effect of effluent recycle ratio in a continuous anaerobic biohydrogen production system

8
Effect of efuent recycle ratio in a continuous anaerobic biohydrogen production system Chin-Chao Chen a, * , Biswarup Sen b, c, d , Yeong-Song Chuang b , Chia-Jung Tsai c , Chyi-How Lay e a Environmental Resources Laboratory, Department of Landscape Architecture, Chung Chou University of Science and Technology, Changhwa 51022, Taiwan b Department of Environmental Engineering and Science, Feng Chia University, Taichung 40724, Taiwan c Green Energy Development Center, Feng Chia University, Taichung 40724, Taiwan d Master Program of Green Energy Science and Technology, Feng Chia University, Taichung 40724, Taiwan e Department of Chemistry and Bioengineering, Tampere University of Technology, 33720, Finland article info Article history: Received 9 February 2011 Received in revised form 8 April 2012 Accepted 9 April 2012 Available online 17 April 2012 Keywords: Mesophilic hydrogen fermentation Biohydrogen Continuously stirrer tank reactor Recycle rate Efuent recycling abstract The effect of efuent recycle on hydrogen production in an anaerobic continuous stirred tank reactor (CSTR) was investigated. The CSTR was fed on sucrose (20 g chemical oxygen demand (COD)/L) and at hydraulic retention time (HRT) of 12 h. The pH and temperature were regulated around 6.7 and 35 C, respectively. The efuent was then recycled at recycle ratios of 0, 0.2, 0.4, 0.6, 0.8 and 1.0. When the recycle ratios increased, the volatile suspended solid (VSS) concentration also increased with a peak of 14.2% at the recycle ratio 0.4 compared with the VSS 3.52 g/L without efuent recycle. However, a drastic drop in hydrogen production performance was observed with increased recycle ratio. The strategy of increasing organic loading rate by increasing the substrate concentration and shorting the HRT was then applied keeping a recycle ratio of 0.2. VSS concentration increased with increasing substrate concen- tration but as the HRT was shortened from 12 h to 2 h stepwise, VSS concentration increased rst and then decreased at HRT 8 h. The maximal hydrogen production was obtained at sucrose concentration of 20 g COD/L and HRT 6 h (organic loading rate (OLR) 66.7 g COD/L-d) with recycle ratio 0.2. The hydrogen yield, hydrogen production rate and specic hydrogen production rate values also peaked at the same conditions (sucrose 20 g COD/L and HRT 6 h) with 3.88 mol H 2 /mol sucrose, 807 mmol H 2 /L-d and 244.3 mmol H 2 /g VSS-d, respectively. This study showed peak hydrogen production performance at OLR 66.7 g COD/L-d in the recycled continuous anaerobic biohydrogen production system by controlling HRT. Thus, efuent recycling at the optimum ratio along with optimum HRT and substrate concentration can maximize the hydrogen production performance in an anaerobic biohydrogen production system. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. 1. Introduction Energy is vital to global prosperity and global energy require- ments are mostly dependent on fossil fuels, which eventually lead to foreseeable depletion due to limited fossil energy sources (Das and Veziroglu, 2008; Venkata Mohan et al., 2007). Recently renewable energy from biomass has received considerable atten- tion because of the noticeable disadvantages of fossil fuel usage. Both developing and industrialized countries consider biofuels (for e.g. ethanol and hydrogen) as important energy sources which can replace fossil fuels in future. Use of biofuels worldwide will solve issues related to energy security, environmental concerns, foreign exchange savings and socioeconomics (Demirbas and Balat, 2006). The production of biofuels in commercial scale is quite challenging and the control of the process design and management is often overlooked or difcult to solve. In most cases the individual components tend to be optimized in isolation without regard to other components (Lindsey, 2011). Often such discrete optimization leads to process failure. Therefore the development of sustainable concepts with minimum operation hurdles is necessary. Biohydrogen is recognized as an important component of the fuel market for the futures low- or non-carbon based energy systems (Urbaniec et al., 2010). Biohydrogen production via dark fermentation is attractive because of the energy saving process when compared with thermal or chemical processes. Besides dark fermentation is economically feasible because it has higher hydrogen production rates than other biological hydrogen production methods such as photo fermentation (Lay et al., 2010). However, to make the dark fermentative hydrogen production * Corresponding author. Tel.: þ886 939328028; fax: þ886 4 8378848. E-mail addresses: [email protected], [email protected] (C.-C. Chen). Contents lists available at SciVerse ScienceDirect Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro 0959-6526/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jclepro.2012.04.006 Journal of Cleaner Production 32 (2012) 236e243

