Fermentative Biohydrogen Production Trends and Perspectives_2008_Revi Ews in Environmental Science and Biotechnology(2)

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R E V I E W P A P E R

Fermentative biohydrogen production:

trends and perspectivesGustavo Davila-Vazquez Sonia Arriaga Felipe Alatriste-Mondragon Antonio de Leon-Rodrguez Luis Manuel Rosales-Colunga Elas Razo-Flores

Received: 15 January 2007 / Accepted: 2 May 2007 / Published online: 6 June 2007

Springer Science+Business Media B.V. 2007

Abstract Biologically produced hydrogen (biohy-

drogen) is a valuable gas that is seen as a future

energy carrier, since its utilization via combustion or

fuel cells produces pure water. Heterotrophic fer-

mentations for biohydrogen production are driven by

a wide variety of microorganisms such as strict

anaerobes, facultative anaerobes and aerobes kept

under anoxic conditions. Substrates such as simple

sugars, starch, cellulose, as well as diverse organic

waste materials can be used for biohydrogen produc-

tion. Various bioreactor types have been used andoperated under batch and continuous conditions;

substantial increases in hydrogen yields have been

achieved through optimum design of the bioreactor

and fermentation conditions. This review explores the

research work carried out in fermentative hydrogen

production using organic compounds as substrates.

The review also presents the state of the art in novel

molecular strategies to improve the hydrogen

production.

Keywords Anaerobic conditions Biohydrogen Biomass Bioreactor Dark fermentation Genemanipulation Hydrogenases Hydrogen production Mixed culture

1 Introduction

A large proportion of the world energy needs are

being covered by fossil fuels, which have led to an

accelerated consumption of these non-renewable

resources. This has resulted in both, the increase in

CO2 concentration in the atmosphere and the rapid

depletion of fossil resources. The former is considered

the main cause of global warming and associated

climate change, whereas the latter will lead to an

energy crisis in the near future (Kapdan and Kargi

2006). For these reasons, large efforts are being

conducted worldwide in order to explore new sus-tainable energy sources that could substitute fossil

fuels. Processes, which produce energy from biomass,

are typical examples of environmentally friendly

technologies as biomass is included in the global

carbon cycle of the biosphere. Large amounts of

biomass are available in the form of organic residues,

such as solid municipal wastes, manure, forest and

agricultural residues. Some of these residues can be

used after minor steps of pre-treatment (usually

G. Davila-Vazquez S. Arriaga F. Alatriste-Mondragon E. Razo-Flores (&)Division de Ciencias Ambientales, Instituto Potosino deInvestigacion Cientfica y Tecnologica, Camino a la PresaSan Jose 2055, Col. Lomas 4a, Seccion, C.P. 78216 SanLuis Potosi, SLP, Mexicoe-mail: [email protected]

A. de Leon-Rodrguez L. M. Rosales-ColungaDivision de Biologa Molecular, Instituto Potosino deInvestigacion Cientfica y Tecnologica, Camino a la PresaSan Jose 2055, Col. Lomas 4a, Seccion, C.P. 78216 SanLuis Potosi, SLP, Mexico

123

Rev Environ Sci Biotechnol (2008) 7:2745

DOI 10.1007/s11157-007-9122-7


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dilution and maceration), while others may require

extensive chemical transformations prior to being

utilized as a raw material for biological energy

production (Claassen et al. 1999). Biological pro-

cesses such as methane and hydrogen production

under anaerobic conditions, and ethanol fermentation

are future oriented technologies that will play a majorrole in the exploitation of energy from biomass.

Using the appropriate microbial mechanisms of

anaerobic digestion, hydrogen (biohydrogen) would

be the desired product of the digestion process while

the organic acids would be the by-products. The

major advantage of energy from hydrogen is the

absence of polluting emissions since the utilization of

hydrogen, either via combustion or via fuel cells,

results in pure water (Claassen et al. 1999).

This mini-review provides an overview of the

state of the art and perspectives of biohydrogenproduction by microorganisms. The review focuses

on the literature published mainly during the years

2005 and 2006 on hydrogen production by hetero-

trophic fermentation (dark hydrogen fermentation).

For a full overview of previous work on this topic

the reader is referred to excellent reviews published

elsewhere (Nandi and Sengupta 1998; Claassen et al.

1999; Hallenbeck and Benemann 2002; Hawkes

et al. 2002; Nath and Das 2004; Kapdan and Kargi

2006).

2 Biohydrogen producing microorganisms

Biohydrogen can be produced by strict and faculta-

tive anaerobes (Clostridia, Micrococci, Methanobac-

teria, Enterobacteria, etc), aerobes (Alcaligenes and

Bacillus) and also by photosynthetic bacteria (Nandi

and Sengupta 1998). Table 1 shows that during the

time period covered by this review, most of the

studies were conducted using mixed cultures, and just

a few of them were pure cultures. Different sources ofinocula were reported (soil, sediment, compost,

aerobic and anaerobic sludges, etc.) and most of

them were heat or acid treated before being used.

These treatments have been used in previous studies

as methods for increasing hydrogen production by

altering the microbial communities present in the

starting mixed population (Cheong and Hansen

2006). The reason for this is that unlike H2-consum-

ing methanogens, H2-producing bacteria are com-

monly tolerant to harsher environmental conditions

(Kawagoshi et al. 2005). For example Clostridium

and Bacillus species tolerate higher temperatures than

H2-consuming methanogens due to the formation of

endospores (Setlow 2000). Also, H2-producing bac-

teria can grow at lower pH than H2-consuming

methanogens (Cheong and Hansen 2006).Various studies have been carried out to identify

the microbial communities present in mixed cultures

used for H2 production (Ueno et al. 2001; Fang et al.

