formula (Eq. (4)). The density of CH4 was ta-ken as 0.000668 g/mL and the density of CO2 as 0.00184 g/mL [27].The TCOD removal was calculated using Eq. (5) [22].

BR ¼ V0 � qmix

mð4Þ

TCOD removal of feed ð%Þ ¼ V0 � CH4 Corr

320�mð5Þ

where BR is the mass of biogas removed per gram TCOD added (g/gTCOD added), V0 is volume of biogas produced (mL, at STP), qmix ismass concentration of CH4 + CO2 in the biogas (g/mL), m is theAmount of TCOD added (g).

64 G.K. Kafle, S.H. Kim / Applied Energy 103 (2013) 61–72

2.5. Kinetic study

Assuming first-order kinetics for the hydrolysis of particulateorganic matter, the cumulative methane production can be de-scribed by means of the following equation:

GðtÞ ¼ Go � ð1� eð�KtÞÞ ð6Þ

where G(t) is the cumulative methane yield at digestion time t days(mL/g TCOD added), Go is methane potential of the substrate (mL/gTCOD added), K is methane production rate constant (first order dis-integration rate constant) (1/day), t is the time (days).

Apart from specific methane yield and the cumulative methaneyield, the duration of the lag phase is also an important factor indetermining the efficiency of anaerobic digestion. The lag phase(k) can be calculated with the modified Gompertz model as de-scribed by Lay et al. [28] as follows:

M ¼ P � exp � expRmax � e

Pðk� tÞ þ 1

� �� �ð7Þ

where M is the cumulative methane production (mL/g TCOD), P ismethane production potential (mL/g TCOD), Rmax is maximummethane production rate (mL/g TCOD-d), k is lag phase (day), t istime (day), e is the exp(1) = 2.7183.

A nonlinear least-square regression analysis was performedusing SPSS program (IBM SPSS statistics 19 (2010)) to determinethe K, Rmax, k, and the predicted methane yield. The predictedmethane yield obtained from the SPSS program was plotted withthe measured methane yield using Matlab software R2011b(7.13.0.564). The statistical indicators, Correlation coefficient (R2)and root mean square error (RMSE) were calculated [29,30].

RMSE ¼ 1m

Xm

j¼1

dj

Yj

� �2 !1

2

ð8Þ

where m is the number of data pairs, j is jth values, Y is measuredmethane yield (mL/g TCOD), d is the Deviations between measuredand predicted methane yield.

2.6. Statistical analysis and data processing

2.6.1. Data processingIt is known that during batch digestion, the biogas production

rates and methane content considerably change over the digestiontime [20]. The methane content of biogas of intermediate days dur-ing batch tests were calculated using linear interpolation by IN-TERP1 function in Matlab software R2011b (7.13.0.564). Theweighted average corrected methane content (CH4 Corr, WA) overthe digestion period was calculated as follows:

CH4 Corr;WAð%Þ ¼Pn

i¼1BPi � CH4 Corr; iPni¼1BPi

ð9Þ

Based on the weighted average corrected methane content andthe interpolated data, the standard deviation (r) was calculated inExcel software 2007 as follows:

r ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPðCH4 Corr; i � CH4 Corr;WAÞ2

n� 1

sð10Þ

where BPi is the biogas production on day i, CH4 Corr, i is correctedmethane content on day i, n is the number of observations.

2.6.2. Statistical analysisThe significance of differences in the average biogas yield,

methane yields, methane content, and pH was determined byusing single factor Analysis of Variance (ANOVA) in Excel software2007. If the calculated F value was higher than the tabulated F

value, Least Significant Difference (LSD) was calculated to judgewhether two or more averages are significantly different or not.LSD was calculated at a = 0.05 (LSD0.05) and at a = 0.01 (LSD0.01)as follows [31,32].

LSDa ¼ ta

ffiffiffiffiffiffiffi2s2

r

rð11Þ

where ta is the t value chosen for the degree of freedom for errorand level of significance(a) desired, s2 is mean square for error(MSE), r is the number of replications on which the means to be sep-arated are based.

