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Page 1: Biogas Production from Two-stage Anaerobic Digestion of               Jatropha curcas               Seed Cake

This article was downloaded by: [Erciyes University]On: 21 December 2014, At: 03:50Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

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Biogas Production from Two-stageAnaerobic Digestion of Jatropha curcasSeed CakeN. Sinbuathong a b , P. Sirirote c , B. Sillapacharoenkul d , J.Munakata-Marr e & S. Chulalaksananukul fa Scientific Equipment and Research Division , Kasetsart UniversityResearch and Development Institute (KURDI), Kasetsart University ,Bangkok , Thailandb KU-Biodiesel Project, Center of Excellence for Jatropha, KasetsartUniversity , Bangkok , Thailandc Department of Microbiology, Faculty of Science , KasetsartUniversity , Bangkok , Thailandd Department of Agro-Industrial Technology, Faculty of AppliedScience , King Mongkut's University of Technology North Bangkok ,Bangkok , Thailande Civil and Environmental Engineering, Colorado School of Mines ,Golden , Colorado , USAf Department of Chemical Engineering, Faculty of Engineering ,Mahidol University, Salaya Campus , Nakornpathom , ThailandPublished online: 24 Sep 2012.

To cite this article: N. Sinbuathong , P. Sirirote , B. Sillapacharoenkul , J. Munakata-Marr & S.Chulalaksananukul (2012) Biogas Production from Two-stage Anaerobic Digestion of Jatrophacurcas Seed Cake, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 34:22,2048-2056, DOI: 10.1080/15567036.2012.664947

To link to this article: http://dx.doi.org/10.1080/15567036.2012.664947

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Page 2: Biogas Production from Two-stage Anaerobic Digestion of               Jatropha curcas               Seed Cake

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Page 3: Biogas Production from Two-stage Anaerobic Digestion of               Jatropha curcas               Seed Cake

Energy Sources, Part A, 34:2048–2056, 2012

Copyright © Taylor & Francis Group, LLC

ISSN: 1556-7036 print/1556-7230 online

DOI: 10.1080/15567036.2012.664947

Biogas Production from Two-stage Anaerobic

Digestion of Jatropha curcas Seed Cake

N. SINBUATHONG,1;2 P. SIRIROTE,3

B. SILLAPACHAROENKUL,4 J. MUNAKATA-MARR,5 and

S. CHULALAKSANANUKUL6

1Scientific Equipment and Research Division, Kasetsart University Research

and Development Institute (KURDI), Kasetsart University, Bangkok, Thailand2KU-Biodiesel Project, Center of Excellence for Jatropha, Kasetsart University,

Bangkok, Thailand3Department of Microbiology, Faculty of Science, Kasetsart University,

Bangkok, Thailand4Department of Agro-Industrial Technology, Faculty of Applied Science, King

Mongkut’s University of Technology North Bangkok, Bangkok, Thailand5Civil and Environmental Engineering, Colorado School of Mines, Golden,

Colorado, USA6Department of Chemical Engineering, Faculty of Engineering, Mahidol

University, Salaya Campus, Nakornpathom, Thailand

Abstract Digestion of Jatropha curcas seed cake was investigated in two-stage (aci-dogenic and methanogenic) anaerobic bioreactors without pH adjustment. Acidogenic

reactors were fed once daily with a slurry of 1:10 Jatropha curcas seed cake:watercontaining approximately 100 g of chemical oxygen demand/l. Organic loading rates

were 2.5, 3.3, 5, 10, and 20 kg chemical oxygen demand/m3.day, which correspondedto hydraulic retention times of 40, 30, 20, 10, and 5 days, respectively, for each

reactor stage. The maximum methane yield (340 l at STP/kg of chemical oxygendemand degraded) was observed at an organic loading rate of 3.3 kg chemical oxygen

demand/m3.day (hydraulic retention times D 30 days for each stage). At this organic

loading rate and hydraulic retention time, the chemical oxygen demand degradationefficiency was 65%. The average pH in the acidogenic and methanogenic reactors was

4.9 and 7.4, respectively. This study demonstrates high methane yield and degradationextent of Jatropha curcas seed cake in a two-stage anaerobic process without chemical

addition for pH adjustment.