2002; Ueno et al. 2004; Kawagoshi et al. 2005; Kim

et al. 2006). Fang et al. (2002) identified the

microbial species in a granular sludge used for H2production from sucrose. They found that 69.1% of

the microorganisms were Clostridium species and

13.5% were Bacillus/Staphylococcus species. Kaw-

agoshi et al. (2005) studied the effect of both pH and

heat conditioning on different inoculums. In theirstudy they concluded that the highest hydrogen

production was obtained with heat-conditioned anaer-

obic sludge. They also found DNA bands with high

similarity (>95%) to Clostridium tyrobutyricum,

Lactobacillus ferintoshensis, L. paracasei, and Cop-

rothermobacterspp. Kim et al. (2006) indicated that

heat-treatment (908C for 20 min) caused a change in

the microbial community composition of a fresh

culture used to produce H2 from glucose in a

membrane bioreactor. They reported that most of

the species found in the fresh sludge were affiliated tothe Lactobacillus sp. and Bifidobacterium sp.; in

contrast a Clostridium perfringens band was observed

in the heat-treated sludge. When mixed cultures are

used as inocula the predominant species in a biore-

actor depends on operational conditions such as

temperature, pH, substrate, inoculum type, inoculum

pre-treatment, hydrogen partial pressure, etc. Kotay

and Das (2006) studied the H2 production with

glucose and sewage sludge as substrates using a

defined microbial consortium consisting of three

facultative anaerobes, Enterobacter cloacae, Citrob-acter freundii and Bacillus coagulans. They carried

out experiments with the consortium (three species)

and the individual species. E. cloacae produced a

higher yield than the other strains, but similar to the

consortium suggesting that E. cloacae dominated the

consortium. Some studies using pure strains have also

been carried out for H2 production. Escherichia coli

(genetically modified strains), Clostridium butyricum,

C. saccharoperbutylacetonicum, C. thermolacticum,

28 Rev Environ Sci Biotechnol (2008) 7:2745

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Table1

Hydrogenproductionratesandyieldcoefficientsfrompureand

complexsubstratesunderbatch,semi-c

ontinuousandcontinuousoperation

System

Inoculum

Substratea

Volumetric

H2production

rate(mmol

H2/lculture-h)f

H2yield

Culture

conditionsa[HRT

(h),Load,pH,

Temperature

(8C),H2in

biogas(%v/v)]

Reference

Batch

ClostridiumbutyricumCGS5

Sucrose(20gCOD/l)

8.2

2.78molH2/mol

sucrose

,,5.56.0

c,37,64

Chenetal.

(2005)

Clostridium

saccharoperbutylacetonicum

ATCC27021

Crudecheesewhey(ca.

41.4glactose/l)

9.4

2.7m

olH2/mol

lactose

,,6.0

d,30,NRb

Ferchichietal.

(2005)

Escherichiacolistrains

Glucose(4g/l)

NRb

*2m

olH2/mol

glucose

,,7.0,37,NR

Bisaillonetal.

(2006)

Escherichiacolistrains

Formicacid(25mM)

11795

1molH2/mol

formate

,,6.5

d,37,NR

Yoshidaetal.

(2005)

Definedconsortium(1:1:1,and

separatelytested):En

terobacter

cloacaeIIT-BT08,C

itrobacter

freundiiIIT-BTL139

,Bacillus

coagulansIIT-BTS1

Glucose(10g/l)

NR

41.23

mlH2/g

CO

Dremoved

,,6.0

d,37,NR

KotayandDas

(2006)

MesophilicbacteriumH

N001

Starch(20g/l)

59

2molH2/mol

glucose

,,6.0

c,37,NR

Yasudaand

Tanisho

(2006)

Aerobicandanaerobicsludges,

soilandlakesedimen

t(acidand

heatconditioned)

Glucose(20g/l)

NR

1.4m

olH2/mol

glucose

,,6.0

c,35,NR

Kawagoshietal.

(2005)

Aerobicsludge(heatco

nditioned)

Glucose(2g/l)

NR

2.0m

olH2/mol

glucose

,,6.2,d30,87.4

Parketal.(2005)

Soil(heatconditioned)

Organicmatterpresent

infourcarbohydrate-

richwastewaters

6.2

100m

lH2/g

CO

Dremoved

,,6.1,d23,60

VanGinkeletal.

(2005)

Anaerobicsludge(acid

treatment

andacclimatedinaC

STR)

Sucrose(20gCOD/l)

96

1.74molH2/mol

sucrose

,,6.1,d40,45

Wuetal.(2005)

Anaerobicsludge(heat

conditioned)

Glucose(10g/l)

27.2mmolH2/gVSS-

lculture-h

1.75molH2/mol

glucose

,,6.0,d37,40

ZhengandYu

(2005)

Anaerobicsludge

(acidtreatment)

Glucose(*21.3g/l)

4.98.6

0.81.0molH2/mol

hex

ose

,,5.7,c34.5,59

66

Cheongand

Hansen(2006)

Rev Environ Sci Biotechnol (2008) 7:2745 29

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Table1

continued

System

Inoculum

Substratea

Volumetric

H2production

rate(mmol

H2/lculture-h)f

H2yield

Culture

conditionsa[HRT

(h),Load,pH,

Temperature

(8C),H2in

biogas(%v/v)]

Reference

Microflorafromacowdung

compost(heattreatment)

Wheatstrawwastes

(25g/l)

2.7mmolH2/gTVS-

lculture-h

2.7m

molH2/gTVS

,,7.0,d36,52

Fanetal.(2006)

Anaerobicsludge(he

attreated)

Sucrose(10g/l)

8

1.9m

olH2/mol

sucrose

,,5.5,c35,NR

Muetal.(2006a)

Anaerobicsludge(he

attreated)

Sucrose(24.8g/l)

20

3.4m

olH2/mol

sucrose

,,5.5,c34.8,64

Muetal.

(2006b)

Anaerobicsludge(he

attreated)

Glucose(3.76g/l)

9

1.0m

olH2/mol

glucose

,,6.2,d30,66

Salernoetal.

(2006)

Anaerobicsludge(he

attreated)

Glucose(2.82g/l)

NR

0.968molH2/mol

glucose

,,6.2,d25,5772

Ohetal.(2003)

Microflorafromsoil(heat

shocked)

Glucose,sucrose,

molasses,lactate,

potatostarch,

cellulose(each:4g

COD/l)

NR

0.92

molH2/mol

glucose,1.8mol

H2/molsucrose,

0.59molH2/mol

po

tatostarch,e

0.01molH2/mol

lac

tate,0.003mol

H2/molcellulosee

,,6.0,d26,62

Loganetal.

