Applied Energy 93 (2012) 648–654

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Applied Energy

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Thermophilic anaerobic co-digestion of oil palm empty fruit bunches with palmoil mill effluent for efficient biogas production

Sompong O-Thong a,b, Kanokwan Boe a, Irini Angelidaki a,⇑a Department of Environmental Engineering, Technical University of Denmark, Kgs Lyngby 2800, Denmarkb Department of Biology, Faculty of Science, Center of Excellent in Sustainable Energy and Environment, Thaksin University, Phatthalung 93110, Thailand

a r t i c l e i n f o a b s t r a c t

Article history:Received 20 August 2010Received in revised form 26 December 2011Accepted 29 December 2011Available online 30 January 2012

Keywords:Empty fruit bunchesHydrothermal pretreatmentPalm oil mill effluentCo-digestionThermophilic conditionBiogas production

0306-2619/$ - see front matter � 2012 Elsevier Ltd. Adoi:10.1016/j.apenergy.2011.12.092

⇑ Corresponding author. Tel.: +45 45251429; fax: +E-mail address: ria@env.dtu.dk (I. Angelidaki).

The effect of pretreatment methods for improved biodegradability and biogas production of oil palm emptyfruit bunches (EFB) and its co-digestion with palm oil mill effluent (POME) was investigated. The maximummethane potential of POME was 502 mL CH4/g VS-added corresponding to 33.2 m3 CH4/ton POME and 98%biodegradability. Meanwhile, the maximum methane potential of EFB was 202 mL CH4/g VS-added corre-sponding to 79.1 m3 CH4/ton EFB with 38% biodegradability. Co-digestion of EFB with POME enhancedmicrobial biodegradability and resulted in 25–32% higher methane production at mixing ratios of 0.4:1,0.8:1 and 2.3:1 on VS basis than digesting EFB alone. The methane yield was 276–340 mL CH4/g VS-addedfor co-digestion of EFB with POME at mixing ratios of 0.4:1–2.3:1, while minor improvement was observedat mixing ratios of 6.8:1 and 11:1 (175–197 mL CH4/g VS-added). The best improved was achieved fromco-digestion of treated EFB by NaOH presoaking and hydrothermal treatment with POME, which resultedin 98% improvement in methane yield comparing with co-digesting untreated EFB. The maximum methaneproduction of co-digestion treated EFB with POME was 82.7 m3 CH4/ton of mixed treated EFB and POME(6.8:1), corresponding to methane yield of 392 mL CH4/g VS-added. The electricity production of 1 ton mix-ture of treated EFB and POME would be 1190 MJ or 330 kW h of electricity. The study shows that there is agreat potential to co-digestion treated EFB with POME for bioenergy production.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Oil palm (Elaeis guineensis) is vastly cultivated as a source of oilin West and Central Africa, where from it is originating, and inMalaysia, Indonesia and Thailand. In Malaysia, oil palm is one ofthe most important commercial crops. The process to extract theoil requires significantly large quantities of water for steam steril-izing the palm fruit bunches and clarifying the extracted oil. Oilpalm mill plant requires a large amount of water for its operationand discharge considerable quantities of wastewater or palm oilmill effluent (POME). POME is an important source of inland waterpollution when released into rivers or lakes without treatment.One ton of oil palm fruit approximately 0.87 m3 POME is generatedor 3.7 ton of effluent per ton of oil produced [1]. Oil palm mill plantalso generates large amount of solids wastes such as empty fruitbunch (EFB) (23%), mesocarp fiber (12%) and shell (5%) for everyton of fresh fruit bunches (FFBs) processed in the mills [2]. Malay-sian palm oil industry produced approximately 52.3 million ton oflignocellulosic biomass from milling process and 12.6 million ton isan EFB with moisture content 65% from milling process [3]. Thus,

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the utilized of EFB and POME has gained interest from manyresearchers due to the abundant amount generated in the mills [4].

