8
Performances comparison between three technologies for continuous ethanol production from molasses Hassib Bouallagui *, Youssef Touhami, Nedia Hana, Amine Ghariani, Moktar Hamdi Laborat ory of Micro bial Ecology and Techn ology , Nation al Insti tute of Applie d Scienc es and Techn ology, BP 676, Tunis 1080, Tunisia a r t i c l e i n f o Article history: Received 7 August 2009 Received in revised form 16 October 2012 Accepted 20 October 2012 Available online 23 December 2012 Keywords: Molasses Ethanol Productivity CSTR ICR MBR a b s t r a c t Molasses are a potential feedstock for ethanol production. The successful application of anaerobic fermentation for ethanol production from molasses is critically dependent to the develop ment and the use of high rate bioreact ors. In this study the fermentat ion of sugar cane molasses by  Saccharomyces cerevisiae  for the ethanol production in a continuously stirred tank reactor (CSTR), an immobilised cell reactor (ICR) and a membrane reactor (MBR) was invest igated. Ethanol productio n and reacto r produc tivities were compared under different dilution rates (D). When using the CSTR, a decent ethanol productivity (Qp) of 6.8 g L 1 h 1 was obtained at a dilution rate of 0.5 h 1 . The Qp was improved by 48% and the residual sugar concentration was reduced by using the ICR. Intensifying the production of ethanol was investigated in the MBR to achieve a maximum ethanol concentration and a Qp of 46.5 g L 1 and 19.2 g L 1 h 1 , respectively. The achieved results in the MBR worked with high substrate concentration are promising for the scale up operation. ª 2012 Elsevier Ltd. All rights reserved. 1. Introduction In the past decades, natural energy resources such as petro- leum have been consumed at high rates. Therefore, alterna- tive resources as ethanol are becoming more important  [1,2]. Microbial ethanol production has been considered as renew- able fuel for future contributing to the reduction of environ- mental impacts  [3e5].  Saccharomyces cerevisiae  is the most use ful micr oor ganism for the ethanol production by the alcoholic fermentation of various raw materials rich in sugars [6]. Molasses is an agro-industrial by-product often used in alcohol distil ler ies due to the pre sence of fermentati ve sug ars, bei ng an opt ima l carbon source for the mic roorga nis m metabolism [7]. The use of molasses generated during the process of sugar cane rening has attracted great interest because they are rich in sucrose, which presents a substrate not requiring pre-treatment prior to the fermentation. During traditional batch fermentation for distilled ethanol produ ction using  S. cere visiae , inhibition of growth can be caused either by product or substrate concentrations, and the productivity is limited to only 1.8e2.3 g L 1 h 1 [8]. This leads us to dene new str at egi esfor inten si ve pr oduct ion in order to maximi se Qp and yie ld. Use of continuous rea ctors for ethanol produ ction has been investigat ed to improv e economics and the performance of fermentation process es [9]. Alt hough, high Qp can be achieved if cell retention is also employed  [10]. General strategies for cell retention include immobilisation of microbial cells within the fermenter or membrane separation from the product stream, followed by recycle to the fermenter [11,12]. *  Corresponding author. Tel.:  þ216 22524406; fax:  þ216 71704329. E-mail address: [email protected]  (H. Bouallagui).  Available online at  www.sciencedirect.com http://www.elsevier.com/locate/biombioe biomass and bioenergy 48 (2013) 25 e32 0961-9534/$ e  see front matter ª 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biombioe.2012.10.018

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Immobilisation of microbial cells for fermentation has

been developed to eliminate inhibition caused by high

concentration of substrate and product, also to uncouple the

hydraulic retention time and the cellular retention time.

Immobilisation of cells to a solid matrix is an alternative

means of high biomass retention. Alginate is widely used in

food, pharmaceutical; textile and paper products. The use of 

alginate is for thickening, stabilising, gel and film forming  [6].Recent works on ethanol production in an immobilised cell

reactor showed that production of ethanol using  Zymomonas

mobilis and  S. cerevisiae was increased significantly [13,14].