Keywords agricultural waste, anaerobic digestion, bioenergy, biogas, Jatropha cur-

cas, methane, two-stage operation

Introduction

Every year in the world several million tons of agricultural wastes are disposed of through

methods, such as incineration, land application, and land filling. This global waste has a

high potential as a biorenewable energy resource and can be turned into high-value by-

Address correspondence to Dr. Nusara Sinbuathong, Scientific Equipment and ResearchDivision, Kasetsart University Research and Development Institute, Kasetsart University, Bangkok10900, Thailand. E-mail: [email protected]

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Anaerobic Digestion of Jatropha curcas Seed Cake 2049

products (Isci and Demirer, 2007). Jatropha curcas is a drought-resistant shrub belonging

to the family Euphorbiaceae, which is cultivated on a large scale in Central and South

America, Southeast Asia, India, and Africa (Schmook and Seralta, 1997). Jatropha curcas

seed cake is one of the agricultural wastes considered as a possible energy source. The

seed cake is a by-product of oil extraction from the seeds; the oil can be used as a

substitute for diesel after transesterification (Singh et al., 2008). With the known high

potential of Jatropha curcas for energy production, researchers have generally focused on

the production of biodiesel (Achtena et al., 2008). A few studies have investigated biogas

production from Jatropha curcas seed cake. Most of these studies were conducted using

batch operation and single stage semi-continuous operation. In general, they conclude

that Jatropha curcas seed cake is a good biogas source, due to the high conversion rates

and efficiencies obtained (Staubmann et al., 1997; Singh et al., 2008; Sinbuathong et al.,

2010, 2011).

The solids concentration of Jatropha curcas seed cake is crucial to ensure sufficient

gas production. However, high solids content may cause a system failure due to the acidic

pH of the seed cake slurry (Gunaseelan, 2009; Sinbuathong et al., 2010, 2011). In the

previous studies, the initial pH of the Jatropha curcas slurry needed to be adjusted to

neutral during the start-up period in order to prevent system failure (Sinbuathong et al.,

2010, 2011). For batch operation, the appropriate Jatropha curcas seed cake-to-water ratio

for methane (CH4) production was found to be in the range of 1:20 to 1:10 (Sinbuathong

et al., 2011). For a single-stage semi-continuous operation, the organic loading rates

(OLRs) were found to be optimal between 1.25 and 1.67 kg chemical oxygen demand

(COD)/m3.d (Sinbuathong et al., 2010). In the present study, higher OLRs were applied

to the two-stage system under the assumption that phase separation may be appropriate

for the digestion of the acidic slurry of Jatropha curcas seed cake, because acidogenic

bacteria favor an acidic aqueous environment in the first phase as described below.

The anaerobic biodegradation is carried out by three groups of bacteria: (1) hydrolytic

and fermentative bacteria, which hydrolyze the long chain molecules and ferment the

resulting monosaccharides to organic acids; (2) acetogenic bacteria, which convert these

acids to acetate, hydrogen (H2), and carbon dioxide (CO2); and (3) methanogenic bacteria,

which convert the end products of acetogenic reactions to methane (CH4) and carbon

dioxide (CO2). Several studies have proposed the physical separation of these phases

in order to increase the degradation of organic matter, improve biogas production, and

attain better control of operating conditions (Derimelk and Yenigun, 2002; Kasapgil and

Ince, 2000; O’Keefe and Chynoweth, 2000; Yu et al., 2002). The metabolic pathways

of the two-stage anaerobic digestion process are the same as those of conventional

digestion; however, they are physically separated in (1) an acidogenic stage (hydrolytic

and acetogenic stage) and (2) a methanogenic stage. The two-stage anaerobic treatment

process has several advantages over conventional processes. First, it permits the selection

and enrichment of different bacteria in each digester; in the first phase, acidogenic bacteria

degrade complex pollutants into volatile fatty acids (VFAs), which are subsequently

converted to CH4 and CO2 by acetogenic and methanogenic bacteria in the second phase.