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Page 1: Effect of effluent recycle ratio in a continuous anaerobic biohydrogen production system

at SciVerse ScienceDirect

Journal of Cleaner Production 32 (2012) 236e243

Contents lists available

Journal of Cleaner Production

journal homepage: www.elsevier .com/locate/ jc lepro

Effect of effluent recycle ratio in a continuous anaerobic biohydrogen productionsystem

Chin-Chao Chen a,*, Biswarup Sen b,c,d, Yeong-Song Chuang b, Chia-Jung Tsai c, Chyi-How Lay e

a Environmental Resources Laboratory, Department of Landscape Architecture, Chung Chou University of Science and Technology, Changhwa 51022, TaiwanbDepartment of Environmental Engineering and Science, Feng Chia University, Taichung 40724, TaiwancGreen Energy Development Center, Feng Chia University, Taichung 40724, TaiwandMaster Program of Green Energy Science and Technology, Feng Chia University, Taichung 40724, TaiwaneDepartment of Chemistry and Bioengineering, Tampere University of Technology, 33720, Finland

a r t i c l e i n f o

Article history:Received 9 February 2011Received in revised form8 April 2012Accepted 9 April 2012Available online 17 April 2012

Keywords:Mesophilic hydrogen fermentationBiohydrogenContinuously stirrer tank reactorRecycle rateEffluent recycling

* Corresponding author. Tel.: þ886 939328028; faxE-mail addresses: [email protected],

(C.-C. Chen).

0959-6526/$ e see front matter Crown Copyright �doi:10.1016/j.jclepro.2012.04.006

a b s t r a c t

The effect of effluent recycle on hydrogen production in an anaerobic continuous stirred tank reactor(CSTR) was investigated. The CSTR was fed on sucrose (20 g chemical oxygen demand (COD)/L) and athydraulic retention time (HRT) of 12 h. The pH and temperature were regulated around 6.7 and 35 �C,respectively. The effluent was then recycled at recycle ratios of 0, 0.2, 0.4, 0.6, 0.8 and 1.0. When therecycle ratios increased, the volatile suspended solid (VSS) concentration also increased with a peak of14.2% at the recycle ratio 0.4 compared with the VSS 3.52 g/L without effluent recycle. However, a drasticdrop in hydrogen production performance was observed with increased recycle ratio. The strategy ofincreasing organic loading rate by increasing the substrate concentration and shorting the HRT was thenapplied keeping a recycle ratio of 0.2. VSS concentration increased with increasing substrate concen-tration but as the HRT was shortened from 12 h to 2 h stepwise, VSS concentration increased first andthen decreased at HRT 8 h. The maximal hydrogen production was obtained at sucrose concentration of20 g COD/L and HRT 6 h (organic loading rate (OLR) 66.7 g COD/L-d) with recycle ratio 0.2. The hydrogenyield, hydrogen production rate and specific hydrogen production rate values also peaked at the sameconditions (sucrose 20 g COD/L and HRT 6 h) with 3.88 mol H2/mol sucrose, 807 mmol H2/L-d and244.3 mmol H2/g VSS-d, respectively. This study showed peak hydrogen production performance at OLR66.7 g COD/L-d in the recycled continuous anaerobic biohydrogen production system by controlling HRT.Thus, effluent recycling at the optimum ratio along with optimum HRT and substrate concentration canmaximize the hydrogen production performance in an anaerobic biohydrogen production system.

Crown Copyright � 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Energy is vital to global prosperity and global energy require-ments are mostly dependent on fossil fuels, which eventually leadto foreseeable depletion due to limited fossil energy sources (Dasand Veziroglu, 2008; Venkata Mohan et al., 2007). Recentlyrenewable energy from biomass has received considerable atten-tion because of the noticeable disadvantages of fossil fuel usage.Both developing and industrialized countries consider biofuels (fore.g. ethanol and hydrogen) as important energy sources which canreplace fossil fuels in future. Use of biofuels worldwide will solveissues related to energy security, environmental concerns, foreign

: þ886 4 [email protected]

2012 Published by Elsevier Ltd. All

exchange savings and socioeconomics (Demirbas and Balat, 2006).The production of biofuels in commercial scale is quite challengingand the control of the process design and management is oftenoverlooked or difficult to solve. In most cases the individualcomponents tend to be optimized in isolation without regard toother components (Lindsey, 2011). Often such discrete optimizationleads to process failure. Therefore the development of sustainableconcepts with minimum operation hurdles is necessary.