3. Results and discussion

3.1. Substrate characterisation

The results of the feedstock characterisation are summarised inTable 1. The AW’s TS percentage was approximately 2.4–4.0 timeshigher than that of the SM, and its VS percentage was 3.4–5.4 timeshigher. Similarly, the AW contained much higher TCOD and SCOD(approximately two and three times higher, respectively) than theSM. The VS/TS ratio of the AW was 0.92–0.95, while that of the SMwas 0.65–0.71, indicating that AW contained more digestible or-ganic matter than SM. The AW was very acidic (pH = 4.05–4.15)compared to the SM (pH = 6.33–7.95). The TVFA concentration inthe SM was 13,371–16,329 mg/L, and its TA concentration was3600–17,096 mg/L. There was no alkalinity (TA) in the apple wastedue to its low pH, which could cause acidification in the reactor ifused alone. The VS/TS ratio and COD values of the SM in this studywere similar to that reported by [33].

The carbon-to-nitrogen ratio (C/N) ratios of the AW (27.3–28.9)were within the optimal range (25–30) for anaerobic digestion[34], but the SM’s C/N ratio (2.7) was very low. The TKN concentra-tions in the SM were more than three times higher than in the AW.Most of the nitrogen in the AW existed as organic nitrogen, whilethe majority of the SM’s nitrogen was ammonia. Ammonical nitro-gen is responsible for maintaining alkalinity in the digester, but theconcentration of ammonical nitrogen inhibits methanogen activitywhen it exceeds 3000 mg/L [35]. The mixture of AW and SM wasexpected to improve the efficiency of the anaerobic digester com-pared to using SM alone. Molinuevo-Salces et al. [36] observed bet-ter digester performance when the high buffer capacity of swinemanure was coupled with the high C/N ratio supplied by vegetablewastes.

3.2. BMP and biogas production rate of AW and SM (Experiment I)

The cumulative biogas yield (mL/g TCOD added), daily biogasproduction (mL/g TCOD-d), methane content, and H2S concentra-tion in the biogas produced during the digestion of AW and SMat an OLR of 2.5 g VS/L and a F/M ratio of 0.5 under mesophilic con-ditions are shown in Fig. 1. Biogas production started immediatelyon the first day of digestion in all of the digesters. The biogas pro-duction was very low in the SM-100% digesters until day 5 (0.1–3.1 mL/g TCOD-d), when the gas production began to continuouslyincrease until day 13. Thereafter, gas production remained almostconstant, with some fluctuations, until day 27 (Fig. 1b). After day27, the biogas production from the SM-100% digesters slowly de-creased and almost ceased after day 34. The peak value of biogasproduction rate for SM-100% was 21 mL/g TCOD-d on day 22.The AW-100% digesters showed different biogas production pat-tern compared to the SM-100% examples. In the AW-100% digest-ers, the biogas production rate abruptly increased until the thirdday of digestion and then rapidly dropped until day 10. The peakvalue of the biogas production rate was 61.5 mL/g TCOD-d on

Fig. 1. (a) Cumulative biogas yield; (b) biogas production rate; (c) methane content; and (d) H2S concentration in biogas produced from apple waste (AW-100%) and swinemanure (SM-100%) at OLR of 2.5 g VS/L and F/M ratio of 0.5 under mesophilic conditions (Experiment I).

G.K. Kafle, S.H. Kim / Applied Energy 103 (2013) 61–72 65

Table 3Initial and final characteristics of the digester contents for batch tests.

Experiment Experiment I Experiment II

Feed composition (VS basis) AW-100% SM-100% SM-100% AW-33% AW-33%

Temperature (�C) 36.5 36.5 36.5 36.5 55OLR (g VS/L) 2.5 2.5 5.0 5.0 5.0F/M ratio 0.50 0.50 1.0 1.0 1.0Number of replications (n) 3 3 2 2 2

TVFA (mg/L) Initial 881 1300 2130 2593 2423Final 682 970 1046 848 1489

TA (mg/L) Initial 9765 9956 9077 10,276 8870Final 10,093 11,815 11,122 11,357 9352

TVFA/TA ratio Initial 0.090 0.131 0.235 0.252 0.273Final 0.068 0.080 0.090 0.075 0.159

pH Initial 8.22 8.25 8.20 8.25 8.10Final 7.77 7.90 7.77 7.8 7.89

NH3–N (mg/L) Initial 2280 2360 2600 2760 2310Final 2333 2560 2760 2370 2090

Values are expressed as mean for Experiment I (n = 3) and Experiment II (n = 2).