(2002)

Fedbatch

Mixedculture

OFMSW-Semisolid

substrate

14.7mmolH2/

gVSdestroyed

NRb

504,11gVS/

kgwmr

d,6.4,55,

58

Valdez-Vazquez

etal.(2005)

AnaerobicPOMEslu

dge

POME(2.5%w/v)

17.82

NR

24,NR,5.5,60,66

Atifetal.(2005)

Windrowyardwaste

compost

Glucose(2g/l)

7.44

1.75

molH2/mol

glucose

76,NR,5.4,55,NR

Callietal.

(2006)

CSTR

Mixedculture

Sucrose(20gCOD/l)

17

3.5m

olH2/mol

sucrose

12,NR,6.8,35,45.9

Linetal.

(2006b)

Mixedculture

Sucrose(40g/l)

20

1.15

molH2/mol

hexose

12,80g/l-d,5.2,35

,

60

Kyazzeetal.

(2006)

Mixedcultureimmob

ilizedin

siliconegel

Sucrose(30gCOD/l)

612.5

3.86

molH2/mol

sucrose

0.5,NR,6.5,40,44

Wuetal.

(2006a)


123


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Table1

continued

System

Inoculum

Substratea

Volumetric

H2production

rate(mmol

H2/lculture-h)f

H2yield

Culture

conditionsa[HRT

(h),Load,pH,

Temperature

(8C),H2in

biogas(%v/v)]

Reference

Mixedculture

Xylose(20gCOD/l)

5

1.1m

olH2/mol

xylose

12,NR,7.1,35,32

LinandCheng

(2006)

Mixedculture

Brokenkitchenwastes

(10kgCOD/m3-d)

andcornstarch

(10kgCOD/m3-d)

1.7

NR

96,NR,5.35.6,35,

NR

Chengetal.

(2006)

Mixedculture

Glucose(15gCOD/l)

13.23

1.93molH2/mol

glucose

4.5,80gCOD/l-d,

5.5,37,67

Zhangetal.

(2004)

Dewateredand

thickenedsludge

Glucose(4gCOD/l)

3.47

1.9m

olH2/mol

glucose

10,NR,5.5,35,67

Salernoetal.

(2006)

Mixedculture

Sucrose(20gCOD/l)

15.6

3.6m

olH2/mol

sucrose

12,NR,5.5,35,50

LinandChen

(2006)

Mixedculture

Organicwastewater

(4000mgCOD/l)

4.96

NR

12,NR,4.4,8kg

COD/m3-d,30,

NR

Wangetal.

(2006)

Sewagesludge

Sucrose(20gCOD/l)

52.6

3.43molH2/mol

sucrose

12,NR,6.8,35,50.9

LinandLay

(2005)

Mixedculture

Sucroseandsugarbeet

5.15

1.9m

olH2/mol

hex

ose

15,16kgsugar/m3-

d,5.2,32,NR

Hussyetal.

(2005)

Mixedculture

Glucose(15g/l)

0.115gH2-COD/g

CODFeed

1.38molH2/mol

hex

ose

10,NR,5.5,35,45

Kraemerand

Bagley(2005)

C.thermolacticum(DSM2910)

Lactose(10g/l)

2.58

2.13

molH2/mol

lactose

17.2,NR,7.0,58,55

Colletetal.

(2004)

Seedsludge

Molasses(3000mg

COD/l)

26.13molH2/kg

CODremoved

NR

11.4,27.98kgCOD/

m3reactor-d,4.5,

35,45

Renetal.(2006)

UASB

Mixedculture

Glucose(10g/l)

2.18

2.47molH2/mol

glucose

26.7,NR,4.85.5,

70,NR

Kotsopoulos

etal.(2006)

Mixedculture

Sucroserichwaste

water

5.93

1.61molH2/mol

glucose

12,NR,7,39,NR

MuandYu

(2006)

Mixedculture

Citricacidwastewater

(18kgCOD/l)

1.23

0.84molH2/mol

hex

ose

12,38.4kgCOD/

m3-d,7,35,NR

Yangetal.

(2006)


123


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Table1

continued

System

Inoculum

Substratea

Volumetric

H2production

rate(mmol

H2/lculture-h)f

H2yield

Culture

conditionsa[HRT

(h),Load,pH,

Temperature

(8C),H2in

biogas(%v/v)]

Reference

Mixedculture

Sucrose(20gCO

D/l)

11.3

1.5mmolH2/mol

sucrose

8,175mmol

sucrose/l-d,6.7,

35,42.4

ChangandLin

(2004)

Mixedculture

Glucose(7.7g/l)

18.4

1.7molH2/mol

glucose

2,NR,6.4,55,36.8

Gavalaetal.

(2006)

(1.3g/l)

19

0.7molH2/mol

glucose

2,NR,4.4,35,29.4

CSTRandUASB

Mixedculture

Starch(10g/l)an

d

xylose(1:1w/w)

4.5

32.9,NR,7,35,68

Camilliand

Pedroni(2005)

4.76

6.7,NR,7,35,68

2.54

20.5,NR,7,35,68

CSTR,UASB,

UFBR

Mixedculture

Glucose(6.86g/l)

37.5

1.6molH2/mol

glucose

12,NR,5.5,60,48

Ohetal.(2004)

TBR

Clostridiumacetobutylicum

(ATCC824)

Glucose(10.5g/l)

8.9

0.9molH2/mol

glucose

0.035,8.3g/l-h,4.9,

30,74

Zhangetal.

(2006)

Mixedculture

Glucose(2g/l)

NR

2.48molH2/mol

glucose

0.5,96kg/m3-d,7.7,

30,NR

Leiteetal.