The Qp can be also improved by increasing the concentra-

tion of viable biomass in the reactor. In this approach many

researches studied cultures at high cell density. The best

performance was obtained using continuous systems with cell

recycling  [15e17]. Membrane filtration is most frequently used

to increase the concentration of viable biomass in the reactor.

We musttake intoaccount the characteristics of the membrane

(material, porosity, size and number of pores, charge of 

surface), the nature and composition of species present in the

solution and the operating conditions (applied pressure, thesolution concentration, temperature, pH and ionic strength).

The aim of this work was to study the alcoholic fermen-

tation of sugar cane molasses by using CSTR, ICR and MBR

technologies. In order to findthe better alternative with higher

ethanol productivity, the reactors performances were inves-

tigated at different loading rates.

2. Materials and methods

2.1. Yeasts and molasses sources

Sugarcane (Saccharum officinarum)  cultivars employed in thiswork were harvested in the central province of Santiago

(Cuba) on 2008. Molasses, from which fermentation medium

were prepared, were obtained from the STS Company of 

Tunis. They have the following composition: dry matter

(72.2%), total sugars (48.5%), ash content (6.7%) and pH of 7.9.

The feedstock solution was boiled for 5 min, centrifuged and

filtered for pre-treatment and clarification. In the clarification

step, a part of the coloured material and unknown toxic

substances frequently included in the molasses were sepa-

rated or inactivated and the molasses were diluted with

distilled water to give a total sugar concentration of 100 g L1.

pH was adjusted to 5 with 10% sulphuric acid concentration.

Medium used for bioreactors feeding consisted of yeastextract (10 g L1), peptone (5 g L1), NH4Cl (2 g L1), KH2PO4,

MgSO4  H2O (0.5 g L1), sugars (100 g L1) and 200 mg L1Na-

thioglyconate as the reducing agent at pH 5 [18]. The culture

medium was sterilized at 121   C for 15 min. The initial

oxidation reduction potential (ORP) of the medium was nearly

250 mV indicating the anaerobic conditions.

The different fermentations have been achieved using 

fresh commercial baker’s yeast S. cerevisiae (Tunisian Society

of Yeasts). The yeast strain viability was determined using 

methylene blue staining technique. It was cultivated using an

incubator shaker under sterile conditions at pH 5, 30   C and

150 revolutions min1. Pure cultures grown under anaerobic

conditions were used forinoculation of experimentalreactors.

2.2. Laboratory reactors configurations and operating

conditions

Three types of reactors were used to carry out the continuous

fermentation at 30  C (CSTR: Fig. 1, ICR: Fig. 2 and MBR: Fig. 3).

The fermentation in the CSTR was conducted in a 1 L Bio-

lafit fermentor using a pre-culture of 200 mL seeded in

a medium with a sugar concentration of 50 g L1. An aeratedbatch phase for 48 h was provided to produce a sufficient

concentration of biomass of 18 g L1. At the end of the batch

phase, the continuous culture was conducted in anaerobic

conditions to ensure the fermentation. The CSTR was fed with

a sugar concentration of 100 g L1. Different dilution rates

were tested. A D of 0.12 h1 (run 1) was applied during the first

246 h of monitoring, a D of 0.25 h1 (run 2) was applied from

time 246 h to time 415 h and a D of 0.5 h1 (run 3) was applied

during the last 85 h of work.

The fermentation set-up of the ICR was comprised of 

a Biorad column packed with beads of immobilised cells. The

immobilisation of  S. cerevisiae was performed by the enriched

cells cultured media harvested at the exponential growthphase. The fixed cell loaded ICR was carried out at initial stage

of operation and the cell were entrapped by calcium alginate

(2%)   [6,8]. The fermentation in the ICR has been done over

a period of 225 h. Ds were adapted to 0.12, 0.25 and 0.5 h 1,

respectively.

