2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 24 -
xylose concentration to 8 hr and 5 g/L, respectively.
Afterwards, it was stepwise increased 30 g-xylose/d L by
feeding with xylose 10 g/L at a 8-hr HRT. The OLR was
further increased to 60 and 120 g-xylose/d L by feeding
10 g-xylose/l at the HRT of 4 and 2 hr, respectively.
Each level of loading was kept, after steady state was
achieved, for 2 HRTs. The process was considered to be
in steady state when the variation of the gas produced
was less than 5% [14].
B. Monitoring and analysis
The volume of produced gas was recoreded by water
displacement gas meter and the composition was
analysed by gas chromatography GC-8APT, Shimadzu,
Japan equipped with a thermal conductivity detector
(TCD). Liquid samples were taken at designated time for
further analysis of pH, volatile fatty acids (VFAs),
alcohols, lactate, formate, and xylose. The VFAs and
alcohol determined by using GC
–
8APF, Shimadzu, Japan
equipped with a flame ionization detector (FID) [15].
Lactate and formate were analyzed by suppressed ion
exclusion chromatography equipped with a high
performance liquid chromatography (HPLC) pump
(L2100 Hitachi). Xylose was analyzed by HPLC
equipped with refractive index (RI) detector [14].
III. R
ESULT AND
D
ISCUSSION
A. UASB reactor performance
The UASB reactor was continuously fed xylose
supplemented with nutrients at different OLRs of 15 30,
60, and 120 g-xylose/d/L. Monitored parameters
(hydrogen production rate, effluent xylose concentration,
pH, and metabolite concentrations) during its continuous
operation are shown in Fig. 1. Through out the operation
period, hydrogen content in the produced gas was varied
in the range of 34
–
47%, the rest being carbon dioxide.
Additionally, the reactor was able to self maintained pH
in the range of 4.9-5.7. At the initial OLR of 15 g-
xylose/d/L, a stable hydrogen production rate of 2129.4 ±
92.3 mL
–
H
2
/d/L was achieved simultaneously with
complete xylose degradation. The hydrogen production
rate increased up to the highest value of 15073.8 ± 675.5
mL
–
H
2
/d/L corresponding to the hydrogen yield of
251.2±11.3 ml
–
H
2
/g-xylose at the OLR of 60 g
–
xylose/d/L. At the same time, xylose in the UASB reactor
was still almost fully degraded with less than 0.5 g/L
remaining. Moreover, high hydrogen production was
accompanied with the generation of butyrate and acetate
as the dominant metabolites. However, further increasing
the OLR to 120 g-xylose/d/L, the hydrogen process
became overload, as indicated by sharp increase of
effluent xylose concentration from less than 0.5 g/L to
approx. 2.7 g/L and slight decrease of the hydrogen
production rate.
B
.
AF reactor performance
The AF reactor was also continuously fed xylose
supplemented with nutrients at the same OLRs fed into
the UASB reactor. No methane was detected and
hydrogen content in the gas phase was in around 35
–
47% during the experiment.
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
H
2
Production Rate (mL/d L)
0
0.5
1
1.5
2
2.5
3
3.5
EffluentXyloseConcentration (g/L)
HydrogenProductionRate
EffluentXylose Concentration
0
5
10
15
20
25
30
35
40
0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60
Time (Day)
MetabolicProducts (mM)
0
1
2
3
4
5
6
7
8
pH
ETOH AA PA BA LA FA pH
Fig. 1. Profiles of hydrogen production rate, effluent xylose
concentration, pH, and metabolite concentrations in the UASB reactor
operated at 55 ºC and the OLR of 15 (
I
), 30 (
II
), 60 (
III
), and 120 (
IV
)
g-xylose/d/L. ETOH: ethanol; AA: acetate; PA: propionate; BA:n-&
iso-butyrate; LA: lactate; FA: formate.
The pH was self maintained within the range of 5.0-
5.6 through out the operation period as well. Fig.2 shows
the profiles of hydrogen production rate, effluent xylose
concentration, pH, and metabolite concentrations during
the continuous operation.
T
he hydrogen production rate
of 2625.6 ± 246.4 ml-H
2
/d/L and the yield of 175.0 ± 16.4
mL-H
2
/g-xylose was achieved under the steady state
condition by feeding with the OLR of 15 g-xylose/d/L.
Increasing OLR up to 120 g-xylose/d/L resulted in
significant increasing of hydrogen production rate up to
19383.5± 693.9 ml-H
2
/d/L, corresponding to hydrogen
yield of 161.5 ± 5.8 mL
–
H
2
/g
–
xylose. Meanwhile, the
remaining xylose concentration of 1.9 g/L in the effluent
was observed simultaneously with stable hydrogen
production rate. This demonstrates that the AF reactor is
more capable to produce hydrogen than the UASB
reactor at high OLR of 120 g
–
xylose/d/L. Thus, the up
–
flow anaerobic reactor is a trade off between economic
efficiency (based on hydrogen production rate) and
technical efficiency (based on hydrogen yield) and when
the immobilized cell configurations of granule
–
based
UASB reactor and biofilm
–
based AF reactor are
compared. Butyrate and acetate were also the main
soluble end products generated during the operation of
AF reactor fed with xylose solution.
I
II
III
IV
