2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 28 -
hydrogen gas production, hydrogen yield and specific
hydrogen gas production rate (SHPR) were reported
and the performance of the test bioreactors was
compared.
III. R
ESULTS AND DISCUSSION
For both sets studied in the experiment, cumulative
hydrogen production gradually increased with
increasing initial pH. Figs. 1 and 2 depict the time
course of variations in the cumulative hydrogen
production at different initial pH’s of the wastewater
used. The cumulative hydrogen production reached the
highest level within 24 hours when the Cell:COD ratio
was 4 (Fig. 1) and it reached the highest level within
120 hours when the ratio was at 0.26 (Fig. 2). More
than 80% of the H
2
gas was produced within 24 hrs.
There was negligible methane (CH
4
) detected in the
biogas under these conditions.
Fig. 1. Time course of H
2
production from bioreactors at different
initial pH with experiments carried out with the Cell : COD at 4
(Batch set 1).
Fig. 2. Time course of cumulative H
2
production from bioreactors at
different initial pH with experiments carried out with the Cell : COD
at 0.26 (Batch set 2).
The highest cumulative hydrogen production was
obtained at the initial pH 7 (24.8 ml for Batch set 1, and
51.3 ml for Batch set 2) and the lowest was at the initial
pH 4 (12 ml for Batch set 1, and 4 ml for Batch set 2).
Increasing the initial pH of the starch wastewater
increased the H
2
production (Figs 1 and 2).
During the operation time of 120 hours, COD
decreased from an initial level of approximately 25000
mg/l to approximately 12000 mg/l for Batch set 1 at
initial pH 7 (Cell: COD ratio = 4) and to approximately
18000 mg/l for Batch set 2 at initial pH 7 (Cell: COD
ratios=0.26). The quantity of COD degraded in the
fermentation was used to calculate the H
2
yield.
H
2
yield and specific hydrogen production rate
(SHPR, ml H
2
/g cells- hr) are important parameters
used to compare the different conditions. The H
2
yield
is defined as the ratio of H
2
produced in the experiment
to the waste degraded. From the study of the Batch set
1, the cumulative H
2
produced was 16.6, 16.4, 19.3 and
24.8 ml at STP (Fig. 1) and the COD degraded in the
serum bottles was 11050, 9843, 9134, 11405 mg/l (at
initial pH 5.5, 6, 6.5 and 7, respectively). Thus, the H
2
yield obtained was 15, 17, 21 and 22 ml at STP/g COD
degraded (at initial pH 5.5, 6, 6.5 and 7, respectively)
(Fig.3) From the study of the Batch set 2, the
cumulative H
2
produced was 2.0, 1.8, 28.8, and 51.3 ml
at STP (Fig. 2) and the COD degraded in the serum
bottles was 2030, 1350, 6240 and 8100 mg/l (at initial
pH 5.5, 6, 6.5 and 7, respectively). Thus, the H
2
yield
obtained was 10, 14, 46 and 63 ml at STP/g COD
degraded (Fig.3). The H
2
yield obtained from Batch set
1 (Cell:COD = 4 g cell/g COD) was compared with that
of Batch set 2 (Cell:COD = 0.26 g cell/g COD) at
different initial pH’s of wastewater and the comparativ
e
results are shown in Fig. 3.
Fig. 3. Comparison of H
2
yield from bioreactors operated with the
Cell : COD at 4 to that of Cell : COD at 0.26 (results show at initial
pH 5.5 -7).
Argun et al. [8] studied the effects of substrate and
cell concentration on bio-hydrogen production from
ground wheat using combined dark and photo-
fermentation at a controlled pH of 6-7. Their
experiments were performed using combined pure
cultures of dark and light fermentation bacteria. In their
study, the highest H
2
yield was 156.8 ml/g starch, a
yield that was higher than that obtained in this study.
This is possible due to differences in the reactions
involved in the fermentation. Combined dark-light
fermentation by pure cultures was much more efficient
than dark fermentations alone. However, dark
fermentation by mixed culture is feasible for
fermentation.
0
5
10
15
20
25
30
0
24
48
72
96
120
144
Time (hours)
Cummulative hydrogen (ml at STP)
)
pH4
pH4.5
pH5
pH5.5
pH6
pH6.5
pH7
Control
0
10
20
30
40
50
60
0
24
48
72
96 120 144
Time (hours)
Cummulative hydrogen (ml at STP)
)
pH4
pH4.5
pH5
pH5.5
pH6
pH6.5
pH7
Control
21
22
14
46
63
15
17
10
0
10
20
30
40
50
60
70
5.5
6
6.5
7
Initial pH
H
2
Yield (ml STP/
g COD degraded)
Cell:COD=4
Cell:COD=0.26