Second, it increases the stability of the process by controlling the acidification phase in

order to prevent overloading and the build-up of toxic material. Third, the first stage

may act as a metabolic buffer, preventing pH shock to the methanogenic population;

in addition, low pH and a high OLR are all factors that favor the establishment of the

acidogenic phase.

This research experimentation was carried out in a semi-continuous, laboratory-

scale, two-reactor system operated at five different OLRs. The research objectives were

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2050 N. Sinbuathong et al.

to observe the CH4 yield and the organic degradation efficiency from two-stage anaerobic

reactors and identify a method of operation that did not require addition of any chemicals

for pH adjustment.

Materials and Methods

Seed Cake Characterization

The most important parameters affecting CH4 production are the composition of feed-

stock. The analyses of the seed cake include moisture content, total solids (TS), total

volatile solids (TVS), organic carbon, organic matter, nitrogen, and phosphorous. The

moisture content of the samples was determined by oven-drying to a constant weight

at 105ıC. Total solid content was calculated as 100% � % moisture content. TVS was

obtained by igniting the TS in a muffle furnace at 550ıC. Organic carbon in the sample

was measured using the Walkley-Black method (Buurman et al., 1996; Walkley, 1947).

Organic carbon was oxidized with a mixture of potassium dichromate and sulfuric acid;

the excess potassium dichromate was titrated with ferrous sulfate. The organic matter

content of soil was indirectly estimated through multiplication of the organic carbon

content by 1.72 (Soil Science Society of America and American Society of Agronomy,

1996). Nitrogen was determined by the Kjeldahl method by digesting samples to convert

organic-N to NHC

4-N and determining NHC

4-N in the digest (Walkley, 1947). Total

phosphorous was determined by digesting samples with sulfuric acid and analyzed by an

ascorbic acid method (Rayment and Higginson, 1992).

Inoculum, Feed Solution, and Test Bioreactors

Fresh cow dung was collected and brought back to the laboratory in bags. For experi-

ments, sufficient water was added to cow dung at a ratio of 1:1 by weight to produce a

slurry. Then biomass (as mixed liquor volatile suspended solids; MLVSS) was measured

in order to start each test bioreactor with the same cell mass. Jatropha curcas seed cake

was collected from Prathumthani Province, Thailand. The seed cake was stored in a

plastic bag at room temperature and was blended prior to use.

Five sets of reactors were constructed with plastic bottles. Each set consisted of two

reactors, an acidogenic and a methanogenic reactor with a working volume of five liters

each (Figure 1). The acidogenic reactor was equipped with two outlet ports, one port for

gas venting and the other port for digested slurry, both of which fed to the methanogenic

reactor. The methanogenic reactor was connected to a gas collection system, which was

based on water displacement by the exiting gases. Sulfuric acid of 0.05 molar was used

to measure the displacement by gas in the gas collection system.

Jatropha curcas seed cake was prepared as a slurry with tap water at a ratio of

1:10 by weight. The initial COD and TVS content of this slurry were 100 and 110 g/l,

respectively. The initial pH of the seed cake slurry was 5.5 and the pH was not adjusted

during the entire period of the experiment. The conditions were 13.8 g/l MLVSS, initial

pH 5.5, and temperature 30 ˙ 1ıC.