Biohydrogen is recognized as an important component of thefuel market for the future’s low- or non-carbon based energysystems (Urbaniec et al., 2010). Biohydrogen production via darkfermentation is attractive because of the energy saving processwhen compared with thermal or chemical processes. Besides darkfermentation is economically feasible because it has higherhydrogen production rates than other biological hydrogenproduction methods such as photo fermentation (Lay et al., 2010).However, to make the dark fermentative hydrogen production

rights reserved.

Page 2: Effect of effluent recycle ratio in a continuous anaerobic biohydrogen production system

Nomenclature

COD chemical oxygen demandCSTR continuous stirred-tank reactorEtOH ethanol, mg COD/LGCeTCD gas chromatographethermal conductivity detectorGCeFID gas chromatographeflame ionization detectorH2 content hydrogen content in biogas, %HPR hydrogen production rate, mmol H2/L-dHRT hydraulic retention time, hHY hydrogen production yield, mol H2/mol sucroseOLR organic loading rate, g COD/L-dORP oxidationereduction potentialSHPR specific hydrogen production rate, mol H2/g VSS-dSMP soluble metabolic product, mg COD/LTVFA total volatile fatty acids, mg COD/LVSS volatile suspended solids, g/L

Gas/liquid

Gascollection

system

Peristaltic pump

Feed

Gas/liquidseparator

EffluentCollector

(Precipitation)

Thermostat

Fermentor

Liquidsample port

Gaspump

Incubator

Gassample port

Recyclepump

Fig. 1. Configuration of the anaerobic bioreactor system for continuous hydrogenproduction.

C.-C. Chen et al. / Journal of Cleaner Production 32 (2012) 236e243 237

process more economical and viable, it is very essential that theprocess design is suitably conceived with novel strategies. Recy-cling the effluent from the fermentor not only allows the efficientmixing of themicroorganismswith the substrate but it also helps toutilize the residual organic fraction and lowering of effluent CODfor discharge. Effluent recycle is widely used in traditional anaer-obic or aerobic reactor systems, such as upflow anaerobic sludgeblanket (UASB) reactor (Yu et al., 2002), anaerobic fluidized bedreactor (Cavalcante de Amorim et al., 2009), anaerobic baffledreactor (Saritpongteeraka and Chaiprapat, 2008), anaerobic/anoxic/oxic (A2/O) system (Chakraborty and Veeramani, 2006; Baeza et al.,2004), two-phase reactor system (Romli et al., 1994) and tricklingbed reactor (Peintner et al., 2010). There have been already somereports regarding the recycling of the outlet from the bioenergyprocess as the seed of another reactor (Banks et al., 2010; Wanget al., 2011; Ngoma et al., 2011). A CSTR system is a commoncontinuous hydrogen production system which can producehydrogen continuously and efficiently at very short HRT. But tooshort HRT leads to washout of the hydrogen producers in CSTR.Therefore, effluent recycle provides a sufficient liquid velocity tofluidize the hydrogen producers to grow stably inside the reactor.On the other hand, effluent recycle could maintain enough alka-linity to reduce the requirement of base addition. Literature oneffluent recycle applied to hydrogen production systems is negli-gible (Foglia et al., 2010; Ngoma et al., 2011).

The purpose of this studywas to investigate the effect of effluentrecycle on hydrogen production in an anaerobic CSTR. The effluentrecycle ratio and organic loading rate were studied to observe thevariations in the hydrogen production by determining the VSS, HY,HPR and SHPR as the index of performance.

2. Material and methods

2.1. Seed sludge and substrate

The seed sludge for the present investigation was the effluentfrom a CSTR fed with sucrose (20e40 g COD/L) and kept inoperation for over 2 years. The initial start-up sludge for the CSTRwas obtained from Li-Ming municipal sewage treatment plant(Taichung, Taiwan) and pretreated with heat (100 �C for 45 min) toinhibit the methanogenic activity. The feed solution contained thefollowing inorganic supplements (Endo formulation, mg/L):NH4HCO3 5240, K2HPO4 125, MgCl2�6H2O 100, MnSO4�6H2O 15,FeSO4�7H2O 25, CuSO4�5H2O 5, CoCl2�5H2O, 0.125 and NaHCO36720 (Endo et al., 1982).