Currently, most of palm oil mill plants use POME as feedstock toproduce biogas in Malaysia and Thailand [5,6]. However, POME haslimited availability during low oil palm fruit production period(December–February). The shortage of feedstock has becomemajor bottleneck for achieving the goals; therefore, feedstockalternatives need to be developed. Although there are a few meth-ods available for solid biomass residues utilization from palm oilindustry, over 90% remains unused [7,8]. EFB is a nonwood ligno-cellulosic material, which can be utilized as a renewable energysource for biogas production [9]. EFB has high content of organicmatter; and has potential to be used for biogas production[10,11]. Co-digestion of EFB and POME in a single treatment stepwould simplify the technical and economical requirements forthe transformation of both residues into biogas. However, the bio-degradability of EFB is low due to its lignocellulosic composition,and it needs to be significantly improved before EFB can be effi-ciently used for biogas production.

Lignocellulosic biomass has a rigid structure with tight associa-tion between lignin and carbohydrates, which makes the carbohy-drates resistant to enzymatic attach. Buffiere et al. [12] found adirect correlation between the absolute biodegradability and thesum of the lignin and cellulose content of different waste streams.

Table 1Characteristics of palm oil mill effluent and empty palm fruit bunches used in theexperiments.

Characteristics POME Raw EFB

Total solids (% w/w) 6.7 ± 0.2 43.6 ± 1.2Volatile solids (% w/w) 5.7 ± 0.3 39.2 ± 0.8COD (g/kg) 97.1 425Total nitrogen (g/kg) 3.2 2.2SS (g/L) 40.6 NDOil (g/kg) 8.4 4.7VFAs (g/L) 3.3 NDEthanol (g/L) 0.59 NDAlkalinity (g CaCO3/L) 2.4 NDpH 4.3 ± 0.1 5.2 ± 0.2Cellulose (% dry basis) 11 ± 1.7 39.1 ± 0.8Hemicellulose (% dry basis) 7 ± 0.6 22 ± 1.2Lignin (% dry basis) 42 ± 0.3 23 ± 0.7

ND = Not determined.

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The higher lignin and cellulose content, the lower biodegradabilitywas obtained. Different pretreatment methods have been provedeffective for improving the biodegradability of lignocellulosic mate-rials [13]. Our previous research showed that steam pretreatmentcould significantly improve biodegradability and enhance biogasproduction of biofibers [14]. Steam treatment with NaOH presoa-king has been reported to increase the biogas production of sortedmunicipal solid waste by 50% [15]. Alkaline hydrolysis with NaOHhas been successfully applied to treat lignocellulosic materials suchas straw or hardwood [16]. Mechanical treatment (milling) provedto be suitable for applications at full scale biogas plants and in-creased the methane yield of lignocellulosic substrates by up to25% [17,18]. Therefore, pretreatments facilitating the accessibilityof holocellulose (cellulose and hemicelluloses) could result in in-crease of the biogas production. The application of pretreatmenton EFB for improved of biogas from EFB has not been reported.

Co-digestion strategies and pretreatments of the substrate to im-prove the degradability of the lignocellulosic materials are thereforerequired to increase biogas yields. Macias-Corral et al. [19] studiedthe anaerobic co-digestion of dairy cow manure, the organic fractionof municipal waste and cotton gin waste. Maximum biogas produc-tion was obtained with cow manure and cotton gin waste (87 m3

CH4/ton of dry waste). The treatment of solid slaughterhouse waste,fruit–vegetable waste and manure in a co-digestion process resultedin methane yields in the range of 0.27–0.35 m3 CH4/kg VS added andVS reductions of between 50% and 67% [20]. Co-digestion of variousorganic wastes for energy production has several advantages com-pared to single substrate digestion such as improved biogas yield,economic advantages derived from the sharing of equipment, easierhandling of mixed wastes, and synergistic effect [21,22].

The current study aimed to determine the potential of usingEFB, a by product from palm oil mill plant for co-digestion withPOME. EFB was treated with different methods (chemical, mechan-ical, hydrothermal, and combination of hydrothermal and chemi-cal) for increasing the methane production potential. Theperformance and synergistic effect of co-digestion of treated EFBwith POME for efficient bioenergy production was determined.