The MBR system (YUASA Membrane Systems: ED-03SPH) is

composed of two reactors: the fermentation tank and

a membrane tank (1 L each) maintained in anaerobic condi-

tions. A recirculation of flux was maintained between the

fermentation tank and the membrane tank with a high flow

rate to maintain similar conditions in both tanks in terms of 

mixed liquor total suspended solids. Then, the dilution rate

was calculated using the 2 L total volume of the fermentor andthe membrane unit. The filtrationunit consisted of an external

filtration module with a nominal cut-off filter of 0.4   mm and

a filtration surface of 0.014 m2. The maximum allowable

pressuredropoverthefilterwas4bars.Thestart-upoftheMBR

culture wasprovidedfrom theoutletof theCSTR with an initial

biomass concentration of 8 g L1 and a pH of 4.8. The

fermentation tank was loaded with an initial sugar concen-

tration of 100 g L1. The system was operated 245 h without

cleaning resulting in a membrane flux of 21.4e35.7 L h1 m1.

There has been a decrease in the permeate flow over time

Feed Tank    Pump Fermenter   Pump

CO2

Product Tank 

Fig. 1  e  Schematic of the laboratory scale continuously

stirred tank reactor.

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which corresponded to the membrane fouling. Consequently

thedilutionrate initiallyset at0.5 h1 fallduringthefirst90hof 

working to stabiliseat 0.31 h1. Allthe experiments occurred in

triplicate and the relative standard deviations among the

results did not exceed 5%.

2.3. Technical analysis

Kinetic of fermentations was monitored by measuring the

concentration of biomass, total reduced sugars (TRS) and

ethanol during the time. The samples were centrifuged

(10,000 g) and the biomass concentration wasdetermined by

measuring the optic density (OD) of diluted sample at 600 nm

using a standard curve of absorbance against dry cell mass

(TSS: total suspended solids). The TRS concentration was

measured by using the phenol acid method [19]. The samples

were analysed in triplicates and results were reproducible. pH

and ORP were measured using a pH metre (WTW).

Ethanol production was analysed by high performance

liquid chromatography on an Agilent Model 1200 series

liquid chromatograph equipped with four solvent pumps,a programmable multi-wavelength detector and a data

module. The mobile phase was 1 mol m3 H2SO4. Aliquots of 

20   mL were injected into a silica column (ZORBAX C18)

(150 mm 3.9 mm) at ambient temperature. The flow rate of 

the mobile phase was 0.3mL min1 and the analysis was done

under isocratic mode (no composition change of the solvent).

Quantification of ethanol was done by using standard ethanol.

Samples were diluted with ultra pure water and filtered with

millipore membranes (0.22  mm pore size).

2.4. Statistical analysis

In order to determine whether the observed differences

between reactors performances were significantly different,

data were subjected to the analysis of variance by ANOVA

tests ( p < 0.05) [20].

3. Results and discussion

3.1. Performances of the CSTR

The alcoholic fermentation provided in the CSTR has been

achieved by varying the D from 0.12 h1 to 0.5 h1.

Fig. 4   shows the variation of total sugar and ethanolconcentrations. During the first run (D   ¼   0.12 h1) the

concentration of residual sugar was stabilised at an average

value of 18.3 g L1 equal to a substrate conversion yield (Ys) of 

81.7%. This condition permitted to succeed to an average

ethanol concentration of 27.9 g L1 and a yield ethanol for

substrate (YP/S) of 0.34 g g 1. Average Qp of 3.34 g L1 h1 has

been determined.

Feed Tank Pump

Valve

Pump

Valve

Product Tank 

     I   m   m   o     b     i     l     i   s   e     d     C   e     l     l     R   e   a

   c    t   o   r

Fig. 2 e  Schematic of the laboratory scale immobilised cell

reactor.

Circulation

Tank 

Feed Tank Feed Pump   Fermenter   Circulation

PumpCirculation

Pump  Membrane

Permeate Tank 

(Permeate)(Concentration)

P

P

CO2

(V)

(V)

(V)(V)

Fig. 3 e

 Schematic of the laboratory scale membrane bioreactor (P [ pressure measurement and V [  valve).