2.2. Experimental design and fermentor

A CSTR as shown in Fig. 1 with a working volume of 4 L wasoperated at a temperature of 35 �C and HRT 12 h. The fermentorwas constantly mixed by using the agitator gas recirculation. Theeffluent from hydrogen fermentor was pumped into a sedimenta-tion tank. The effluent in the sedimentation tank was recycled backto the CSTR containing biomass and soluble metabolic products.The effluent recycle ratio was defined as recycled-effluent quantitydivided by feed quantity. The pH of 6.7 (favourable value forhydrogen production) was adjusted by adding 0.8 g NaOH/L solu-tion. The amount of biogas produced was measured using a plasticgas bag at room temperature (25 �C) and 1 atm pressure(760 mmHg). When a steady-state condition was achieved and thedesired data were obtained, such as the hydrogen gas content andbiogas production, the HRT was then reduced. Three series ofcontinuous experiments were conducted to study the following:effect of effluent recycle ratios (experiment I), effect of substrateconcentration (experiment II) and effect of HRT (experiment III).

2.2.1. Effect of effluent recycle ratio (experiment I)The effluent recycle ratios of 0, 0.2, 0.4, 0.6, 0.8 and 1.0 were

investigated at HRT 12 h and substrate concentration of 20 g COD/L.

2.2.2. Effect of organic loading rate

a. Effect of substrate concentration (experiment II)The substrate concentrations from 20 to 40 g COD/L were

investigated at HRT 12 h using the optimal recycle ratio 0.2which was obtained in experiment I.

b. Effect of HRT (experiment III)The CSTR was started at HRT 12 h and substrate concentra-

tion of 20 g COD/L. The HRT was shortened from 12 h to 2 hstepwise.

2.3. Monitoring and analysis

Standard methods were used to determine pH, ORP, alkalinity,total COD and VSS (APHA, 1995). Ethanol and VFAs (acetate,propionate and butyrate) were analyzed in a gas chromatographequipped with flame ionization detector GCeFID (Shimadzu GC-14,Japan) and biogas composition was analyzed in a gas chromato-graph equipped with thermal conductivity detector GCeTCD(China Chromatograph 8700T). Other analytical procedures for

Page 3: Effect of effluent recycle ratio in a continuous anaerobic biohydrogen production system

Table 1Experimental data of recycle reactor at various recycle ratios at steady-state (HRT 12 h and substrate concentration 20 g COD/L).

Recycle ratio 0 0.2 0.4 0.6 0.8 1.0

ORP (�mV) 500� 32 476� 35 474� 31 469� 38 342� 27 346� 25Alkalinity (mg/L as CaCO3) 5370� 439 4870� 410 4780� 406 4480� 285 4400� 314 4260� 397H2 content (%) 50.3� 2.5 47.1� 2.9 42.3� 4.3 39.7� 2.4 34.9� 1.8 34.2� 2.3VSS (g/L) 3.52� 0.22 3.74� 0.19 4.02� 0.32 3.78� 0.22 3.55� 0.24 3.24� 0.04Sucrose degradation (%) 98.4� 0.6 99.1� 0.5 99.1� 0.4 98.8� 0.5 99.2� 0.2 99.2� 0.04Ethanol (mg COD/L) 999� 102 2568� 237 3144� 346 6975� 411 5099� 604 5479� 329Acetate (mg COD/L) 2746� 287 3067� 270 3039� 235 2627� 216 2794� 243 3267� 274Propionate (mg COD/L) 594� 64 665� 91 840� 88 795� 69 817� 81 986� 76Butyrate (mg COD/L) 9274� 982 8074� 674 5311� 637 2787� 220 3042� 313 3377� 271TVFA (mg COD/L) 12,614� 990 11,806� 709 9190� 655 6209� 423 6653� 624 7630� 343SMP (mg COD/L) 13,613� 1090 14,374� 715 12,334� 673 13,184� 430 11,752� 665 13,109� 389Butyrate/acetate ratio (mol/mol) 3.38 2.63 1.75 1.06 1.09 1.03

n¼ 5e20 (number of sampling); TVFA¼ acetateþ propionateþ butyrate; SMP¼ TVFAþ EtOH.