2. Materials and methods

2.1. Materials and pre-treatment procedures

The palm oil mill effluent (POME) and empty fruit bunches(EFB) used in this study were collected from a palm oil mill plant,in Malaysia. POME was stored at the temperature of 4 �C for lateruse. POME is brownish, slurry, viscous, acidic and high oil andgrease containing. The characteristics of POME and EFB wereshown in Table 1. EFB had a moisture content of 60–75%. EFB werecrushed in a blender for reducing the particle size and resulted in amixture of threads and particles of a size smaller than 5 mm. Forchemical treatment, EFB was treated with 1% NaOH (w/w). Forhydrothermal treatment, 200 g wet weight of EFB was soaked withwater and subsequently steam treated at temperature of 230 �C for15 min in pressure vessel. The pressure vessel was equipped with atemperature sensor, a valve for steam introduction, a valve forsteam release and valve for collection of the treated liquid fraction(called hydrolysate) [13]. Combination of hydrothermal and chem-ical, EFB was presoaking with 0.1% NaOH and then placed in thelaboratory at ambient temperature for 1 day and process withhydrothermal treatment described above.

2.2. Experimental design

The biodegradability and biogas potential of EFB, POME, co-digestion of EFB with POME and co-digestion of treated EFB with

POME were determined in batch assays under thermophilic condi-tion as described previously by Angelidaki et al. [23]. For new sub-strates with unknown degradation characteristics, a number ofdifferent VS concentration of the substrate are required, to ensurethat the methane potential of the substrates is not underestimateddue to overload or potential inhibition [24]. POME was tested atdifferent VS content at 1.3%, 2.6% and 3.9% of VS correspondingto initial organic load of 13, 26 and 39 g VS/L, respectively. EFBwas tested at different VS content of 2% and 4% of VS correspondingto initial organic load of 20 and 40 g VS/L, respectively. Co-diges-tion of raw EFB with POME was tested at different mixing ratiosof EFB/POME (0.4:1–11:1). Biochemical methane potential assaytreated EFB samples (a mixture of solid and liquid fraction of trea-ted EFB) was co-digested at mixing ratios of EFB/POME (6.8:1). Themaximum VS in batch tested was 4.6% of VS corresponding to or-ganic load of 46 g VS/L, in order to minimize the effect from shockload and avoid acidification of the process [23]. The assay was con-ducted as batch cultivations in 320 mL serum bottles. In each bot-tle, 80 mL of inoculum and 20 mL of substrate/water mixture wereadded. The inoculum for the batch assays was collected from ther-mophilic biogas plant (55 �C) digesting manure. Thermophilicinocula placed in an incubator for 5 days until the biogas produc-tion ceased, in order to minimize the contribution from residual or-ganic materials contained in the inoculum. The inoculumcontained 7.9% total solids (TS), 6.1% volatile solids (VS), and53.5 g/L volatile suspended solids (VSS). Inoculum and sampleswere inserted in the test bottles and were subsequently purgedwith N2:CO2 (80:20) to ensure anaerobic conditions. Afterwardsthe bottles were closed with butyl stoppers and placed in a 55 �Cincubator for 45 days. Controls, with water added instead of sam-ple were included. Additionally, for testing the inoculum qualitypositive controls with added glucose and cellulose instead of sam-ple were also included. Methane production in the headspace ofthe vials was monitored. The gas produced by the control vialswith inoculum only, was subtracted from the actual gas producedthrough digestion of each treatment.

2.3. Analytical methods and calculation

The methane content was analyzed by a gas chromatograph(GC) equipped with a flame ionization detector [25]. Gas measure-ment was reported in STP conditions (standard temperature andpressure, 273 K, 1.01325 Pa). Methane and energy yields are ex-pressed as m3 CH4/ton wet weight and KW h/ton wet weight,respectively, where wet weight indicates the wet weight of un-treated substrates. The thermal energy content of the methanewas calculated using the lower calorific value 50.1 MJ/kg CH4.