Table 4Gas potential, BR and TCOD removal for batch tests.

Experiment Experiment I Experiment II

Feed composition (VS basis) AW-100 % SM-100% SM-100% AW-33% AW-33%

Temperature (�C) 36.5 36.5 36.5 36.5 55.0OLR (g VS/L) 2.5 2.5 5.0 5.0 5.0Biogas yield (mL/g TCOD added) 510 329 342 398 505Methane yield (mL/g TCOD added) 252 268 259 267 276Methane (%)a 49.3(20.0) 81.3(14.2) 75.8(5.5) 67.0(13.0) 54.6(20.0)COD removal (%) 79 84 81 83 86BR (g/g COD added) 0.644 0.292 0.325 0.420 0.606

Values are expressed as mean for Experiment I (n = 3) and Experiment II (n = 2).a Values in parenthesis are standard deviations.

66 G.K. Kafle, S.H. Kim / Applied Energy 103 (2013) 61–72

day 3. After day 10, the biogas production rate slowly increasedagain until day 31, with some fluctuations, and thereafter, thegas production rate declined and almost ceased after day 35. Thespecific biogas yield increased until approximately day 33 andday 31 in the AW-100% and SM-100% digesters, respectively, grad-ually levelling off thereafter. Approximately 90% of the biogas yieldwas obtained within 31 days and 30 days of digestion for the AW-100% and SM-100% digesters, respectively. The average biogasyields from the digesters with feed compositions of SM-100% andAW-100% were 510 and 329 mL/g TCOD added, respectively (Ta-ble 4). Thus, the biogas yield from 100% AW was significantly high-er (p < 0.01) than that of 100% SM (LSD0.05 = 41 mL/g TCOD addedand LSD0.01 = 51 mL/g TCOD added). The higher biogas yield fromAW-100% illustrates the possibility of improving the biogas yieldfrom an SM digester with the addition of AW.

The methane content in the biogas produced from the SM-100%digesters was 13% on day 1 and increased rapidly until day 12(approximately 79%), after which it remained almost constantthroughout the rest of the test period (Fig. 1c). The AW-100%digesters showed a different pattern of methane content than theSM-100% digesters. The methane content in biogas produced fromthe AW-100% digesters was 4.5% on day 2 and increased continu-ously until end of the test, reaching a value of 72% on day 32.The methane content increased slowly until day 10 and then in-creased rapidly, similar to gas production. The methane concentra-tion and gas production pattern in the AW-100% digesters showedthat methanogenesis was inhibited during days 4–10 due to acid-ification in the digester, even at a low F/M ratio of 0.50 (OLR = 2.5 gVS/L). The anaerobic digestion of AW alone at higher F/M ratios(>0.50) and a higher OLR (>2.5 g VS/L) can cause acidification inthe digester and digester failure. Thus, the results suggested that

when running digesters with AW at higher OLRs (and F/M ratios),additional alkalinity must be added or co-digested with alkaline-rich substrates such as SM to achieve better digester performance.The average methane content of the biogas produced was 49.3%and 81.3% for AW-100% and SM-100% digesters, respectively (Ta-ble 4). Thus, the methane content of biogas produced from SMwas significantly higher (p < 0.01) than that produced from AW(LSD0.05 = 7% and LSD0.01 = 10%).

Fig. 1d shows the pattern of H2S concentrations in the biogasesproduced from SM and AW. The H2S concentration in biogas pro-duced from SM was almost constant (1896–2280 ppm (ppm)) dur-ing days 1–17 and then decreased slowly. The H2S concentration inbiogas produced from AW-100% digesters was higher than thatfrom SM-100% digesters (Fig. 1d) and increased rapidly duringthe digester start-up period (days 1–6), after which it decreased(Fig. 1d). The H2S concentration in the biogas produced from AWdigesters fell within the range of 1120–3300 ppm and reached itspeak value on day 6 (3300 ppm). In an anaerobic digestion process,there is competition between sulphate-reducing bacteria (SRB) andmethanogens. Some SRB may oxidise VFAs (e.g., propionic and bu-tyric acid) completely to CO2 in the presence of sulphates and gen-erate sulphide during the process. Others may break down VFAsincompletely to acetate, also producing sulphide. SRB are espe-cially effective in competing with methanogens for acetate andhydrogen in the presence of sulphate [37,38]. The rapid drop inbiogas production, slow increase in methane content, and rapid in-crease in H2S concentration during the digester start-up period(days 4–10) of the AW-100% digesters showed that SRB activitywas higher than methanogen activity during this start-up period.The rapid acidification of AW may explain the low methanogenactivity during the digester start-up period.