(2006)

CIGSB

Mixedculture

Sucrose(17.8g/l)

0.298

3.88molH2/mol

sucrose

0.5,NR,6.7,40,4

2

Leeetal.(2006)

PBR

Cowdung

POME(560gC

OD/l)

0.42lbiogas/g

CODdestroyed

NR

37,NR,5,NR,53

56

Vijayaraghavan

andAhmad

(2006)

UACF

Cowdung

Jackfruitpeel(22

.5g

VS/l)

0.72lbiogas/gVS

destroyed

NR

288,NR,5,NR,5

6

Vijayaraghavan

etal.(2006)

MBR

Mixedculture

Glucose(10g/l)

71.4

1.1molH2/mol

glucose

0.79,NR,5.5,37,70

Kimetal.(2006)


123


7/19

Table1

continued

System

Inoculum

Substratea

Volumetric

H2production

rate(mmol

H2/lculture-h)f

H2

yield

Culture

conditionsa[HRT

(h),Load,pH,

Temperature

(8C),H2in

biogas(%v/v)]

Reference

FBRDTFBR

Mixedculture

Sucrose(20gCOD/l)

50.27

2.10molH2/mol

sucrose

2,NR,6.9,40,40

Wuetal.

(2006b)

95.23

1.22molH2/mol

sucrose

0.5,NR,7,40,35

a

Whenoptimizationtrialswerecarriedout,optimumvaluesarereported

b

NR:Notreported

c

Controlledvalue

d

Initial,notcontrolled

e

Starch,celulose:[(C6H10O5)n]

f

Insomecasesunitconversionsw

eremadeaccordingtotheconditionsreportedbytheauthors.CIGSB:Carrierinducedgranularsludgebed.COD:Chem

icaloxygendemand.

CSTR:Continuousstirredtankreactor.DTFBR:Drafttubefluidizedbedreactor.FBR:Fluidizedbedbioreactor.kgwmr:

Kilogramsofwetmassinthereac

tor.MBR:Membrane

bioreactor.OFMSW:Organicfractionofmunicipalsolidwastes.PBR:Pa

ckedbedreactor.POME:Palmoilmilleffluent.TBR:Tricklingbiofilter.TVS

:totalvolatilesolids.

UACF:Up-flowanaerobiccontact

filter.UASB:Upflowanaerobicsludge

blanket.UFBR:Up-flowfixedbedreactor.VS:volatilesolids.VSS:volatilesuspendedsolids


123


8/19

and C. acetobutylicum are among the microorganisms

used (Table 1).

3 Biohydrogen producing substrates

The main criteria for substrate selection are: avail-ability, cost, carbohydrate content and biodegradabil-

ity (Kapdan and Kargi 2006). Glucose, sucrose and to

a lesser extent starch and cellulose, have been

extensively studied as carbon substrates for biohy-

drogen production (Table 1). They have been used as

model substrates for research purposes due to their

easy biodegradability and because they can be

present in different carbohydrate-rich wastewaters

and agricultural wastes. Other substrates suitable for

biohydrogen production are protein- and fat-rich

wastes. Although they are less available than carbo-hydrate-rich wastes, they represent potential feeds for

the biological conversion of organic wastes to

hydrogen (Svensson and Karlsson 2005).

A maximum theoretical yield of 12 mol of H2 per

mol of hexose is predicted from the complete

conversion of glucose:

C6H12O6 6H2O ! 12 H2 6CO2 DG00 25 kJ

It should be noted that essentially no energy isobtained from this reaction to allow microbial growth

(Hallenbeck2005). Actual yields in metabolisms that

lead to H2 production are lower compared to the

maximum theoretical yield; ca. 2 and 4 mol of

hydrogen per mol of glucose and sucrose, respec-

tively, are obtained (Table 1). Fermentation of

organic compounds is considered to have an imme-

diate potential for economical hydrogen production,

but only if conversion efficiency could be increased

to 6080% (Benemann 1996). This means that from

each mol of glucose fermented, a minimum of 7 molof hydrogen should be obtained. Nonetheless, due to

thermodynamic constrains, only 4 mol of hydrogen

are released from 1 mol of glucose if acetate is

produced, and 2 mol of hydrogen are obtained when

butyrate or more reduced compounds (lactic acid,

propionic acid, or ethanol) are produced (Hallenbeck

and Benemann 2002; Angenent et al. 2004). As

discussed by Logan (2004), although genetic engi-

neering of bacteria could increase hydrogen recovery,

it is now considered that the maximum conversion

efficiency will still remain below 33%. This so called

fermentation barrier is maintained regardless of the

fermentation system used for H2 production e.g.

batch, semi-continuous or continuous one step-pro-

cesses (Logan 2004).

Another important feature of hydrogen fermenta-tion is volumetric H2 production rate (VHPR). As

suggested by Levin et al. (2004), we agree that in

order to assess the potential for practical application

of biological hydrogen production systems, an effort

should be made in order to report VHPR in

standardized units. By doing this, it would be easier

to calculate and compare the size of a bioreactor

that would be needed to supply hydrogen to a

specific fuel cell for electricity generation. For this

reason an effort was made in this review to report

VHPR in Table 1 in the same units whenever this waspossible.

4 Biohydrogen production in batch, continuous

and semi-continuous systems

Biohydrogen production by dark fermentation is

highly dependent on the process conditions such as

temperature, pH, mineral medium formulation, type

of organic acids produced, hydraulic residence time

(HRT), type of substrate and concentration, hydrogen

partial pressure, and reactor configuration (Table 1).