Two-Stage Operation

Each bioreactor was filled with a liter of mixture of the culture that contained 13.8 g

MLVSS/l. Initially, the Jatropha curcas seed cake slurry was added to the acidogenic

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Anaerobic Digestion of Jatropha curcas Seed Cake 2051

Figure 1. Two-stage experiment set up; reactors I and II were acidogenic and methanogenic

reactors, respectively. (color figure available online)

reactor at a rate of 2 liters/day for 2 days. This acidogenic reactor was operated for 2

days in a batch mode before feeding with the Jatropha curcas seed cake slurry semi-

continuously in an upflow mode by feeding once per day at a feed rate of 1,000, 500,

250, 167, and 125 ml/day in each set of reactors, giving rise to OLRs of 20, 10, 5,

3.3, and 2.5 kg COD/m3.day and the corresponding increasing hydraulic retention times

(HRTs) of 5, 10, 20, 30, and 40 days in both acidogenic and methanogenic reactors. The

ambient temperature of all reactors was 30 ˙ 1ıC and the reactors functioned without

pH control. When the system reached steady state, the total gas production was recorded

(at room temperature) daily and the CH4 content was determined by a Shimadzu GC-

14B gas chromatograph equipped with a thermal conductivity detector. The CH4 volumes

were then adjusted to standard temperature and pressure (STP). The operating period was

approximately 100 days. The digested slurry from the methanogenic reactor was analyzed

for COD, TVS, and pH according to the procedure of the Standard Methods (APHA,

AWWA, and WEF, 2005). All experiments were conducted in duplicate and the results

were calculated using the mean of the experimental values. Organic waste degradation

(in terms of COD and TVS degradation) and CH4 production from the system at various

OLRs were used as indicators of reactor performance. The CH4 yield was calculated and

reported in terms of CH4 produced/kg COD (and kg TVS) degraded and CH4 produced/kg

seed cake added to the reactor.

Results and Discussion

The most important factors impacting biogas production are the composition and quantity

of substrate. Both parameters were influenced by the substrate used and the OLR applied.

In this study, analysis of the seed cake showed that it had 8.8% moisture, 91.2% TS, of

which 82% was TVS. The cake was fairly high in organic matter (68.9%) and, therefore,

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2052 N. Sinbuathong et al.

Table 1

Performance data of two-stage anaerobic digestion of Jatropha curcas seed cake at

various organic loading rates

Organic loading rate,

kg COD/m3.day

Parameters 2.5 3.3 5 10 20

HRT (days) for each acidogenic and

methanogenic reactor

40 30 20 10 5

Reactor volume (ml) for each

acidogenic and methanogenic

reactor

5,000 5,000 5,000 5,000 5,000

Daily feed (ml slurry/day) 125 167 250 500 1,000

Initial COD (g/l) 100 100 100 100 100

COD at steady state (g/l) 34.2 35.4 51.5 86 85.5

COD degradation efficiency (%) 66 65 48.5 14 14

Initial TVS (g/l) 110 110 110 110 110

TVS at steady state (g/l) 27.6 27.9 38.4 52.0 56.0

TVS degradation efficiency (%) 75 75 65 53 49

Initial pH of the slurry 5.5 5.5 5.5 5.5 5.5

Average pH of acidogenic reactor at

steady state

4.9 4.9 4.8 4.7 4.7

Average pH of methanogenic

reactor at steady state

7.4 7.4 7.0 4.8 4.8

Average CH4 (%) 63 62 61 11 9

CH4 production (ml at STP/day) 2,740 3,680 3,660 175 140

CH4 production rate (ml at

STP/liter.day)

550 736 732 35 28

CH4 yield (liter at STP/kg COD

degraded)

333 340 302 25.4 9.6

CH4 yield (liter at STP/kg TVS

degraded)

265 267 203 6 2.5

CH4 yield (liter at STP/ kg seed

cake added)

240 242 161 4 2

had good potential for biogas generation. The C and N contents were 39.9 and 3.3%,

respectively; thus, the C/N ratio was 12. The results of the digester performance in

terms of organic waste reduction and biogas production at various OLRs are shown in

Table 1.

Organic Waste Reduction

In this study, COD and TVS were used to quantify the organic strength of the waste.