C.-C. Chen et al. / Journal of Cleaner Production 32 (2012) 236e243238

VFAs, ethanol and biogas assayswere the same as those indicated inour previous studies (Chen et al., 2001). Anthrone-sulphuric acidmethod was used to measure total carbohydrate concentration(Ludwig and Goldberg, 1956). For the chemostat reactor, steady-state conditions were considered to be established when theproduct concentrations were stable (less than 15% variation). The

HY

(mol

H2/m

ol s

ucro

se)

0

1

2

3

HPR

(m

mol

H2/L

-d)

0

100

200

300

SHPR

(m

mol

H2/g

VSS

-d)

020406080

100120140

VSS

(mg/

L)

3.0

3.2

3.4

3.6

3.8

4.0

4.2

Recyc0.0 0.2 0.4

HBu

/HAc

(mol

/mol

)

0

1

2

3

4

Fig. 2. Hydrogen production performance (HY, HPR and SHPR), HBu/HAc ratio and bioma20 g COD/L).

hydrogen production efficiency was evaluated using the hydrogencontent in the biogas, hydrogen yield (the ability of convertingsucrose into hydrogen), hydrogen production rate (the rate ofhydrogen production from the reactor volume, HPR) and specificHPR (the hydrogen production ability of the biomass in the reactor,SHPR).

le ratio0.6 0.8 1.0 1.2

ss (VSS) concentrations at each recycle ratio (HRT 12 h and substrate concentration

Page 4: Effect of effluent recycle ratio in a continuous anaerobic biohydrogen production system

C.-C. Chen et al. / Journal of Cleaner Production 32 (2012) 236e243 239

3. Results and discussion

3.1. Effect of recycle ratio

Table 1 summarizes the experimental data under steady-stateconditions for each recycle ratio at HRT 12 h and substrateconcentration 20 g COD/L. The results showed that the biomass(VSS) increased from 3.52 g/L to 4.02 g/L while increasing recycleratio from 0 to 0.4, which then decreased to 3.24 g/L when thereactor was operated at recycle ratio 1.0 (Fig. 2). The alkalinity,hydrogen content, HPR and SHPR decreased with an increase inrecycle ratio (Table 1 and Fig. 2). HYs of 3.39, 2.72, 1.81, 1.16, 0.70and 0.35 mol H2/mol sucrose were obtained at recycle ratios of 0,0.2, 0.4, 0.6, 0.8 and 1.0, respectively. HPRs ranged from 72 to289 mmol H2/L-d when the recycle ratio was varied from 0.2 to 0.8.The SHPR of 112.5 mmol H2/g VSS-d peaked at recycle ratio 0.Whenthe recycle ratio was increased to 0.8, SHPR decreased to17 mmol H2/g VSS-d, probably due to decreased availability of freshsubstrate for the hydrogen producing bacteria. The effluent of darkfermentation will mostly contain organics acids which are appro-priate substrate for methanogens and can produce methane withhigh rate (Kraemer and Bagley, 2005). In fact, without recycling theeffluent, best performance was observed with various HRTs andsucrose concentrations. In this study the HY of 1e3.5 mol H2/molsucrosewas obtained, which is slightly less than our previous study

VSS

(g/L

)

4

5

6

7

8

HY

(mm

ol H

2/m

ol s

ucro

se)

2.0

2.2

2.4

2.6

2.8

3.0

HPR

(mm

ol H

2/L

-d)

260

280

300

320

340

360

Substrate concentrat

15 20 25

SHPR

(mm

ol H

2/g

VSS

-d)

40

50

60

70

80

90

Fig. 3. Hydrogen production performance (HY, HPR and SHPR) and biomass (VSS) co

results (0.7e3.9 mol H2/mol sucrose) (Lin et al., 2006). This revealsthat hydrogen production without recycling of effluent could givehigher yields.

VFAs and alcohols were the main soluble metabolic products.The main SMP observed was butyrate in this study. The butyrateconcentration decreased from 9274 mg COD/L to 3042 mg COD/Lwith increasing recycle ratio from 0 to 0.8. However, the ethanolconcentration of 999 mg COD/L at recycle ratio of 0 increased to5099 mg COD/L at recycle ratio of 0.8. This result showed that themicrobial metabolism shifted from butyrate fermentation toalcohol fermentation that is unsuitable for hydrogen production.Thus, recycling of effluent can lead to bioethanol production whichis also an ideal renewable source and widely studied (Lin and Hung,2008). Hawkes et al. (2002) reported that butyrate is usuallyassociated with high hydrogen production whereas ethanol isusually associated with low hydrogen production. Similar results ofhydrogen and SMP productions were found in this study. Fewstudies (Chen et al., 2001; Lin et al., 2006; Ayhan, 2007) indicatedthat the ratio of acetate to butyrate concentration (HAc/HBu ratio)may vary with microbial growth during fermentation and has beenused to indicate the progress of hydrogen production. The HBu/HAcratio decreased with increasing recycle ratio (Fig. 2). The peak HBu/HAc ratio of 3.38 was obtained when the reactor was operatedwithout recycling with the highest hydrogen production perfor-mance (Table 1). The high correlation coefficient value

ion (g COD/L)

30 35 40 45

ncentrations at each substrate concentrations (recycle ratio 0.2 and HRT 12 h).