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In the second run, the increase of the D to 0.25 h1 caused

a disruption of the system balance which resulted in a signif-

icant drop in the residual sugar concentration, accompanied

by an increase in the alcohol concentration. The important

input of substrate excited the fermentation metabolism of 

cells to produce high quantity of ethanol which reached

a maximum concentration of 40.9 g L1. This result showed

the coupling between biomass and ethanol productionsbecause the biomass concentration has risen in this phase to

reach a maximum value of 9 g L1. At the end of this run

ethanol and residual sugar concentrations were stabilised at

an average values of 21.6 g L1 and 31.1 g L1, respectively,

equal to a Ys yield of 68.9%. The YP/S   yield decreased to

0.31 g g 1 and the Qp increased to 5.4 g L1 h1 by the appli-

cationofa D of0.25h1. In fact,the Qp depends on the ethanol

concentration and the dilution rate that has been increased.

The highest Qpof 6.8 g L1 h1 for theCSTRwas observedat

a D of 0.5 h1 (run 3). Applying a high D caused a progressive

decrease in the concentration of ethanol to achieve an average

value of 11.6 g L1, accompanied by an increase in the

concentration of TRSs (51.5 g L1). Then, the Ys fell to 48.5%corresponding to an YP/S of 0.24 g g 1. At high D, the assimi-

lation declined and a significant part of sugars was washed

out. This result showed that cells fail to produce high quantity

of ethanol at high D due to the wash out of cells at high flow

rate. The same Qp in order of 5.98 g L1 h1 with a high YP/S

yield of 0.47 g g 1 were obtained by Purwadi and Taherzadeh

[9]   in a continuous fermentation of glucose by applying 

a dilution ratio of 0.86 h1. However, using hydrolysed ligno-

celluloses wood after an acid pre-treatment has led to a high

Qp in the order of 20 g L1 h1 by conducting a continuous

fermentation reactor in a volume equal to 0.45 L, with a D of 0.3 h1 [21].

At run 1 (D ¼ 0.12 h1), the biomass concentration dropped

considerably from 18 g L1 to 4.8 g L1 after passing through

fluctuations (Fig. 4). This decrease corresponds to a transi-

tional state of cells adaptation characterised by a very

important wash out of cells out side the reactor. The stabili-

sation of biomass concentration was reached after 75 h of 

operation of the system. The increase in the D from 0.12 to

0.25 h1 caused an acceleration of cell multiplication to ach-

ieve a biomass concentration of 9 g L1, followed by a fall, then

stabilised. The fall of the biomass concentration was caused

by the important flow rate that created a significant wash out

of cells. After 108 h of operation under this condition thebiomass stabilised and reached steady state at a concentra-

tion of 4.9 g L1. The second change of the D (run3) led to

a further reduction in the biomass concentration to be

0

2

4

6

8

10

12

14

16

18

20

0 50 100 150 200 250 300 350 400 450 500

Time (h)

   B   i  o  m  a  s  s  c  o  n  c  e  n   t  r  a

   t   i  o  n

   (  g   L  -   1   )

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

5,5

6

  p   H

  a  n   d   D

   (   h  -   1   )

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350 400 450 500

Time (h)

   T   R   S  a  n   d  e   t   h  a  n

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

   (  g

   L  -   1   )

0

0,1

0,2

0,3

0,4

0,5

0,6

   D

   (   h  -   1   )

a

b

Fig. 4 e Evolution of (a): total reducing sugar (TRS) ( > ), ethanol ( B ), (b): biomass ( > ) and pH ( B ) during CSTR fermentation of 

sugar cane molasses under different Ds ( D ).

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2.4 g L1. The application of a high D of 0.5 h1 caused

a significant wash out of cells. The continuous increase of the

D to a value exceeding the maximum specific growth rate

(mmax) induced the phenomenon of leaching. In these condi-

tions growth does not compensate the biomass washed out

due to the increase of the flow rate. However, if the out flow of 

biomass exceeds the growth yield of yeasts, the continuous

cultivation fails and the biomass is washed out. Taherzedehet al.   [22]   showed that the continuous fermentation of 

a fermentable hydrolysate by yeast can be successful at a D of 

0.1 h1, but fails at a D of 0.2 h1 or higher.