G.K. Kafle, S.H. Kim / Applied Energy 103 (2013) 61–72 67

The initial and final characteristics of the digester contentsduring Experiment I are shown in Table 3. The TVFA, TVFA/TA ratio, and pH decreased, while the TA and NH3–N increasedafter anaerobic digestion. The final pH value was significantlyhigher (p < 0.01) (LSD0.05 = 0.04 and LSD0.01 = 0.06) in the SM-100% digester than in the AW-100% digester. The final pH va-lue of 7.77 and TVFA/TA ratio of 0.068 in the AW-100% diges-

Fig. 2. (a) Cumulative biogas yield; (b) biogas production rate; (c) methane content;mesophilic and thermophilic conditions at OLR of 5 g VS/L and F/M ratio of 1.0 (Experim

ter showed that the available alkalinity was sufficient tomaintain stability in the reactor, although VFA inhibition wasobserved during the start-up period. Raposo et al. [39] ob-served stable digestion operation under batch conditions, witha TVFA/TA ratio in the range of 0.30–0.40 or less, but destabil-isation of the digester was observed at a TVFA/TA ratio ofapproximately 0.70.

and (d) H2S concentration in biogas produced from SM-100% and AW-33% underent II).

Table 5Results of kinetic study using first-order kinetic model.

Experiment Feed compositionVS basis (%)

OLR g VS/L F/M ratio Temperature(�C)

Ka

(1/day)R2 Methane yield (45 days digestion time)

Measured (mL/g TCOD) Predicted (mL/g TCOD) Difference (%)

Experiment I AW-100 2.5 0.5 36.5 0.042 0.844 242 213 12SM-100 2.5 0.5 36.5 0.044 0.823 264 238 9.8

Experiment II SM-100 5.0 1.0 36.5 0.072 0.899 263 249 5.3AW-33 5.0 1.0 36.5 0.032 0.828 249 204 18.1AW-33 5.0 1.0 55.0 0.053 0.894 263 251 4.6

a Calculated at 95% confidence interval.

Table 6Results of kinetic study using modified Gompertz model.

Experiment FeedcompositionVS basis (%)

OLR gVS/L

F/M ratio Temperature(�C)

Rma

(mL/g TCOD-d)Lag phaseka

(days)

R2 T90

(days)Tef

(days)Methane yield (45 days digestion time)

Measured(mL/g TCOD)

Predicted(mL/g TCOD)

Difference(%)

Experiment I AW-100 2.5 0.5 36.5 8.9 5.5 0.947 31.0 25.5 242 236 2.5SM-100 2.5 0.5 36.5 14.2 9.4 0.994 30.0 20.6 264 271 2.7

Experiment II SM-100 5.0 1.0 36.5 16.3 4.3 0.996 25.5 21.2 263 258 1.9AW-33 5.0 1.0 36.5 9.8 10.1 0.995 41.5 31.4 249 246 1.2AW-33 5.0 1.0 55.0 13.0 5.1 0.992 25.0 19.9 263 272 3.4

T90 – Time taken for 90% biogas production.Tef – Effective biogas production duration (T90 –k).

a Calculated at 95% confidence interval.

68 G.K. Kafle, S.H. Kim / Applied Energy 103 (2013) 61–72

3.3. Co-digestion of AW and SM under mesophilic and thermophilicconditions (Experiment II)

The results of Experiment I showed that SM could be used at anOLR higher than 2.5 g VS/L, but the use of 100% AW at a higher OLR(>2.5 g VS/L) can cause acidification and possible failure of the di-gester process. Therefore, tests were conducted only with SM (SM-100%) and a mixture of AW and SM (AW-33%) at an OLR of 5 g VS/Land a F/M ratio of 1.0 under both mesophilic and thermophilic con-ditions during Experiment II.

