2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 91 -
The equation demonstrated that interaction between
inoculum size and KH
2
PO
4
showed significance. It could
be proved by
Fig.1E
that the isoresponse contour between
these two items showed interaction.
Fig. 1F
showed the
effect of inoculum size and yeast extract on the ethanol
concentration. The convex response surface suggested
well-defined optimum variables (inoculum size and yeast
extract) and that the ethanol concentration increased to
the peak with the increase of inoculum size and yeast
extract up to 15% and 0.9 g/l, respectively; then declined
with the further increase of these two parameters. This
result demonstrated that the response surface had a
maximum point for ethanol production. The optimum
condition for ethanol production under non-sterile
condition was 1.44 g/l (NH
4
)
2
SO
4
, 0.93 g/l KH
2
PO
4
, 0.9
g/l yeast extract and 15 %v/v inoculums size.
Ethanol
yield in batch fermentation using co-culture was
predicted at 0.16 g-ethanol/g-food waste (79.5 g/l), which
was 97% of the theoretical yield.
The ethanol production
increased by 57.2% as compared with the use of raw food
waste hydrolysates (34 g/l).
B. Model Validation and Confirmation
To confirm the validity of the statistical experimental
strategies and gain a better understanding of ethanol
production from FWH, a confirmation experiment with
duplicate set was performed at the specified optimum
condition. Experiments conducted at the optimum
condition (1.44 g/l (NH
4
)
2
SO
4
, 0.93 g/l KH
2
PO
4
, 0.9 g/
yeast extract and 15 %v/v inoculums size) demonstrated
that the ethanol concentration (77.6 g/l) was closer to the
predicted value (79.5 g/l). Corresponding to the ethanol
concentration of 77.6 g/l, the ethanol yield was calculated
as 0.15 g-ethanol/g-food waste. This showed that the
model was useful to predict the ethanol concentration as
well as optimize the experimental conditions.
Z. mobilis
alone yielded 0.11 g-ethanol/g-food waste (54.2 g/l),
which is 65% of the theoretical yield and
C. shehatae
alone yielded 0.09 g-ethanol/g-food waste (48 g/l),
reaching more than the published value for
C. tropicalis
with starch
[35]
.
T
ABLE IV
C
OMPARISON OF ETHANOL FERMENTATION AMONG THE INDIVIDUAL
AND MIXED STRAINS FERMENTATION IN
250
FLASK AND
1
L FERMENTOR
250 ml flask fermentation
1 l
fermentor
Z.
mobilis
C.
shehatae
Z. mobilis
+
C. shehatae
Z. mobilis
+
C. shehatae
Ethanol
production (g/l)
54.2
48
77.6
78.8
Ethanol
yield
(g-ethanol/ g-
Food waste)
0.11
0.09
0.15
0.16
Ethanol
yield
(g-ethanol/ g-
reducing sugar)
0.33
0.29
0.47
0.48
Theoretical
ethanol
yield
(%)
65
58.6
94.6
96
Process was scaled up with FWH had shown higher
ethanol yield than batch fermentation (
Table IV
). Ethanol
yield in batch fermentation using mixed culture was 0.15
g-ethanol/g-food waste in 72 h, which was 94.6% of the
theoretical yield. Ethanol yield by
Z. mobilis
alone was
0.11 g-ethanol/g-food waste (54.2 g/l), which is 65% of
the theoretical yield and
C. shehatae
alone yielded 0.09
g-ethanol/g-food waste (48 g/l) which was 58.6% of
theoretical yield in 72 h (
Table IV
). However, in the 1 l
fermentor using mixed culture, the ethanol yield was 0.16
g-ethanol/g-food waste (78.8 g/l) which was 96% of the
theoretical yield. Reproducibility of the process was
checked in repeat runs with the above conditions.
IV. C
ONCLUSIONS
A significant improvement in ethanol yield (0.48g/g-
reducing sugar) was demonstrated, resulting in very low
sugar and fewer by-products. CCD design have shown
that yeast extract and inoculum size are the key
parameters that influence ethanol production from FWH,
while (NH
4
)
2
SO
4
and KH
2
PO
4
showing a little effect.
Maximum ethanol concentration of 79.5 g/l was obtained
at the optimum condition of 1.44 g/l (NH
4
)
2
SO
4
, 0.93 g/l
KH
2
PO
4
, 0.9 g/l yeast extract and 15 %v/v inoculums
size. The ethanol concentration at the optimum
experimental condition (77.6 g/L) agreed well with the
predicted one (79.5 g/L). This indicated the suitability of
the model employed and the success of RSM to optimize
the conditions of ethanol production from FWH. The
ethanol yield was reproduced in 1 l fermentor with 0.16
g-ethanol/g-food waste (78.8 g/l) which was 96% of the
theoretical yield. The results from the investigation
showed that FWH can be used as an alternative substrate
for ethanol production, in comparison to virgin biomass
resources such as energy-rich crops, if sterilized suitably
prior to fermentation by some low cost energy sources
such as excess heat or waste heat from some industrial
processes adjacent to ethanol production facility.
A
CKNOWLEDGMENT
This work was financial supported by Thaksin
University and National Research Council of Thailand
(NRCT).
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