The initial pH of feed was regulated to 5.5 by adding 

a diluted sulphuric acid. Fig. 4 showed that the pH decreased

slightly during the fermentation and stabilised around values

of 4.6e5. This slight decrease can be explained by the CO 2

production during the fermentation and the accumulation of 

ions HCO

3  which represent the soluble form of CO2. It can be

also explained by the production of metabolites other than

ethanol (acetate, succinic acid, etc.) by the yeast during the

fermentation.   S. cerevisiae   have been reported to increase

ethanol production at pH 5.0 and 5.5 as opposed to pH 4.0 and4.5 and its optimum pH is from 5.0 to 5.2  [23].

3.2. Performances of the ICR

The ICR fermentation monitoring has been done over a period

of 225 h. The applied Ds were adapted to 0.12, 0.25 and 0.5 h1.

The results of the different runs are presented in  Fig. 5 which

showed that the production of ethanol was steady after 18 h of 

operation and maximum concentration of ethanol was

reachedduring the first 115 h of the fermentation. The average

concentration of ethanol during the stationary phase was

44.1 g L1. In fact, Qp reached during this D was 5.28 g L1 h1.

The concentration of ethanol was affected by the mediaflow rates and the residence time distribution. Following 

a first increase in the D from 0.12 h1 to 0.25 h1 after 115 h of 

fermentation, a reduction in the ethanol concentration was

observed and the Qp increased significantly (P < 0.05). Indeed,

with the increase of the D, yeasts fail to completely consume

sugar in the culture medium. Therefore the YP/S yield reduced

and the concentration of residual sugars become more

important (9.6 g L1). The decrease in the concentration of 

ethanol could be caused by the combined effect of increasing 

the medium flow rate and the loss of cell viability over time.

The maximum Qp of 10.1 g L1 h1 was obtained with a D of 

0.5 h1. Goksungur and Zorlu   [24]  achieved a Qp very close

(10.16 g L1 h1) to that of our work by applying a D of 0.22 h1.While, Baptista et al. [8]  achieved a higher Qp of 16 g L1 h1

with a D of 0.4 h1, with the same type of bioreactor but using 

cells of  S. cerevisiae immobilized on polyurethane cubes.

Monitoring of pH during this fermentation showed

a decrease of values from 5.5 to 4.3e5.3.This decrease was due

to the increase of dissolved CO2 in the reactor as mentioned

previously. The dissolved CO2   in the solution reacts with

water to form carbonate ions and proton Hþ. According to the

literature, the intracellular pH varies slightly for an extracel-

lular pH ranging between 4 and 7. In this range of pH, cell

viability can not be affected by the pH because its value

remained constant and greater than 4  [23].

3.3. Performances of the MBR

Fig. 6 shows a very important increase in the concentration of 

biomass in the MBR from an initial value of 8 g L1 to a final

value of 41.1 g L1 after 245 h of work. This increase in the

biomass concentration was accompanied with a gradual

decrease in the permeate flux due to the membrane fouling.

The concentration of biomass at the outlet of the MBR was

very low; it is equal to 0.05 g L1. This result confirms the

proper functioning of the membrane system that can retain

all the cells inside the bioreactor. pH within and in the outlet

of the bioreactor were the same which showed that the

membrane system does not affect the pH.Fig. 6 shows an almost consumption of total sugars. The Ys

yield was close to 97.7e99.3% with a residual concentration

lower than2.5g L1. The consumed sugar was headed towards

the growth of yeasts resulting in a very important activity

and ethanol production with average concentrations of 

0

10

20

30

40

50

60

0 25 50 75 100 125 150 175 200 225

Time (h)

  s  n  o   i   t  a  r   t  n  e

  c  n  o  c   l  o  n  a   h   t  e   d  n  a

   S   R   T

   (  g   L  -   1   )

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

5,5

6

   D

   (   h  -   1   )  a  n   d  p   H

Fig. 5 e Evolution of total reducing sugar (TRS) ( > ), ethanol ( , ) concentrations and pH ( D ) during ICR fermentation of sugar

cane molasses under different Ds ( B ).