The cumulative biogas yield (mL/g TCOD added), biogas produc-tion rate (mL/g TCOD-d), methane content and H2S concentrationin biogas for SM-100% under mesophilic conditions and AW-33%under both mesophilic and thermophilic conditions are shown inFig. 2. Experiment II lasted for 85 days, and the gas production ratewas measured daily for the first 60 days, after which gas produc-tion was measured in 3- to 5-day time intervals due to low gas pro-duction (that had almost ceased), as shown in Fig. 2a. As inExperiment I, biogas production commenced immediately on thefirst day of digestion. The biogas production rate (mL/g TCOD-d) in-creased continuously for the SM-100% until day 10 with some fluc-tuations, after which the biogas production rate started decliningslowly with some fluctuations. Under mesophilic conditions, thebiogas production rate for AW-33% digesters increased until day2 and then decreased rapidly until day 8. After day 9, the biogasproduction rate increased until day 24 and then declined slowly,similar to the SM-100% from days 10–33. Under thermophilic con-ditions, the biogas production rate from the AW-33% digesters in-creased until day 2, similar to its behaviour under mesophilicconditions, and then dropped rapidly on day 3. A constant low bio-gas production rate was achieved from days 3–6, and thereafter, itincreased rapidly until day 10. The biogas production rate againdropped rapidly until day 17. From day 18 on, the biogas produc-tion rate increased and reached its peak on day 22. Finally, the ratedecreased slowly from day 23 until day 43 with some fluctuations.These results showed that the gas production trend was very dif-ferent in thermophilic conditions and mesophilic conditions. Threemajor peaks in daily biogas production were obtained under ther-

mophilic conditions, compared to two peaks during mesophilicconditions. The peak value in the biogas production rate was 21,18 and 45 mL/g TCOD-d on days 10, 24 and 10 for SM-100%, AW-33% under mesophilic conditions and AW-33% under thermophilicconditions, respectively.

The specific biogas yield (mL/g TCOD added) increased untilapproximately day 30 for SM-100% digesters and approximatelydays 45 and 38 for AW-33% digesters under mesophilic and ther-mophilic conditions, respectively, after which it gradually levelledoff in all the digesters. Approximately 90% of the biogas yield wasobtained within 26 days of digestion in the SM-100% digesters andwithin 42 and 38 days in the AW-33% digesters under mesophilicand thermophilic conditions, respectively. The average biogas yieldfrom the digester with SM-100% was 342 ml/g TCOD added and398 and 505 ml/g TCOD added in the AW-33% digesters undermesophilic and thermophilic conditions, respectively (Table 4).Thus, the biogas yield from the mixed feed (AW-33%) increasedsignificantly (p < 0.01) under both mesophilic (by 16%) and ther-mophilic conditions (by 48%) compared to the SM-100% material.The increase in biogas yield with the mixed feed was due to thehigher biogas potential of AW than SM along with the synergeticeffect, which was found to be higher under thermophilic condi-tions than under mesophilic conditions.

The methane concentration in biogas produced from SM-100%was significantly (p < 0.01) higher than that produced from AW-33% (Fig. 2b). The rapid drop in biogas production rate during thedigester start-up period and the very low methane content inAW-33% compared with that in SM-100% showed some inhibitionduring the digester start-up period in AW-33% digesters underboth mesophilic and thermophilic conditions. This inhibition maybe due to the rapid acidification of apple waste. Thus, the resultssuggested that for high rate anaerobic digesters (F/M ratio > 1.0and OLR > 5.0 g VS/L) the AW should not exceed 33%, or else addi-tional alkalinity should be added. The average methane contentswere calculated to be 75.8% for SM-100% and 67.0 and 54.6% forAW-33% under mesophilic and thermophilic conditions, respec-tively (Table 4). The average methane yields were calculated tobe 259 mL/g TCOD added for SM-100% and 267 and 276 mL/g

Fig. 3. Plot of measured and predicted methane yield with statistical indicators: (a) Experiment I and (b) Experiment II.

G.K. Kafle, S.H. Kim / Applied Energy 103 (2013) 61–72 69

Table 7Performance of continuous reactor with different feed mixture ratios at mesophilic temperature.