Page 5: Effect of effluent recycle ratio in a continuous anaerobic biohydrogen production system

VSS

(g/L

)

2.0

2.5

3.0

3.5

4.0

4.5

HY

(mol

H2/m

ol s

ucro

se)

1

2

3

4

5

HR

2 4 6

SHPR

(mm

ol H

2/g

VSS

-d)

0

50

100

150

200

250

300

HPR

(mm

ol H

2/L

-d)

0

200

400

600

800

1000

Fig. 4. Hydrogen production performance (HY, HPR and SHPR) and biomass (VSS) conc

Table 2Experimental data of recycle reactor at various sucrose concentrations under steady-state (HRT 12 h and recycle ratio 0.2).

Sucrose concentration(g COD/L)

20 25 32 40

OLR (g COD/L-d) 40 50 64 80

ORP (�mV) 481� 38 546� 39 513� 33 443� 15Alkalinity

(mg/L as CaCO3)4870� 410 5970� 279 4350� 434 4890� 434

H2 content (%) 47.1� 2.9 48.0� 1.2 46.8� 1.1 47.2� 1.2VSS (g/L) 3.74� 0.19 3.98� 0.35 5.82� 0.26 6.73� 0.40Sucrose

degradation (%)99.1� 0.5 99.2� 0.4 94.3� 3.7 75.4� 3.9

Ethanol (mg COD/L) 2568� 237 4363� 91 2593� 208 2049� 128Acetate (mg COD/L) 3067� 270 3842� 280 2746� 247 2238� 184Propionate

(mg COD/L)665� 91 768� 63 399� 58 270� 24

Butyrate (mg COD/L) 8074� 674 10,131� 1003 7692� 730 7310� 776TVFA (mg COD/L) 11,806� 709 14,741� 1107 10,837� 758 9818� 810SMP (mg COD/L) 14,374� 715 19,104� 1263 13,430� 810 11,867� 832Butyrate/acetate

ratio (mol/mol)3.38 2.97 2.59 6.72

n¼ 5e20 (number of sampling); TVFA¼ acetateþ propionateþ butyrate;SMP¼ TVFAþ EtOH.

C.-C. Chen et al. / Journal of Cleaner Production 32 (2012) 236e243240

(r2¼ 0.9244), obtained from fitting linear equation to the data,indicated that the HBu/HAc ratio varied proportionately with therecycle ratio.

3.2. Effect of substrate concentration

Fig. 3 depicts the hydrogen production performance at varioussubstrate concentrations and at HRT 12 h and recycle ratio of 0.2. Ahigh value of the correlation coefficient (r2¼ 0.9756) of linearequation indicated that VSS concentration was affected signifi-cantly by substrate concentration. The VSS concentration of 3.74 g/Lat substrate concentration of 20 g COD/L increased to the peak VSSof 6.73 g/L at substrate concentration of 40 g COD/L. High substrateconcentration provided sufficient energy and carbon source forbacterial growth leading to higher cell biomass. The HY, HPR andSHPR peaked at sucrose concentration of 25 g COD/L, which thendecreased when the substrate concentrationwas further increased.HYs were 2.72, 2.77, 2.38 and 2.24 mol H2/mol sucrose at sucroseconcentration of 20, 25, 32 and 40 g COD/L, respectively and HPRsranged from 295 to 362 mmol H2/L-d. The SHPR value increasedfrom 74.3 mmol H2/g VSS-d at sucrose concentration of 20 g COD/Lto 93.4 mmol H2/g VSS-d at sucrose concentration of 25 g COD/L.However, at sucrose concentration of 40 g COD/L the SHPRdecreased to 51.9 mmol H2/g VSS-d. Thus, the substrate

T (h)

8 10 12

entrations at each HRT (recycle ratio 0.2 and substrate concentration 20 g COD/L).

Page 6: Effect of effluent recycle ratio in a continuous anaerobic biohydrogen production system

Table 3Experimental data of recycle reactor at various HRTs under steady-state (recycle ratio 0.2 and substrate concentration 20 g COD/L).