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41.4e46.5 g L1. The high consumption of sugars and

production of ethanol are independent to the loading flow

rate. They are the result of the high concentration of biomass

in the bioreactor, which provided an important YP/S   with

a constant rate, even during the change of flow. The stabili-sation of biomass concentration in the MBR can be explained

by the fact that the high concentration of ethanol becomes

a limiting factor for the multiplication of cells and induced the

yeast metabolism for the alcoholic fermentation.

During the MBR system monitoring results showed two

phases characterised by two levels of Qp. The first Qp corre-

sponds to a D of 0.31e0.5 h1 during the first 90 h which was

about 12.83e19.2 g/L h. The second phase was characterised

by a Qp of 14.4 g L1 h1 which corresponds to a D of 0.31 h1.

After the membrane fouling, the Qp decreased significantly

( p < 0.05). The decrease of the Qp can be avoided by studying 

the membrane behaviour during the operation to learn the

moment when it must be regenerated by flow reversal or

chemical cleaning.

The intensification of ethanol production from hydrolysed

wood in a membrane bioreactor was studied by Lee et al.  [15].

They obtained a similar Qp of 16.9 g L1 h1 with ethanol

concentration of 76.9 g L1 and a YP/S of 0.43 g g 1. However,higher Qp has been also obtained byBen Chaabene et al. [17] in

a two stage MBR. They mentioned an important Qp in the

order of 41 g L1 h1 and an ethanol concentration of 80 g L1.

Escobar et al. [25] have implemented a membrane bioreactor

of 7000 L. They obtained a Qp of 3.4 g L1 h1 with ethanol

concentrations ranged between 80 and 90 g L1. However, the

authors mentioned some problems of membrane fouling with

biomass concentrations above 100 g L1. They also observed

a drop in the cell viability due to the stress caused by the

pump.

3.4. Comparison of reactors performances

A comparison between performances of bioreactors used for

the ethanol production has been made.

Table 1   summarizes the results obtained with different

configurations of reactors. The average ethanol concentra-

tions decreased by increasing the D however, productivities

increased. The Qp depends not only to the ethanol concen-

tration but also to the D hence the interest in working with

high D exceeding the   mmax. This was achievable only by

working with the MBR which provided high cell densities

within the reactor.

Data showed that ethanol concentration and Qp during the

fermentation of sugar cane molasses in the ICR are higher

than those obtained with the CSTR. In addition, YP/S yields aremore important in the ICR. This was mainly caused by the

immobilisation of cells in the reactor which slowed cell

proliferation and promotes the metabolism to the alcoholic

fermentation. Subsequently, yeasts consumed the substrate

in advantage to produce ethanol.

The fermentation of molasses through the MBR bioreactor

improved the Qp, the concentration of ethanol and the YP/S

yield. In fact, Qp achieved in this reactor was three times

higher than that in the CSTR and it was two times higher than

that in the ICR. The Ys yield showed very high levels

0

5

10

15

20

25

30

35

40

45

0 30 60 90 120 150 180 210 240

Time (h)

   B   i  o  m

  a  s  s  c  o  n  c  e  n   t  r  a   t   i  o  n   (  g   L  -   1   )

  a  n   d

  p  e  r  m  e  a   t  e   f   l  u  x   (   L   h  -   1   m

  -   2   )

0

10

20

30

40

50

60

0 30 60 90 120 150 180 210 240

Time (h)

   E   t   h  a  n  o   l  c  o  n  c  e  n   t  r  a   t   i  o  n   (  g   L  -   1   )

0

1

2

3

4

5

6

7

8

  p   H

  a  n   d   T   R   S   (  g   L  -   1   )

a

b

Fig. 6  e  Evolution of (a): biomass concentration ( > ),

permeate flux rate ( , ), (b): total reducing sugar (TRS) ( > ),

ethanol concentrations ( D ) and pH ( , ) during MBR 

fermentation of sugar cane molasses.