Feed composition(%), VS basis

OLR (g VS/L/d) Days Biogas yield(mL/g TCOD added)

Methanecontent (%)

pH TVFA (mg/L) TA (mg/L) TVFA/TA ratio BA (mg/L)

AW-25 1.0 1–28 317(13) 76.3(0.7) 7.85(0.07) 6688(560) 13,297(789) 0.507(0.071) 8549AW-25 1.0 29–49 260(18) 76.4(1.4) 7.76(0.03) 7155(331) 13,385(160) 0.534(0.023) 8305AW-25 1.4 50–74 201(21) – 7.37(0.03) 10,705(1134) 11,277(769) 0.954(0.118) 3676AW-25 1.6 75–90 197(28) 74(2.9) 7.56(0.07) 11,132(1039) 11,953(654) 0.936(0.119) 4049AW-33 1.6 91–98 182(8) 76.3(0.88) 7.82(0.091) 9310(1403) 14,146(626) 0.662(0.127) 7536AW-33 1.6 99–123 241(17) 78.7(0.72) 7.81(0.02) 8373(215) 16,580(148) 0.505(0.016) 10,635AW-50a 1.7 124–146 109 44 7.13 14,656 11,975 1.224 1569

Values are expressed as mean (standard deviation).a Data of day 146.

70 G.K. Kafle, S.H. Kim / Applied Energy 103 (2013) 61–72

TCOD added for AW-33% under mesophilic and thermophilic con-ditions, respectively (Table 4). González-Fernández et al. [40] re-ported a methane yield of 246 mL/g COD added from SM, whichis similar to the results of our study. The H2S concentration in bio-gas was higher for AW-33% than for SM-100% (Fig. 2d). The H2Sconcentration in biogas produced from mixed feed (AW-33%)was lower under thermophilic conditions than under mesophilicconditions.

The initial and final characteristics of the digester contents inExperiment II are shown in Table 3. The final TVFA and TVFA/TA ra-tio in AW-33% digesters were higher under thermophilic condi-tions than under mesophilic conditions. The final NH3–Nconcentration increased in SM-100% but decreased in AW-33%.The decrease in NH3–N concentration may be due to the low nitro-gen content in AW. The TCOD removal increased slightly withmixed feed (AW-33%) compared to in SM only (Table 4). Similarly,the BR was higher in mixed feed than SM only (Table 4).The reasonfor this higher BR may be due to the higher quantity of readilydegradable compounds and higher organic content (VS/TS ratio)in the AW.

3.4. Kinetic study results

Tables 5 and 6 summarise the results of a kinetic study using afirst-order kinetic model and modified Gompertz model, respec-tively. The kinetic constants were calculated for 45 days of diges-tion time because the time needed for 90% biogas production(T90) fell within the range of 25–42 days (Table 6). The methaneproduction rate constant (K) for AW and SM was found to be al-most identical under mesophilic conditions at an OLR of 2.5 g VS/L and a F/M ratio of 0.5 (Table 5). The K value of mixed feed(AW-33%) was increased by approximately 1.7 times under ther-mophilic conditions compared to mesophilic conditions at anOLR of 5.0 g VS/L and a F/M ratio of 1.0 (Table 5).

The effective biogas production period (Tef) was calculated bysubtracting the lag time (k) from T90 (Table 6). The Tef value forSM was found to be lower than that for AW at an OLR of 2.5 gVS/L and a F/M ratio of 0.5 under mesophilic conditions. The Tef

for mixed feed (AW-33%) at an OLR of 5.0 g VS/L was larger thanthat for SM-100% under mesophilic conditions, but it decreased un-der thermophilic conditions. One possible reason for the increasedTef under mesophilic conditions could be the VFA’s inhibition tomethanogens due to AW’s rapid acidification and slow rate ofdigestion. However, the rate of digestion is faster under thermo-philic conditions, so the acidification of AW should have less ofan effect than under mesophilic conditions. The acidification ofAW during thermophilic digestion should also reduce ammoniainhibition and further improve the methanogen activity. The pre-dicted methane yield derived from the first-order kinetic modeland modified Gompertz model are shown in Tables 5 and 6,respectively. The difference between the predicted and measured

methane yields was higher in the first-order kinetic model (4.6–18.1%) than in the modified Gompertz model (1.2–3.4%). Thus,the modified Gompertz model was found to have the best fit tothe substrates used. To evaluate the soundness of the model resultsin the modified Gompertz model, the predicted values of methaneyield were plotted against the measured values, as shown in Fig. 3.The statistical indicators (RMSE and R2) are also shown in Fig. 3.The RMSE value fell within the range of 0.1563–0.4879 and theR2 value fell within the range of 0.9521–0.9971.