Temperature is an operational parameter that

affects the growth rate and metabolic activity of

microorganism. Fermentation reactions can be oper-

ated at mesophilic (25408C), thermophilic (40

658C), extreme thermophilic (65808C), or hyper-

thermophilic (>808C) temperatures. Most of the

results presented in Table 1 were obtained under

mesophilic conditions and some under thermophilic

conditions. The effect of temperature on VHPR can

be explained thermodynamically by considering the

changes in Gibbs free energy and in standard

enthalpy of the conversion of glucose to acetate and

assuming a maximum theoretical yield of 4 mol H2per mol glucose (Vazquez-Duhalt 2002):

C6H12O6 2H2O ! 2CH3COOH + 4H2 2CO2

DG 176.1 kJ/mol

DH 90.69 kJ/mol


123


9/19

The changes in Gibbs free energy and in enthalpy

of the reaction indicate that the reaction can occur

spontaneously and the reaction is endothermic. The

vant Hoff equation can be used to explain the effect

of the temperature on the equilibrium constant (Smith

et al. 2000):

lnK1

K2

DH

R

1

T1

1

T2

1

K1

K2

H2 41 CO2

21 CH3COOH

21

H2 42 CO2

22 CH3COOH

22

2

If temperature increases, the equilibrium kinetic

constant also increase because the reaction is endo-

thermic (DH8 has positive sign, see Eq. 1). Therefore,

increasing the temperature of the glucose fermenta-tion, maintaining reactants concentration constant

(see Eq. 2) would enhance H2 concentration. Two

reports provide support to the previous thermody-

namic considerations. In one of them, Valdez-Vaz-

quez et al. (2005) studied the semi continuous H2production at mesophilic and thermophilic condi-

tions. They found that VHPR was 60% greater at

thermophilic than at mesophilic conditions. The

authors suggested that this behavior may be explained

by the optimal temperature for the enzyme hydrog-

enase (50 and 708C) present in thermophilic Clostri-dia. Also, Wu et al. (2005) reported that VHPR was

greater at 408C than at 308C in batch tests using

immobilized sludge in vinyl acetate copolymer. In

addition, fermentation at temperatures above 378C

may inhibit the activity of hydrogen consumers (Lay

et al. 1999) and suppress lactate-forming bacteria (Oh

et al. 2004) yielding in both cases higher VHPR.

Thermophilic conditions also enhance the efficiency

of pathogens removal (de Mes et al. 2003), which are

present in some residues, allowing the use of these

residues as fertilizers for application on agriculturalsoil.

On the other hand, high temperatures can induce

thermal denaturation of proteins affecting the micro-

organism activity. Lee et al. (2006) studied the effect

of temperature on hydrogen production in a carrier

induced granular sludge bed bioreactor (CIGSB).

They found that increasing the temperature from 35

to 458C may inhibit cell growth or granular sludge

formation due primarily to denaturation of essential

enzymes to paralyze normal metabolic functions or

decreasing in production of extracellular polymeric

substances. Another potential disadvantage of a

thermophilic process is the increased energy costs.

In some of the studies presented in Table 1 the

maximum VHPR was obtained between pH 5.0 and

6.0. However, in other studies the maximum VHPRwas found around pH 7.0 (Lee et al. 2006; Lin and

Cheng 2006; Mu and Yu 2006). Recent studies have

indicated that in order to inhibit methanogenesis,

increase VHPR and enhance stability of continuous

systems, moderate acid pH and high temperatures

should be used (Oh et al. 2004; Atif et al. 2005;

Kotsopoulos et al. 2006). For the operation of batch

systems an optimum initial pH of 5.5 was reported

(Fan et al. 2006; Fang et al. 2006; Mu et al. 2006b;

Mu et al. 2006c). However, the final pH in batch

systems is around 45 regardless of the initial pH.This is due to the production of organic acids, which

decreases the buffering capacity of the medium

resulting in a low final pH. Mu et al. (2006c) found

that volatile fatty acids (VFA) and ethanol formation

was pH dependant. Ethanol production decreased

when pH was decreased from 4.2 to 3.4. On the other

hand, butyrate concentration decreased when pH was

increased from 4.2 to 6.3. It has been reported that

high VHPR is associated with butyrate and acetate

production and that inhibition of hydrogen production

is associated with propionic acid formation (Oh et al.2004; Wang et al. 2006). Therefore, control of pH at

the optimum level (56) is critical. Initial pH also

influences the extent of lag phase in batch hydrogen

production. Some studies reported that low initial pH

in the range of 44.5 causes longer lag periods than

high initial pH levels around 9 (Cai et al. 2004).

However, the yield of hydrogen production decreased

at high initial pH.

The mineral salt composition (MSC) also affects

hydrogen production. Lin and Lay (2005) found an

optimal MSC by using the Taguchi fractional designm et h od a nd v ar y in g t h e p ro p or t io n s o f

MgCl2 6H2O, CoCl2 6H2O, CaCl2 2H2O,NiCl2 6H2O, MnCl2 6H2O, KI, NH4Cl, NaCl,ZnCl2 , F eS O4 7H2O , K Mn SO4 4H2O,CuSO4 5H2O, Na2MoO4 2H2O. The VHPRobtained with the optimal MSC was 66% greater

than the value obtained with conventional acidogenic

n u tr i e nt f o r mu l at i on ( N H4HCO3, K2HPO4,

MgCl2 6H2O, MnSO4 6H2O, FeSO4 7H2O,


123


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CuSO4 5H2O, CoCl2 5H2O, NaHCO3). Also, theyfound that magnesium, sodium, zinc and iron were

important trace metals affecting VHPR due to the fact

that these elements are needed by bacterial enzyme

cofactors, transport processes and dehydrogenase.

Recent studies reported the effect of sulfate and

ammonia concentrations on VHPR in CSTR systemswith sucrose and glucose as substrate (Lin and Chen

2006; Salerno et al. 2006). Increasing sulfate con-

centration from 0 to 3000 mg/l, at pH 6.7, reduced

VHPR and shifted the metabolic pathway from

butyrate to ethanol fermentation due to the occur-

rence of sulfate reduction. However, increasing the

sulfate concentration to 3000 mg/l, at pH 5.5, raised

the VHPR to 40% and 98.6% of residual sulfate

concentration was found in the effluent, indicating

that the acidic environment was not favorable to

sulfate reducing bacteria and sulfate reduction did notoccur (Lin and Chen 2006). A decrease of 40% on

VHPR and hydrogen yield was observed at ammonia

concentrations of 7.8 g N-NH4/l compared with the

value obtained at 0.8 g N-NH4/l, which was the

optimal ammonia concentration for hydrogen pro-

duction (Salerno et al. 2006).