650 S. O-Thong et al. / Applied Energy 93 (2012) 648–654

The mass balance was made on VS basis as described by Cullis et al.[26]. Chemical oxygen demand (COD), total solids (TS), volatile sol-ids (VS), volatile suspended solids (VSS), pH, total nitrogen andalkalinity were measured according to standard methods for theexamination of water and wastewater [27]. In case of POME isacidic wastewater, TS and VS determination were made afterincreasing the pH of the POME and drying was performed at a max-imum temperature of 90 �C instead of 105 �C, until constant weightto decrease volatility of volatile fatty acids [23]. The contents of lig-nin, cellulose, and hemicellulose were determined according to theprocedures proposed by Van Seot et al. [28]. Theoretical methanepotential was calculated according Bushwell’s formula which is de-rived by stoichiometric conversion of the compound to CH4, CO2

and NH3 [29]. It is assumed that the averaged chemical composi-tions of VS in POME and EFB are VSlipid(C57H104O6),VSprotein(C5H7O2N), VScarbohydrate(C6H10O5), and VSVFA (C2H4O2),assuming glycerol trioleate, gelatine, cellulose and acetate as therepresentatives for oil, proteins, carbohydrates and VFA respec-tively. The synergetic effect was calculated using the proportionsof EFB and POME in the mixtures and methane production of pureEFB and POME as a control [30].

3. Results and discussion

3.1. Methane potential of EFB and POME

The analysis results of POME and EFB in Table 1 shows thatPOME was a concentrated substrate with high content of lipidwhich could potentially inhibit the process. The fresh POME newlyreleased from the process has a temperature of 80–90 �C, thus itwould be economical to apply thermophilic digestion due to lesscooling requirement and faster bacterial growth rate [31]. Cumula-tive methane production under thermophilic condition obtainedfrom POME and EFB are shown in Fig. 1. Methane production fromPOME was relatively high and produced earlier than EFB. Themethane production yield per amount of organic waste (VS) ofPOME at initial organic loading of 13, 26 and 39 g/L were foundto be 503, 482 and 378 mL CH4/g VS-added, respectively (Table 2).The methane potential of POME was decreased when increased ini-tial VS loading. Low methane yield at high substrate concentrationwhich indicated that they had potential to inhibit the processwhen overloaded. POME was a concentrate substrate with highcontent of lipid and low pH which could potentially inhibit or over-load the process and resulting to decrease in biodegradability [32].The maximum methane potential of POME (33.2 m3 CH4/ton-POME) was from the batch with initial organic loading of

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Fig. 1. Cumulative methane production of palm oil mill efflue

13 g VS/L. The methane production from POME was excellent bothin respect to methane yield (503 mL CH4/g VS-added) and biogasquality with methane content higher than 65%. The methane pro-duction yield from the control (initial organic loading of 20 g-cellu-lose/L), was 415 mL CH4/g cellulose-added which fits well with thetheoretical methane yield thus ensuring of the validity of the assay(405 mL CH4/g cellulose). Due to the high oil content in POME, themethane production was higher than in a control with cellulose[33]. The methane production from POME was also high produc-tion rate, 90% volume of biogas was produced the first 12–14 days.

The methane yield of EFB at initial organic loading of 20 and40 g VS/L was 202 and 153 mL CH4/g VS-added, respectively (Ta-ble 2). Low methane yield at high substrate concentration was alsoobserved in EFB, which indicated that they had potential to inhibitthe process when overloaded. The explanation could be that thehigh content of long-chain fatty acid (LCFA) in the EFB could alsoinhibit the degradation process. It has previously been reportedthat the lipid-rich waste contains long chain fatty acids; especiallypalmitate (higher than 50 mg/g dry weight) and oleate (higherthan 200 mg/L) could inhibit bacterial growth and methane forma-tion [34,35]. The maximum methane potential (79.1 m3 CH4/tonEFB) per mass of EFB was found at initial organic loading of20 g VS/L (Fig. 1). The volatile solid (VS) of the EFB was 39.2% w/w of wet weight indicated that EFB was a high energy contentsubstrate.

At the end of the 45 days digestion, the methane yield obtainedin POME was 1–3 times greater than that of EFB. POME was easilydegradable, and the methane development was fast. On the con-trary EFB was poorly biodegradable due its lignocellulosic struc-ture and VS of EFB consists of a higher portion of biofibers.Biofibers in EFB are more recalcitrant compared to the organicmatter in POME. Besides EFB low biodegradability the total nitro-gen is 2.2 g/kg EFB which might be might be too low to supportmicrobial growth [36]. In terms of the composition of POME andEFB in Table 1, they had theoretical methane potential, 525 and422 mL CH4/g VS, respectively. However, not all of organic mattercontained in POME and EFB could be completely degraded andconverted into methane. As indicated by the biodegradability (Ta-ble 2), 72–95% of theoretical methane potential was achieved forPOME, while only 36–48% of theoretical methane potential wasachieved for EFB.