3.5. Performance of the continuous digester with a mixture of AW andSM

The batch test results from Experiment I showed AW as poten-tial substrate for biogas production, but there was a possibility ofVFA inhibition in the digester when using only AW. Similarly, usingAW-33% (AW:SM = 33:67,% VS basis) during Experiment II resultedin inhibition during the start-up period (Fig. 2b and c), which sug-gested that AW content greater than 33% disturbed the digestionprocess at higher OLRs. Thus, based on the results of the batch test,the apple waste content in the feed mixture started at 25% (by VSbasis) in the continuous test, after which the AW percentage wasincreased to 33% and 50%. The results of the batch test showed thatbiogas production from a mixture of SM and AW increased signif-icantly under thermophilic conditions, but there was no significantdifference in the methane yield between mesophilic and thermo-philic temperatures. Thus, the continuous test was operated atmesophilic temperature.

The operation and performance parameters of the continuouslyfed single stage CSTR are summarised in Table 7. The continuoustest was conducted for 146 days and the characteristics of theSM were different for different test periods, as shown in Table 1.The digester was started with a mixture of AW and SM. The AWcontent was fixed at 25% (based on VS) for days 1–90. During thestart-up period, the OLR was kept at 1.0 g VS/L/d and thereafter in-creased to 1.4 and 1.6 g VS/L/d at a constant HRT for 30 days (Ta-ble 2). A biogas yield of 260–317 mL/g TCOD added was obtainedwith AW-25% (AW:SM = 25:75,% VS basis) at an OLR of 1.0 g VS/L/d (days 1–49). The average methane content in the biogas fellwithin the range of 76.3–76.4%, and the pH was between 7.76and 7.85. The TVFA and TVFA/TA ratio in the digester were6688–7155 mg/L and 0.507–0.534, respectively. The BA concentra-tion in the digester was within the range of 8305–8549 mg/L, thedigester operation was stable, and no inhibition was observed dur-ing days 1–49. McCarty [25] proposed that a BA concentration inthe range of 2500–5000 mg/L was more desirable for anaerobic di-gester operation. Similarly, Georgacakis et al. [41] reported that theBA in a SM-fed digester should be greater than 6000 mg/L to main-tain higher biogas production. When the OLR was increased to1.4 g VS/L/d during days 50–74 at the same AW-25% feed mixture,the biogas yield dropped from 260 to 201 mL/g TCOD added, and

G.K. Kafle, S.H. Kim / Applied Energy 103 (2013) 61–72 71

the pH dropped from 7.76 to 7.37. Similarly, the TVFA increasedabove 10,000 mg/L, and the TVFA/TA ratio reached up to 1.0. Thedigester operation was still stable even though the gas yield de-creased, and the TVFA accumulation and TVFA/TA ratio increased.After day 75, the OLR was further increased to 1.6 g VS/L/d untilday 90. As a result, the biogas yield dropped slightly further andthe TVFA accumulation increased. However, the methane contentwas maintained at 73.2%, and the pH (7.56) of the digester contentincreased, showing that the methanogen acclimated to a higherTVFA concentration. The bicarbonate alkalinity during days 75–90 (4049 mg/L) was higher than during days 50–74 (3676 mg/L).The increase in BA and pH with the increase in OLR from 1.4 to1.6 g VS/L/d may have been due to use of SM with a higher TA con-centration and pH during days 75–90 than during days 50–74 (Ta-ble 1). During co-digestion of the AW with slaughterhouse waste,Llaneza Coalla et al. [11] observed stable reactor conditions whenthe VFA concentration was below 4000 mg/L, but inhibition tomethanogen was then observed when the VFA concentration wasincreased to 7000 mg/L. Similarly, Ehimen et al. [42] observedinstability in an anaerobic digester when the TVFA concentrationwas higher than 5000 mg/L during the co-digestion of microalgaeresidues with glycerol. However, during anaerobic co-digestion ofChinese cabbage silage with SM (25:75 VS basis), Kafle and Kim[43] observed stable digester operation for TVFA concentrationsof up to 10,000 mg/L whenever the TVFA/TA ratio was below 1.0.