Initial COD and TVS as well as the average COD and TVS of each reactor at steady

state period are as reported in Table 1. Organic waste degradation efficiency at various

OLRs is calculated and shown in Figure 2. The general trend showed a decrease of COD

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Anaerobic Digestion of Jatropha curcas Seed Cake 2053

Figure 2. COD and TVS degradation efficiency at various OLRs.

and TVS degradation efficiency with increased loading (Figure 2). An increase in OLR

from 3.3 to 20 kg COD/m3.day resulted in a decrease in COD and TVS degradation

efficiency (Figure 2 and Table 1). Increasing OLR, corresponding with decreasing HRT,

resulted in reduction of the reactor performance (Figure 2 and Table 1). However, the

reactor performance at OLR of 2.5 kg COD/m3.day is quite comparable with that at the

OLR of 3.3 kg COD/m3.day.

Reactor pH

The average pH of the slurry in both the acidogenic and methanogenic reactors of various

OLRs at steady state is shown in Figure 3. In the reactors that received OLRs of 2.5, 3.3,

and 5 kg COD/m3.day, phase separation was achieved, as evidenced by the average pH

in the acidogenic and methanogenic reactor of 4.9 and 7.3, respectively (Figure 3). These

pH values indicate the good performance of the acidogenic and methanogenic reactors.

On the other hand, in the reactors that received the OLRs of 10 and 20 kg COD/m3.day,

the environment in both reactors remained acidic. The average pH in the acidogenic and

methanogenic reactor was 4.7 and 4.8, respectively (Figure 3 and Table 1). At these higher

loading rates, the lower treatment efficiency and CH4 production were probably caused

Figure 3. pH in acidogenic and methanogenic reactors that received various OLRs during the

steady state.

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2054 N. Sinbuathong et al.

by accumulation of VFAs (because of acidic pH). This suggests that phase separation

of acidogenesis and methanogenesis was not achieved in the two reactors at these high

OLRs.

Biogas Production

Differences in the CH4 production at each OLR were observed. When the OLRs were

in the range of 2.5–5 kg COD/m3.day, the CH4 content was acceptably high (61–63%

CH4) (Table 1) but at OLRs 10 and 20 kg COD/m3.day, the reactors fed with the slurry

of Jatropha curcas seed cake produced biogas with much lower CH4 content (11 and 9%

CH4) (Table 1).

The CH4 production rate is shown in Table 1. In this study, with increasing OLRs

from 3.3 to 20 kg COD/m3.day, the CH4 production rate decreased, reaching a maximum

at OLR of 3.3 kg COD/m3.day (736 ml at STP/liter.day) with an average CH4 content

of 62% CH4 (Table 1). This is probably due to the system containing too concentrated

organic substrate at OLR of 10 and 20 kg COD/m3.day, so it cannot convert VFAs to CH4

as rapidly as they form, causing the pH to drop and resulting in lower CH4 production.

At 2.5 kg COD/m3.day, the CH4 production rate was lower (550 ml at STP/liter.day).

When the system contained less organic substrate (OLR 2.5 kg COD/m3.day) than the

optimum concentration (OLR 3.3 kg COD/m3.day), the CH4 production rate was likely

lower because readily transformed carbon sources were depleted at the lowest OLR.

Methane Yield

Based on the CH4 production and organic waste degraded, CH4 yield obtained at various

OLRs was calculated. The digester performance at various OLRs is summarized in

Table 1. The CH4 production yield reached a maximum value of 340 liters at STP/kg of

COD degraded (267 liters at STP/kg of TVS degraded or 242 liters at STP/kg seed cake

added) at the OLR of 3.3 kg COD/m3.day and significantly dropped at OLRs of 10 and 20

kg COD/m3.day. In this study, the two-stage anaerobic digestion of Jatropha curcas seed

cake without external pH control at 10 and 20 kg COD/m3.day was not feasible because

of the low CH4 yield (25.4 and 9.6 liters at STP/kg of COD degraded, respectively, or

6 and 2.5 liters at STP/kg of TVS degraded, respectively). The reactor performance of

Jatropha curcas seed cake digestion could be improved by lowering OLRs (or increasing

HRTs).