HRT (h) 12 8 6 4 2OLR (g COD/L-d) 40 60 80 120 240

ORP (�mV) 476� 35 478� 14 503� 36 444� 21 402� 24Alkalinity (mg/L as CaCO3) 4870� 410 4680� 152 4635� 208 5320� 400 5470� 375H2 content (%) 47.1� 2.9 46.4� 1.4 50.6� 1.4 45.3� 1.6 21.8� 3.5VSS (g/L) 3.74� 0.19 4.11� 0.23 3.81� 0.42 3.00� 0.21 2.11� 0.24Sucrose degradation (%) 99.1� 0.5 99.2� 0.1 95.7� 4.4 69.1� 5.6 54.5� 4.3Ethanol (mg COD/L) 2568� 237 1843� 102 1647� 210 1255� 136 469� 45Acetate (mg COD/L) 3067� 270 1244� 163 1829� 136 404� 64 443� 66Propionate (mg COD/L) 665� 91 387� 35 437� 56 426� 51 367� 41Butyrate (mg COD/L) 8074� 674 4818� 461 7176� 788 1922� 154 719� 58TVFA (mg COD/L) 11,806� 709 6449� 477 9442� 801 2752� 155 1529� 70SMP (mg COD/L) 14,374� 715 8292� 498 11,089� 822 4007� 162 1998� 77Butyrate/acetate ratio (mol/mol) 3.72 3.87 3.92 4.76 1.62

n¼ 5e20 (number of sampling); TVFA¼ acetateþ propionateþ butyrate; SMP¼ TVFAþ EtOH.

C.-C. Chen et al. / Journal of Cleaner Production 32 (2012) 236e243 241

concentration of 25 g COD/L was optimum for peak SHPR at HRT12 h and effluent recycle ratio 0.2, and any further increase insubstrate concentration would lead to hydrogen productioninhibition.

The production of VFA could be affected by various factors, suchas pH, substrate concentration and seed sludge pretreatment.Table 2 indicates the total VFA production during reactor operationand it is interesting to note that VFA production varied with thesubstrate concentrations. The butyrate concentration increased to10,131 mg COD/L at the sucrose concentration of 25 g COD/L andthen decreased to 7310 mg COD/L at the sucrose concentration of40 g COD/L. The peak ethanol concentration of 4363 mg COD/L atthe sucrose concentration of 25 g COD/L was found along with peakbutyrate concentration. Table 2 reveals that at the highest substrate

HY

(mol

H2/m

ol s

ucro

se)

1

2

3

4

5

HPR

(mol

H2/L

-d)

200

400

600

800

1000

OLR (g0 50 100

SHPR

(mol

H2/g

VSS

-d)

0

50

100

150

200

250

300

350

Fig. 5. Hydrogen production performance at various organic loading rates calculated basedconstant sucrose concentration 20 g COD/L.

concentration of 40 g COD/L the amounts of produced VFAs andethanol were lower than those measured at a substrate concen-tration of 32 g COD/L. This might be due to inhibition of catabolicpathways in microorganisms at very high substrate concentrations.The sucrose degradation efficiency decreased to 75% at substrateconcentration 40 g COD/L (Table 2). The HBu/HAc ratio increasedfrom 3.38 at 20 g COD/L to 6.72 at 40 g COD/L.

3.3. Effect of HRT

The biohydrogen production will be inhibited at too highsubstrate concentration (Ayhan, 2007). Therefore, shortening HRTis another strategy to increase OLR keeping the substrate concen-tration constant. Fig. 4 illustrates the peak VSS concentration of

Sucrose concentration 20 --> 40 g COD/LHRT 12 --> 2 h

COD/L-d)150 200 250

on, sucrose concentration 20e40 g COD/L at constant HRT 12 h, and HRT 12e2 h with

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4.11 g/L was obtained at HRT 8 h, which was slightly higher than3.74 g/L at HRT 12 h. At HRT lower than 8 h the VSS decreased whenthe HRT was shortened. The lowest VSS concentration was 2.11 g/Lobtained at HRT 2 h. As shown in Fig. 4, HY, HPR and SHPRincreased while HRT was shortened from 12 h to 6 h. However,when HRT was shorter than 6 h, the HY, HPR and SHPR decreased.HYs were 2.72, 2.90, 3.88, 1.90 and 0.34 mol H2/mol sucrose in caseof HRT of 12, 8, 6, 4 and 2 h, respectively. The peak HPR and SHPRwere 807 mmol H2/L-d and 244.3 mmol H2/g VSS-d respectively,which were 3.0 and 4.8 times higher than values at HRT 2 h.