3.2. Methane potential of co-digestion raw EFB with POME

The methane production rate of EFB was low and slow, this wasdue to its lignocellulosic nature, which makes it difficult to

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Table 2Summary of biochemical methane potential methane yield and biodegradability of EFB, POME, co-digestion EFB with POME and co-digestion treatedEFB with POME.

Initial loading(g VS/L)

BMP yield(mL CH4/g VS)

Theoretical yield(mL CH4/g VS)

Biodegradability(%)

Control (cellulose) 20 405 415 98Control (glucose) 20 317 350 90POME 13 503 525 95POME 26 482 525 92POME 39 378 525 72EFB 20 202 422 48EFB 40 153 422 36

Co-digested raw EFB with POMERaw EFB 39 161 422 38EFB:POME (0.4:1) 46 340 482 70EFB:POME (0.8:1) 46 307 470 65EFB:POME (2.3:1) 46 276 437 63EFB:POME (6.8:1) 46 197 431 45EFB:POME (11:1) 46 175 427 40

Co-digestion treated EFB with POME at ratios 6.8:1NaOH- EFB 39 220 422 52NaOH-EFB:POME 46 308 431 72Hydrothermal-EFB 39 208 422 49Hydrothermal-EFB:POME 46 277 431 64Hydrothermal-NaOH-EFB 39 252 422 60Hydrothermal-NaOH-EFB:POME 46 392 431 91

Ratios based on weight of volatile solid (VS).

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Fig. 2. Methane production potential of co-digestion raw EBF and POME contributed by EFB, POME and synergetic effect; (A) cumulative methane production and (B)methane yield (T-MP) total methane potential (POME-MP) methane potential from POME (EFB-MP) methane potential from EFB (Syn-MP) synergetic methane potential.

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652 S. O-Thong et al. / Applied Energy 93 (2012) 648–654

degrade. This along with the slow biodegradability of EFB was an-other indication of the refractory nature of the EFB. Therefore, co-digestion EFB with easy biodegradable organic (POME) would berequired for increasing the biodegradability of EFB. Empty fruitbunches (EFB) was tested for the feasibility of co-digestion witheasy biodegradable organic (POME) for enhanced microbialhydrolysis by thermophilic acidogens. Methane production wasincreased by all the mixing ratios. The methane yield was 276–340 mL CH4/g VS-added for co-digestion EFB with POME at ratiosof 0.4:1–2.3:1, while minor improvement was observed at ratiosof 6.8:1 and 11:1 (175–197 mL CH4/g VS-added). The best resultsof methane production were achieved when co-digestion EFB withPOME at ratios of 0.4:1 (Fig. 2). Co-digestion of EFB with POME en-hanced microbial biodegradability and resulted in 25–32% highermethane production at EFB/POME mixing ratios of 0.4:1, 0.8:1and 2.3:1 on VS basis than digesting EFB alone. The methane po-tential from the EFB and POME was separately digestion in eachmixing ratios, the increasing of methane production achieved byco-digestion was attributed to increasing in biodegradability ofEFB by co-digestion or synergetic methane potential. The syner-getic methane potential of co-digestion of EFB with POME at mix-ing ratios of 0.4:1, 0.8:1, 2.3:1, 6.8:1 and 11:1 were 26.3, 27.8, 18.6,0.5 and �4.4 mL CH4/g VS-added, respectively. The increasing inbiodegradability of EFB was calculated according to Speece [30]

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Fig. 3. Methane production potential of co-digestion treated EBF and POME at ratio oproduction and (B) methane yield (T-MP) total methane potential (POME-MP) methanemethane potential.

and their increasing was 38%, 27% and 25% of VS contained inEFB when co-digestion of EFB with POME at mixing ratios of0.4:1, 0.8:1 and 2.3:1, respectively. The increased biodegradabilitycould have been due to an active microbial activity supported bythe readily biodegradable organics in POME. The microbial activitywould contribute to higher hydrolytic capacity of the digestioncontributing to higher release of holocellulose from the lignocellu-losic composition of EFB [34,37].