Hydrogen and VFA can be produced during

exponential and stationary growth phases. However,

various authors have shown that VFA and hydrogen

production are maximal during the exponential

growth phase, and decrease during the stationaryphase due to alcohol production (Lay 2000; Levin

et al. 2004). Hydrogen production in continuous and

discontinuous systems depends on both biomass and

substrate concentrations. Yoshida et al. (2005) stud-

ied the effect of biomass concentration on hydrogen

production. They found that the specific hydrogen

production rate (SHPR) increased 67% by increasing

cell density from 0.41 g/l to 74 g/l.

The maximum hydrogen yield of 4 mol/mol has

not been reached because in nature fermentation

serves to produce biomass and not hydrogen. Also,hydrogen production by fermenting cells is a waste of

energy for the bacterial metabolism, and therefore

elaborated mechanisms exist to recycle the evolved

hydrogen in these cells. Additionally, hydrogen yield

is negatively affected by the partial pressure of the

product. Theoretically, up to 33% of the electrons in

hexose sugars can go to hydrogen when growth is

neglected and at least 66% of the substrate electrons

remain on VFA production.

The most appropriate parameter to analyze con-

tinuous systems is the mass loading rate (L), which is

function of substrate concentration (S) and the

hydraulic retention time (HRT):

L

S

HRT 3

VHPR increase when substrate concentration

increase and HRT diminishes. However, at low

HRT microbial washout might be greater than

microbial growth. Thus, the low concentration of

biomass in the reactor leads to decreased VFA

production and a higher pH. High substrate concen-

trations may result in the accumulation of VFA and

a low pH in the reactor. Accumulation of VFA may

cause hydrogen producing bacteria to switch to

solvent production and cell death, reducing theproduction of hydrogen (Lin and Chang 1999; Wang

et al. 2006; Hawkes et al. 2007). Low pH in the

reactor could inhibit the activity of microorganisms

if they come from non acidic environment. Also, the

proportions of undissociate acids (i.e. acetic, buty-

ric) increases as the pH decreases, the undissociated

forms can pass across the cell membrane, collapsing

the membrane pH gradient, and the cell will

sporulate or die (Hawkes et al. 2007). In addition,

when substrate concentrations increase in batch

systems the partial pressure of hydrogen rises andthe microorganism would switch to alcohol produc-

tion, thus inhibiting hydrogen production (Fan et al.

2006). Park et al. (2005) showed that chemical

scavenging of CO2 increased hydrogen production

by 43% in batch glucose fermentation. Also apply-

ing vacuum, gas sparging or CO2 scavenging may

all be effective methods to increase hydrogen

production (Levin et al. 2004; Valdez-Vazquez

et al. 2006).

For most of the results presented in Table 1,

optimal HRT between 0.5 and 12 h and substrateconcentrations around 20 g/l were reported. Chang

and Lin (2004) studied the effect of HRT on

hydrogen yield, VHPR and SHPR in an up-flow

anaerobic sludge blanket (UASB) reactor fed with

sucrose. They found that hydrogen yield was inde-

pendent of HRT, in the range of 8 to 20 h, but that

VHPR and SHPR were dependent on HRT. Oh et al.

(2004) showed that decreasing the HRT to 4 h and

simultaneously increasing the substrate concentration


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from 6.86 to 20.6 g/l resulted in an increased lactate

concentration, which reduced the VHPR.

The reactor configuration is another parameter that

affects VHPR as is shown in Table 1. The VHPR in

different reactor configurations varied, showing the

best performance with immobilized cell bioreactors.

High cell densities are needed to maximize hydrogenproduction rates. Therefore, major improvements are

expected in systems with biomass retention, e.g. by

immobilized cells. Oh et al. (2004) studied hydrogen

production in a trickling biofilter (TBR) with glucose

as substrate and found a maximum VHPR of

37.5 mmol/l-h. The TBR could maintain a high

biomass density of 1824 g VSS/l, which is higher

than other immobilized systems and significantly

higher than most of the cell suspension reactors like

the continuous stirring tank reactor (CSTR). Re-

cently, fluidized bed reactor (FBR) and draft tubefluidized bed reactor (DTFBR) systems with effluent

recycle and immobilized cells were studied for the

production of hydrogen using sucrose (Lin et al.

2006a; Wu et al. 2006a). A VHPR of 95.23 mmol/l-h

was obtained with the DTFBR, which was 50%

greater than the one obtained with FBR. However,

when using immobilized systems it could be impor-

tant to consider the gas (hydrogen and carbon

dioxide) hold up in the reactor, because if this is

excessive could induce inhibition of microorganism

due to changes of pH.The maximum VHPR that has been obtained is

612.5 mmol/l-h by using a CSTR containing silicone

immobilized sludge (10% v/v) and sucrose as

substrate (Wu et al. 2006a). This VHPR is at least

six times greater than any other VHPR reported

(Table 1). This study demonstrated that an appropri-

ate process design, containing simultaneously gran-

ular, immobilized and freely suspended sludge, had a

pronounced positive effect on hydrogen production.

In the same study, a hydrogen yield of 3.86 mol/mol

was obtained, which is similar to the highest yield of3.88 mol/mol obtained in a CIGSB (Carrier induced

granular sludge) system for fermentation of sucrose

(Lee et al. 2006). Ren et al. (2006) reported a

satisfactory performance of a pilot scale CSTR to

produce hydrogen from molasses. However, CSTR

problems could be present when high dilution rates

(low HRT) are used and washout of the cells is

experienced as the specific biomass growth rate is

proportional to the dilution rate in a steady state

operation. If easily biodegradable substrates (sucrose)

are used for hydrogen production, it could be

important to operate at the optimal dilution rate (near

to wash out) to allow optimal VHPR, small reactor

size and lower capital cost. However, high HRT

should be required when complex substrates (cellu-

lose) are used for the production of hydrogen.Gavala et al. (2006) obtained similar VHPR in

CSTR and UASB reactors for glucose fermentation,

but the hydrogen yield attained in the CSTR was

greater than that obtained with the UASB. Overall,

similar VHPR are obtained by using UASB and

CSTR systems (Table 1) (Chang and Lin 2004;

Gavala et al. 2006; Lin and Chen 2006; Zhang et al.