A previous study reported that the CH4 yield from de-oiled cake of Jatropha curcas

by biochemical methane potential assay was 230 liters/kg TVS added (Gunaseelan, 2009).

Singh et al. (2008) studied the biogas generation from Jatropha curcas seed cake digestion

in a single-stage operation; 333 liters of gas were produced per kg seed cake added,

which contained 66% CH4, resulting in a CH4 yield of 220 liters/kg Jatropha curcas

seed cake added. From this current study, the CH4 yield from the two-stage operation

of Jatropha curcas seed cake digestion at optimum OLR of 3.3 kg COD/m3.day is 242

liters at STP/kg seed cake added (Table 1), higher than the CH4 yield obtained from

a single-stage from their study. In the current study, the COD degradation efficiencies

were 65%, while the average pH of the digested slurry was 7.4 under the conditions

used. Sinbuathong et al. (2010) reported that for the single-stage anaerobic digestion of

Jatropha curcas seed cake with chemical pH control, an optimal CH4 yield of 340 liters

at STP/kg COD degraded was obtained at an OLR of 1.25 kg COD/m3.day. Compared

with the current study, the optimal OLR of Jatropha curcas seed cake applied in the

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Anaerobic Digestion of Jatropha curcas Seed Cake 2055

two-stage anaerobic reactor system was 2.6 times (3.3/1.25) higher than that applied to a

single-stage anaerobic reactor, and the two-stage reactor functioned without external pH

adjustment.

The advantages of the single-stage anaerobic system are that it has only one reactor

and is easier to operate. The two-stage set-up used is suitable for anaerobic digestion of

Jatropha curcas seed cake, enabling better conditions for the methanogenic phase and

allowing a high organic loading rate of Jatropha curcas seed cake. However, specific care

must be taken in transferring the slurry from the acidogenic reactor to the methanogenic

reactor because the high solids content can cause blockage of pipes. The two-stage

digestion of high solids content is technically more complex and requires a higher

capital investment. For the reasons described above, when comparing the two-stage semi-

continuous digestion system with the single-stage batch digestion for Jatropha curcas

seed cake, the advantage of single-stage batch digestion is its technical simplicity and

portability. In single-stage batch digestion, the plant solids do not need to flow through

pipes, because the reactor is loaded once and only discharged at the end of the anaerobic

process. The tank is opened, old slurry is removed, and the new charge is added. The tank

is then resealed and ready for operation. However, both batch digestion and single-stage

semi-continuous digestion of Jatropha curcas seed cake need pH adjustment. As for the

two-stage digestion system, if the system is managed properly with the recommended

OLRs, the digester can function without external pH adjustment, and it can generate high

CH4 production and high extent of organic waste degradation.

Conclusion

Two-stage anaerobic digestion improves the digestion of Jatropha curcas seed cake by

having separate reactors for the acidogenic and methanogenic stages, thus providing

flexibility to optimize each of these reactions. The system can be operated with an

acidic slurry of the Jatropha curcas seed cake at a high organic loading rate without

external pH control at OLR less than 5 kg COD/m3.day, which provides a stable system

and high methane yield. The system reached maximum efficiency at the OLR of 3.3

kg COD/m3.day. The methane production rate, the methane content of the biogas, and

the methane yield were satisfactorily high. The COD degradation efficiencies were

approximately 65%, while the digested slurry pH was just over 7.

Acknowledgments

This research is part of the KU-Biodiesel Project and was supported by Kasetsart Uni-

versity Research and Development Institute (KURDI), Kasetsart University, Bangkok,

Thailand and Toray Science Foundation (TSF), Japan.

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