After day 91, the AW content was increased from 25% to 33% atconstant OLR and HRT. The biogas yield at AW-33% (182 mL/g VS)dropped slightly compared to that of AW-25% during days 91–98.The pH and methane content increased slightly, and the TVFA andTVFA/TA ratio decreased slightly. The BA also increased from 6418to 7536 mg/L. New SM with a high pH (7.8–7.85) and alkalinityconcentration (11,713–12,100 mg/L) was used from day 99 on (Ta-ble 1). The biogas yield increased from 182 to 241 mL/g TCODadded (an increase of 28%) at the same feed mixture (AW-33%)and OLR (1.6 g VS/L/d). The methane content also increased from76.3% to 78.7%, and the TVFA and TVFA/TA ratio decreased further.The TVFA/TA ratio (0.505) during days 99–123 was almost thesame as that during days 1–28 (AW-25% at 1.0 g VS/L/d). The BA in-creased from 7536 to 10,635 mg/L, which was a higher concentra-tion than at any previous time in the experiments. Thus, the resultsshowed that SM with a lower TVFA and higher alkalinity concen-tration allowed the digester to be operated stably with a higherpercentage of AW and OLR compared to SM with a lower alkalinityconcentration.

At day 124, the AW content was further increased to 50%, andthe OLR was fixed at 1.7 g VS/L/d. The OLR was increased with anincrease in AW content to maintain constant HRT. The SM usedduring this period was similar to that used during days 99–123.Gas production increased slightly (242–262 mL/g TCOD added)over the next week (days 125–130) and then dropped regularly.The TVFA concentration and TVFA/TA ratio increased regularlyand the TA, BA, pH, and methane content dropped accordingly.Within 24 days (days 124–146), the pH dropped from 7.81 to7.13 and the methane content dropped from 78.7% to 44%. TheTVFA concentration increased from 8373 to 14,653 mg/L, the TAdecreased from 16,580 to 11,975 mg/L, and the TVFA/TA ratio in-creased from 0.505 to 1.224 within those 24 days. Similarly, theBA concentration decreased from 10,635 to 1569 (Table 7). Thus,the process in the reactor was disturbed and heading towards fail-ure. The digester operation was stopped on day 146.

The mixture of AW and SM showed a positive synergetic effecton biogas production when the AW content in the feed was in-creased from 25% to 33% (VS basis), whereas the further increasein AW content from 33% to 50% had a negative synergetic effecton the biogas production. The main reason for this negative syner-getic effect was the rapid TVFA accumulation and drop in pH value.

Thus, the results of the continuous test suggested that for SM witha TVFA concentration in the range of 13,371–16,329 mg/L, the AWcontent used the feed mixture could measure up to 25–33% (VS ba-sis) based on the alkalinity (TA) of the swine manure at an OLR of1.0–1.6 g VS/L/d and a HRT of 30 days. Llaneza Coalla et al. [11] rec-ommended less than 10% (TS basis) apple pulp as the optimum forco-digestion with slaughterhouse waste.

4. Conclusions

Apple waste was anaerobically treated with swine manure inboth batch and continuous modes. The AW was found to be a po-tential substrate for co-digestion with the SM for biogas produc-tion. Biogas production in the batch test improved when using amixture of AW and SM (AW:SM = 33: 67,% VS basis) as opposedto SM only. The biogas production from mixed feed was signifi-cantly higher under thermophilic conditions than under meso-philic conditions, but there was no significant difference inmethane production. The modified Gompertz model fitted theexperimental results better than a first-order kinetic model. Theresults of the continuous tests showed positive synergetic effectson biogas production when the AW content in the feed was in-creased from 25% to 33% (VS basis) at an OLR of 1.6 g VS/L/d anda HRT of 30 days, but a further increase in AW content from 33%to 50% had a negative synergetic effect due to the rapid accumula-tion of TVFA and the drop in pH.

Acknowledgements

This work was supported by research funds supported by theRural Development Administration (RDA), South Korea.

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