2006).

Kim et al. (2006) showed that the us e of

membrane bioreactors (MBR) for hydrogen produc-

tion allows advantages, such as high cell density.They used a MBR system with glucose as substrate

and found a maximum VHPR of 71.4 mmol/l-h.

Nevertheless, the use of MBR systems has been

limited to laboratory scale due to high investment

costs. Although immobilized cells and MBR systems

have shown the highest VHPR, it is not easy to

compare different configurations of reactors to draw a

conclusion in regard to what configuration is better,

even under a specific set of conditions. This is due to

the fact that many factors, such as VHPR, hydrogen

yield, long-term stability of the reactor, scale up, etc.,have an impact on the economics of fermentative

hydrogen production. In particular, the VHPR and

hydrogen yield change significantly depending on

experimental conditions including temperature, pH,

substrate concentration, type of substrate and HRT.

5 Molecular approach

Among the few microorganisms genetically modified

reported for biohydrogen production, Escherichiacoli is the most used, because its metabolic pathways

and genomic sequence are known. Also, there are

molecular tools for its manipulation. The metabolic

pathway for biohydrogen production by enterobacte-

ria is shown in Fig. 1. Under anaerobic conditions, a

fraction of pyruvate can be transformed to lactate by

the lactate dehydrogenase (LDH), but most of it is

hydrolyzed by the pyruvate formate liase (PFL) into

acetyl-CoA and formate. PFL cleaves pyruvate only


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when cells grow fermentatively, while pyruvate

dehydrogenase (PDH) decarboxylates pyruvate under

aerobic conditions. Both enzymes are active under

oxygen limiting conditions. The acetyl-CoA is par-tially converted into ethanol and acetate. Formate is

the electron donor in anaerobic metabolism for nitrate

reduction or can be transformed into hydrogen by the

formate-hydrogen lyase complex (FHL). In E. coli

there are three formate dehydrogenases (FDH)

denominated O, N and H. The FDH-H (encoded by

the fdhF gene) forms part of the FHL complex. The

enzymes required for formate metabolism are en-

coded in the formate regulon (Leonhartsberger et al.

2002; Sawers 2005).

The formate regulon includes genes hycB-I, hypA-E, hycA and hypF. HycB-G proteins are the structural

proteins forming the FHL and Hyp proteins are

involved in the maturation of the FHL, whereas

HycA is the negative transcriptional regulator for the

formate regulon. FhlA (encoded by fhlA gene) is the

positive transcriptional regulator for the expression of

fdhFgene (Fig. 2). Then, manipulating these genes it

is possible to control the production and activity of

FHL complex and therefore the biohydrogen produc-

tion (Leonhartsberger et al. 2002). Thus HycA

defective are hydrogen overproducing strains (Pen-

fold et al. 2003; Yoshida et al. 2005). A description offormate regulon has been published elsewhere

(Sawers 2005).

The hydrogenases 1 and 2 and formate dehy-

drogenase N and O also consume formate, but

they do not produce hydrogen. These isoenzymes

are located on the periplasmic space, and they

must be transported by Twin arginine translocation

(Tat) protein system to be active. Whereas HycE

(also known as hydrogenase 3) and FDH-H are

located on cytoplasm and hence are not to be

transported. Thus, Tat defective mutants arehydrogen overproducing strains. Penfold et al.

(2006) reported that mutant strains defective of

Tat transport (DtatC or DtatA-E) showed a hydro-

gen production comparable to E. coli strain

carrying a DhycA allele. However, DtatC DhycA

double mutant strain did not increase hydrogen

production. Thus, it is possible that hydrogen

production by E. coli could be increased by

discarding activities of the uptake hydrogenases,

which recycle a portion of hydrogen, and the

formate hydrogenases N and O, which oxidize theformate without hydrogen production.

Penfold and Macaskie (2004) transformed E. coli

HD701, a hydrogenase-upregulated strain and

FTD701 (a derivative of HD701 that has a deletion

of the tatC gene), with the plasmid pUR400 carrying

Fig. 2 The formate regulon

of E. coli: formate isgenerated by the pfl geneproduct. Genes or operons

positively regulated byformate through the action

of the transcriptionalregulator FhlA aredesignated by + (Modifiedfrom Sawers 2005)

Fig. 1 Metabolic routes of pyruvate and formate in E. coli.

Key reactions in the generation of hydrogen are shown in bold


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the scrregulon to yield E. coli strains, which produce

hydrogen from sucrose, as an alternative to coupling-

in an upstream invertase. The parenteral strains did

not produce hydrogen, whereas recombinant strains

produced 1.27 and 1.38 ml H2/mg dry weight-lculture.

Mishra et al. (2004) overexpressed a [Fe]-hydrog-

enase from Enterobacter cloacae (obtained withdegenerate primers designed from the conserved

zone of hydA gene) in a non-hydrogen producing

E. coli BL21. The resultant recombinant strain

showed the ability to produce hydrogen. Yoshida

et al. (2005) constructed an E. coli strain overex-

pressing FHL by combining hycA inactivation with

fhlA overexpression. With these genetic modifica-

tions, the transcription offdhF (large-subunit formate

dehydrogenase) and hycE(large-subunit hydrogenase

3) increased 6.5- and 7-fold, respectively, and

hydrogen production increased 2.8-fold comparedwith the wild-type strain. The effect of mutations in

uptake hydrogenases, in lactate dehydrogenase gene

(ldhA) and fhlA was studied by Bisaillon et al. (2006).

They reported that each mutation contributed to a

modest increase in hydrogen production and the

effect was synergistic.