Co-digestion of EFB with POME was found to be feasible in thethermophilic acidogenic hydrolysis of lignocellulosics in the EFB upto mixing ratio of 2.3:1 on VS basis corresponding to 0.3:1 on wetweight basis. The methane yields and biodegradability of co-diges-tion EFB with POME at mixing ratio more than 6.8:1 on VS basiscorresponding to 1:1 on wet weight basis was not significant dif-ference (P < 0.05) in methane production when compared withdigestion EFB alone. From large amount of EFB and POME at millsco-digestion of EFB with POME at mixing ratio 6.8:1 could be moreeconomic benefit, then pretreated EFB is need for further improv-ing biodegradability of EFB and methane production.

3.3. Methane potential of co-digestion pre-treated EFB with POME

Co-digestion of treated EFB with POME resulted in betterimprovement on biodegradability and methane production. Meth-

FB+POME HTNaOH-EFB+POME

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S. O-Thong et al. / Applied Energy 93 (2012) 648–654 653

ane yield has improved by 98% counting up to 392 mL CH4/g VS-added after pretreatment by NaOH presoaking and hydrothermaltreatment compared with that of untreated EFB (197 mL CH4/g VS-added). On the other hand, co-digestion NaOH treated EFBand hydrothermal treated EFB with POME resulted in 56% and40% improvement of methane production, respectively. Amongthe pretreatment methods, hydrothermal treatment of NaOHpresoaking EFB resulted in the highest conversion rate (91%) andresulted in 46% increased in biodegradability compared to co-digestion with raw EFB. The methane production contributed bytreated EFB, POME, and the synergetic effect in the digestion werecalculated and shown in Fig. 3. The 35–65% increasing in methaneproduction was attributed by pretreatment, while the 5–33%increasing in methane production was attributed by the synergisticeffect of co-digestion with POME.

Co-digestion of treated EFB by NaOH presoaking and hydrother-mal treatment with POME at mixing ratio of 6.8:1 was feasible, thehighest methane production achieved was 82.7 m3 CH4/ton ofmixed EFB and POME, corresponding to methane yield of 392 mLCH4/g VS-added. Considering an energy content of 36 MJ per m3

CH4, the energy content in the produced methane would be2977 MJ/ton with a conversion efficiency of approx. 40% in a gasmotor, the electricity production of mixture 0.5 ton of EFB and0.5 ton POME (1:1) would be 1190 MJ or 330 KW h of electricity.The study shows that there is a great potential to co-digestion trea-ted EFB with POME for bioenergy production. Hydrothermal treat-ment with NaOH presoaking may have converted part of lignin intoacetic acid due to oxidative treatments in alkaline conditions con-vert carbohydrates and lignin into carboxylic acids [37]. The pre-sented of the acetic acid in the steam treated material explainsthe high conversion rate of this material into methane, as aceticacid can directly be utilized by aceticlastic methanogens.

4. Conclusions

POME easily biologically converted to methane with maximummethane potential of 502 mL CH4/g VS-added corresponding to33.2 m3 CH4/ton POME and 98% biodegradability. Raw EFB resistantbiologically converted to methane with maximum methane poten-tial of 202 mL CH4/g VS-added corresponding to 79.1 m3 CH4/tonEFB with 38% biodegradability. Co-digestion of treated EFB by NaOHpresoaking and hydrothermal treatment together with POME atmixing ratios of 6.8:1 on VS basis corresponding to 1:1 on volumebasis had a high synergetic effect with highest methane potential392 mL CH4/g VS-added corresponding to 82.7 m3CH4/m3 mixture.Methane recovery from hydrothermal pretreated with NaOH pres-oaking EFB was 91% of VS. Biodegradability was improved by 46%compared to untreated EFB. Pretreatment improved the energy gainby 98% compared to untreated EFB. Co-digestion of treated EFB byNaOH presoaking and hydrothermal treatment together with POMEcould be effective way to improve buffer capacity and could also bean effective way to achieve high biogas production. The addition ofreadily biodegradable organic matter (POME) into treated EFB couldsignificantly increase biogas production.

Acknowledgements

I would like to thank Hector Garcia for technical assistance. Thiswork was supported by the Danish Energy Council and JST-ARDAInternational Project.

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