Table 1 e Performances of the CSTR, ICR and MBR bioreactors used for the ethanol production at different dilution rates.

Reactors CSTR ICR MBR   P

Run 1 Run 2 Run 3 Run 1 Run 2 Run 3 Run 1 Run2

Dilution rate D (h1) 0.12 0.25 0.5 0.12 0.25 0.5 0.31e0.5 0.31   e

Ethanol concentration (g L1) 27.9 1.9 21.6 1.9 11.6 1.6 44.06 1.9 32 1.1 20.2 0.6 41.4 1.8 46.5 1.5 0.0032

Productivity: Qp (g L1 h1) 3.34 0.2 5.4 0.3 6.8 0.3 5.28 0.2 8.0 0.4 10.1 0.6 12.83e19.2 14.41 0.6 0.0061

Substrate inlet (g L1) 100 100 100 100 100 100 100 100

Substrate outlet (g L1) 18.3 1.7 31.1 2.9 51.5 1.2 2.9 0.4 9.6 0.6 26.8 1 2.3 0.2 0.7 0.1 0.004

Sugar consumption: Ys (%) 81.7 2.1 68.9 2.4 48.5 1.8 97.1 3.2 90.4 4.1 73.2 3.8 97.7 2.8 99.3 2.9 0.0012

Biomass in the reactor (g L1) 4.8 0.4 4.9 0.16 2.4 0.13   e e e   8.4e26.7 26.13e41.1   e

YP/S yield (g ethanol

g substrate1)

0.34 0.01 0.31 0.01 0.24 0.01 0.45 0.02 0.35 0.01 0.27 0.01 0.42 0.02 0.46 0.01 0.0056

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exceeding 98%, especially for MBR and ICR configurations

which indicated that sugars were completely consumed by

yeast.

The experimental YP/S  yield compared to the theoretical

performance is more important in both ICR and MBR reactors

than in the CSTR because the microbial activity in both cases

is more important than for theCSTR. The best YP/S of0.46gg 1

and the Ys of 99.3% were obtained with the MBR. However, inthe case of CSTR YP/S   decreased as the cell concentration

within the reactor was diluted by increasing the D.

Themass balancein the systemswas calculated. The mass

substrate was converted in to ethanol, carbon dioxide,

biomass, glycerol and organic acids. The total feed mass was

calculated from the mass of sugars feeding per day. The

biomass generated may be accumulated in the reactors or

washed out in theeffluent daily.It was measured byTSS in the

CSTR and the MBR and by volume variation in the ICR. Results

showed that mass recovery from substrates ranged between

78% for the CSTR to 99% for the MBR. It is very likely that the

low mass balance in the CSTR was due to an ethanollostin the

carbon dioxide event and the secondary metabolites produc-tion like glycerol and organic acids.

4. Conclusion

Results showed that the performance of the MBR was more

advantageous than the CSTR and the ICR for anaerobic sugar

cane molasses fermentation since a higher concentration of 

ethanol (41.4 g L1) and Qp (12.83e19.2 g L1 h1), with low

residual substrate content were achieved. These results

showed the interest of using the MBR to reach a high ethanol

concentration which confirmed the effectiveness of thistechnology.

Acknowledgements

The authors wish to acknowledge the Ministry of Superior

Education and Scientific Research and Technology, which has

facilitated the carried work.

Abbreviations

CSTR continuously stirred tank reactor

D dilution rate (h1)

ICR immobilised cell reactor

MBR membrane bioreactor

OD optic density

m max (mu-max) maximum specific growth rate (h1)

ORP oxidation reduction potential (mV)

Qp ethanol productivity (g L1 h1)

TRS total reduced sugars (g L1)

YP/S   mass ratio between ethanol and sugar (g g 1)

WTW WTW GmbH, Weilhem, Germany

Ys substrate conversion yield (%)

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