As expected, the amino acid sequence of FDH-H

(E.C.1.2.1.2) from E. coli was highly homologous to

the FDH-H sequences reported by NCBI for other

Enterobacteria (Fig. 3). FDH-H sequence identities of

98.5%, 98.3% and 79.4% were calculated for Salmo-nella enterica YP_153159, Salmonella typhimurium

NP_463150, and Erwinia carotovora YP_049356

respectively. The partial sequence available for

Enterobacter aerogenes CAA38512 showed high

homology as well. In addition, the identity with the

FDH-H from the Archaeas Methanocaldococcus

jannaschii NP_248356 and Thermoplasma acidophi-

lum NP_393524 were 58.1% and 54.9%, respectively

and the FDH-H from Photobacterium profundum

ZP_01218756 (belongs to Vibrionaceae family) was

59.8%. The high homology of FDH-H sequenceamong diverse bacteria suggests a high evolutive

conservation of FDH-H.

The hydrogen production by Gram-positive bac-

teria such as Clostridium is shown in Fig. 4. The

pathway for hydrogen production uses two enzymes:

ferredoxin-NAD reductase (FNR) and [Fe]-hydroge-

nase (FR). The overexpression of hydA gene encod-

ing the FR has been used as a strategy to enhance

hydrogen production. For instance, Morimoto et al.

(2005) reported that the hydrogen yield increased 1.7-

times in recombinant Clostridium paraputrificum

overexpressing hydA gene with respect to a wild-

type strain. Harada et al. (2006) proposed to disrupt

thl gene encoding thiolase (THL), which is involved

in the butyrate formation from Clostridium butyri-

cum, like novel molecular strategies to improve thehydrogen production. Since, THL defective mutant

do not uptake NADH, it could be used for the

hydrogen production by the FNR. Nevertheless, at

this time, results on hydrogen production are not

available. Overexpression of FNR could improve

hydrogen production. However, strains overexpress-

ing FNR have not been reported.

6 Economics of biohydrogen production and

perspectives

Even when there are many reports in the literature

about biohydrogen production, only few of them are

related to the economic analyses of the biohydrogen

production. In general, the molar yield of hydrogen

and the cost of the feedstock are the two main barriers

for fermentation technology. The main challenge in

fermentative production of hydrogen is that only 15%

of the energy from the organic source can typically be

obtained in the form of hydrogen (Logan 2004).

Consequently, it is not surprising that mayor effortsare directed to substantially increase the hydrogen

yield. The U.S. DOE program goal for fermentation

technology is to realize yields of 4 and 6 mol

hydrogen per mol of glucose by 2013 and 2018,

respectively, as well as to achieve 3 and 6 months of

continuous operation for the same years (http://

www1.eere.energy.gov/hydrogenandfuelcells/mypp/

). Additionally, some integrated strategies are now

being under development such as the two step

fermentation process (acidogenic + photobiological

or acidogenic + methanogenic processes) or the useof modified microbial fuel cells (de Vrije and

Claassen 2003; Logan and Regan 2006; Ueno et al.

2007). Through these coupled processes more hydro-

gen or energy (in the form of methane) per mol of

substrate are achieved in a second step (Fig. 5).

Hydrogen molar yields may be increased through

metabolic engineering efforts. At this moment the

acceptability of genetically modified microorganisms

is a challenge, since the possible risk of horizontal


123
http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/


14/19

Fig.

3

Multiplealignmentofthe

FDH-HproteinofE.coliwiththeFD

H-Hoftwoarcheas(M.

jannaschiiandT.acidophilum),avibrionales(P.p

rofundum)andother

enterobacteria.Sequencefromlast

linerepresentsthetotalconservedaminoacids


123


15/19

Fig.

3

continued


123


16/19

transference of genetic material. However, this can be

ruled out by chromosomal integration and the elim-

ination of plasmids containing antibiotic markers

with available molecular tools (Datsenko and Wanner

2000). Moreover, the improvement of hydrogen

production by gene manipulation is mainly focused

on the disruption of endogenous genes and not

introducing new activities in the microorganisms.

New pathways must be discovered to directly takefull advantage of the 12 mol of H2 available in a mol

of hexose.

de Vrije and Claassen (2003) reported the cost of

hydrogen production using a locally produced ligno-

cellulosic feedstock. The plant was set at a production

capacity of 425 Nm3 H2/h and consisted of a thermo-

bioreactor (95 m3) for hydrogen fermentation fol-

lowed by a photo-bioreactor (300 m3) for the

conversion of acetic acid to hydrogen and CO2.

Economic analysis resulted in an estimated overall

cost of2.74/kg H2. This cost is based on acquisition

of biomass at zero value, zero hydrolysis costs and

excludes personnel costs and costs for civil works, all

of them with potential cost factors. Current estima-

tion for hydrogen production cost is 4/kg H2 or 30/

GJ H2. The estimation is done on the basis of processparameters which seem presently feasible (http://

www.biobasedproducts.nl/UK/5%20Projects/

frame%205%20projecten.htm).

Regarding feedstock costs, commercially pro-

duced food products, such as corn and sugar are not

economical for hydrogen production (Benemann

1996). However, by-products from agricultural crops

or industrial processes with no or low value represent

a valuable resource for energy production. Neverthe-

less, besides hydrogen biological production, other

biofuels (bioethanol, biodiesel, biobutanol, etc.) pro-cesses are being under development (Reisch 2006)

and, eventually, the demand of agricultural by-

products would increase its present low value.

According to the US DOE, for renewable H2 to be

cost competitive with traditional transportation fuels,

the sugar cost must be around $0.05/lb sugar

feedstock and provide a H2 molar yield approaching

10 (http://www1.eere.energy.gov/hydrogenandfuel-

cells/mypp/). Wastewater has a great potential for

economic production of hydrogen; only in the United

States the organic content in wastewater producedannually by humans and animals is equivalent to

0.41 quadrillion British thermal units, or 119.8 terra-

watt h (Logan 2004).

Currently, biologically produced hydrogen is more

expensive than other fuel options. There is no doubt

that many technical and engineering challenges have

to be solved before economic barriers can be

meaningfully considered.

Acknowledgements This work was supported by the Fondo

Mixto San Luis Potos Consejo Nacional de Ciencia yTecnologa (FMSLP-2005-C01-23).

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