2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 54 -
III. R
ESULT AND
D
ISCUSSION
A. Hydrogen Sulfide Removal
The effect of EBRT and liquid recirculation rateon the
efficiency of H
2
S removal was studied by introducingthe
H
2
S concentration of 6,272±1,729ppm into the biofilter,
which the results are shown in Fig. 2. The H
2
S removal
was increased about 22 percent with increasing EBRT
from 100 to 180 s. The maximum H
2
S removals achieved
were in a range of 89.7-96.8 percentacross all 3 liquid
recirculation rates when EBRT was 180 s. Along contact
time between H
2
S gas and biofilm could effectively
provide H
2
Smass transfer into biofilm where is oxidized.
The removal of H
2
S was slightly affected by the liquid
recirculation rate, which at 4.7 m
3
/m
2
/h, the highest H
2
S
removals at each EBRT of 100, 140 and 180 s were 77.5,
86.4 and 96.8 percent, respectively. Increasing liquid
recirculation rate to 7.1 m
3
/m
2
/h resulted in a decline in
H
2
S removal because the excessive liquid content had
reduced media porosity and H
2
S mass transfer[13].It was
interesting to note that our medium liquid recirculation
rate of 4.7m
3
/m
2
/h gave the highest removal while it
decreased when the liquid recirculation rate was at our
lowest tested value of2.4 m
3
/m
2
/h. This could be due to
the O
2
limitation.
Fig. 2. The effect of liquid recirculation rate and EBRT
on H
2
S removal efficiency
The highest H
2
S removal achieved was 96.8
percentwhen EBRT and the liquid recirculation rate were
180 s and 4.7m
3
/m
2
/h, respectively.Compared to 100
percent removal in Aroca et al. [11] which used H
2
S
diluted in the air prior to feeding, our H
2
S removal was
slightly lower because of the limit on dissolved O
2
that
the liquid could carry and deliver into the biofilm.Noted
that saturation level of O
2
solubility was 7 mg/L at 35°C
in fresh water under 1 atm of pressure. Also, the higher
inlet H
2
S concentration in our work (6,272 ppm without
air mix versus 4,600 ppm in mix with air) could cause
higher stress to the microbial communities in the
biofilm.However, the outlet biogas contained only
approximately 200 ppm H
2
S which is considered
acceptable for industrial use. Constraint on this air
delivery method still persists particularly for biogas with
higher H
2
S concentration that will require higher O
2
amount to the biofilter.
B. Sulfuric Acid Production
Sulfuric acidisametabolic product under complete
oxidation of H
2
S by SOB (Eq. 2). Sulfuric acid
production efficiency was determined by the mass of
sulfuric acid produced permass of inlet H
2
S on sulfur
basis.
The effect of EBRT in the biofilter on the sulfuric acid
production efficiency is shown in Fig. 3. The efficiency
of sulfuric acid production was increased byat least 2
times whenincreasing EBRT from 100s to 180 s
(1.8times). Sulfuric acid production was maximum at
EBRT 180 s,0.20, 0.26 and0.39 gH
2
SO
4
-S/gH
2
S-S for the
liquid recirculation rate of 2.4, 4.7 and 7.1 m
3
/m
2
/h,
respectively. Thishigh contact time of gaseous H
2
S
withliquid film and the biofilm clearly enhanced H
2
S
absorptionand allowed time for biochemical reaction.
Longer EBRT could also allowhigher O
2
to H
2
S ratio in
the bed, which is suitable for complete oxidation.
The liquid recirculation rate affectedsulfuric acid
production efficiency(Fig. 3). At EBRT of 100 and 140 s,
increasing the liquid recirculation rate did not show a
clear improvement on sulfuric acid production. At EBRT
180 s,the increases were obvious. The liquid recirculation
rate rise from 2.4 to7.1 m
3
/m
2
/hgave almost double
insulfuric acid production, i.e. from 0.20 to 0.39 gH
2
SO
4
-
S/gH
2
S (2 times) since increasing the liquid recirculation
ratecould supply moreO
2
into biofilter while the H
2
S
loading was fixed, higher O
2
to H
2
S ratioobtained had
induced more complete oxidation.
Fig. 3. The effect of EBRT and liquid recirculation rate
on sulfuric acid production efficiency
In terms of sulfur mass balance, in case of asulfuric
acid production of 0.39 gH
2
SO
4
-S/gH
2
S-S, 39 percent of
inlet H
2
S was completely oxidized into sulfuric acid. The
rest left thebiofilterinoutlet biogas and accumulated on
media as elemental sulfur (S
0
), seen as yellowish particles
(Fig. 4). This S
0
indicated the partial oxidation occurrence
according to Eq. 1, which is similar to an air fed
biofiltration[9] obtaining 60 percent of H
2
S removed in a
form of elemental sulfur.
50
60
70
80
90
100
50
100
150
200
Empty bed retention time (s)
H
2
S Removal efficiency (%)
Liquid recirculation rate 2.4 m3/m2/h
Liquid recirculation rate 4.7 m3/m2/h
Liquid recirculation rate 7.1 m3/m2/h
0.00
0.10
0.20
0.30
0.40
0.50
2.4
4.7
7.1
Liquid recirculation rate (m
3
/m
2
/h)
Sulfuric aicid production/Inlet H
2
S
(gH
2
SO
4
-S/gH
2
S-S)
EBRT 100 s
EBRT 140 s
EBRT 180 s