Table 3 summarizes the experimental data at various HRTs withrecycle ratio 0.2 and substrate concentration 20 g COD/L, obtainedupon achieving steady-state conditions. The VSS concentrationwasfound to relate with sucrose degradation efficiency. This might bedue to the washout of hydrogen producing bacteria at lower HRT.The SMPs concentration decreased at low HRT, except at HRT 6 h,with low sucrose degradation efficiency. The HBu/HAc ratios variedfrom 3.72 to 4.76 when the HRT was decreased from 12 h to 4 h.When the HRT was 2 h the HBu/HAc ratio decreased to 1.62 whichindicatedmicrobial metabolism shift from butyrate fermentation tomixed acid fermentation due to low HRT (Wu et al., 2006).

According to the results (Tables 1e3), the major solublemetabolites during hydrogen fermentation were butyrate (21e68%of SMPs), acetate (10e25% of SMPs) and ethanol (7e52% of SMPs)along with propionate (2e18% of SMPs). These results are inagreement with the previous findings that show the hydrogenproduction by anaerobic bacteria is often through butyrate-typefermentation and mostly produces butyrate and acetate as themajor soluble metabolites (Lin et al., 2011). The production ofethanol and propionate is unfavourable for hydrogen productiondue to consumption of hydrogen and electrons from NADH (Leeet al., 2007).

Fig. 5 illustrates the relationship between the hydrogenproduction performance and OLR. The OLR was calculated based onsucrose concentration and HRTas shown in Fig. 5. The peak HY, HPRand SHPR were 3.88 mol H2/mol sucrose, 807 mmol H2/L-d and244.3 mmol H2/g VSS-d, respectively at OLR 66.7 g COD/L-d withsucrose concentration 20 g COD/L and HRT 6 h (Fig. 5).

3.4. Significance of the experimental results

Recycle CSTR system can significantly increase total energyrecovery and reduce the use of alkaline solutions to control pH inhydrogen reactor. However, such a system is not trouble-free. Ourresults indicated that recycling effluent will have a negative impacton hydrogen production performance probably because hydro-genotrophic methanogens and organisms that produce reducedfermentation end-products (e.g. propionic acid and ethanol) will berecycled. This work may indirectly show that high hydrogenproductivity from a non-sterile wastewater may be difficult toachieve. Our results showed that organic load in the reactor can beincreased by shortening the HRT and keeping the substrateconcentration constant. This strategy not only provides highnutrient availability to the microorganisms but in addition it alsoreduces the effect of inhibitory compounds on the microbialmetabolism due to short residence time. Moreover, shorter HRTcanhelp to maintain the amount of consumed substrate by hydrogenproducing bacteria in a continuous system (Elefsiniotis andOldham, 1994). Nevertheless, a combined treatment (acidþ heat)of the recycled effluent can be used but it would likely beimpractical at full scale and would not affect the many hydrogenconsuming acetogens that are spore forming Clostridium spp.(Ljungdahl and Hugenholtz, 1989). Therefore, using the recycledeffluent with its optimized ratio to produce hydrogenwould be costeffective and beneficial to the environment.

4. Conclusions

The present study evaluated the effect of effluent recycling intoa CSTR system producing biohydrogen using sucrose as the feed-stock. The results showed that when the recycle ratiowas increasedthe VSS concentration also increased with a peak of 14.2% at theeffluent recycle ratio 0.4 compared with the VSS of 3.52 g/L withouteffluent recycle. However, the effluent recycling increased the VSSconcentration but it reduced the hydrogen production perfor-mance. Applying the strategy of increasing OLR by increasingsubstrate concentration and shortening HRT, we could enhance thehydrogen production performance. The best performance ofhydrogen production was obtained at OLR of 66.7 g COD/L-d withsucrose 20 g COD/L and HRT 6 h at recycle ratio 0.2. The peak HY(3.88 mol H2/mol sucrose), HPR (807 mmol H2/L-d) and SHPR(244.3 mmol H2/g VSS-d) were obtained at HRT 6 h.

Acknowledgements

The authors gratefully acknowledge the financial support byTaiwan’s Bureau of Energy (grant no. 100-D0204-3), Taiwan’sNational Science Council (NSC-99-2221-E-035-024-MY3, NSC-99-2221-E-035-025-MY3, NSC-99-2632-E-035-001-MY3) and FengChia University, Taiwan (FCU-09G27102).

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