2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
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efficiency was decreased to 43.9 percent when H
2
S
concentration was increased H
2
S concentration (at 1:1
biogas to air ratio). Besides, sufficient O
2
also plays an
important roleinH
2
S removal efficiency.However, a
heavy air feeding to the biofilterdiluted CH
4
content in
the biogas,which was reduced toonly 14.4 percent, a level
much lower than a general requirement to substitute for
conventional fuel at 60 percent CH
4
[3].
A liquid content in the biofilter is also akey factorfor
enhanced mass transfer of gaseous contaminantsand
metabolic productsinto and from a biofilm for its optimal
performance[12, 13]. Insufficient liquid or moisture
content will cause a detrimentaldecline in microbial
activity [8, 12, 14], while excessive liquid content
reduces media porosity and increase the mass transfer
limitation [13]. Both causes are possible to reduce the
removal efficiency of biofiltration.Sun et al.[14]reported
that an initial liquid contentincrease from 50 to 60 percent
leads to a sharp rise in degradation rate of toluene from
23.8 to 84.0 percent at 6 h operation due to a microbe in
wetter media had a betteradaptability to tolueneand
astrong activity of toluene degradingenzymes.Lee et al.
[13]controlled liquid content bed though liquid
recirculation in biotrickling filters removing ethylene
(C
2
H
4
). Approximately 90 percent of C
2
H
4
was
removedat2.7L/hof liquid recirculation,while decreasing
inC
2
H
4
removal was observed at increasing liquid
recirculation rate to 5.4L/h, due to the lower bed porosity
which limited the contact surface and mass transfer.
Thus, liquid recirculation with dissolved O
2
is another
way to supply O
2
to the biofilm which could solve the
biogas dilution and O
2
availability problems. In addition,
it is advantageous to remove an accumulated sulfuric acid
product off the biofilm, which is harmful to SOB on the
media. In our survey, the effluent discharged fromthe
digester in a rubber latex factoryalready contains essential
nutrientswhich are needed for SOB growth.Thus, the
nutrient addition is not required for use as recirculation
liquid.
The aim of this research was to investigate the
effects of EBRT and liquid recirculation rate on the
efficiency of H
2
S removal in the acidic biofilter
operation. In addition, sulfuric acid production was
monitored and used as indicator for complete oxidation.
II. M
ATERIAL AND
M
ETHOD
A.Biofilter System and Operation
A biofilter (Fig. 1) was made of cylindrical acrylic
with 4.4 cm inner diameter and 661 cm in height (a
working volume of 456.3 cm
3
). The biofilter was packed
with an 1:8 mixture of 30-cm longRaschig ring and
bacterial-immobilized coconut fibers that had been
submerged in a concentrated latex wastewater for 2
weeks. Biogas (Table I) and effluent wastewaterfrom full
scale digester at Chalong Latex Industry Co., Ltd. were
used in this study.
The factorial design was used for this study (9
treatments) in the variation of EBRT of biogas at 100,
140 and 180 s and liquid recirculation rateat 2.4, 4.7 and
7.1 m
3
/m
2
/h. The effluent wastewater,previously adjusted
to pH 4.0±0.05 with 6N HCl,was used as recirculation
liquid. During operation, O
2
was supplied into the liquid
by aerator at all time to ensure O
2
saturation. Each
experiment was performed until pH of the recirculation
liquid reached0.5±0.1 and was replaced by a fresh pH-
adjusted liquid for next run. At least triplicate runs at
each condition were conducted to gain representative
data.
Outlet biogas
Biogas storage
Liquid storage
H
e
a
d
i
n
g
Packing
media
Recirculation
liquid
Inlet biogas
Biogas pump Air pump
Fig. 1. Biofilter system
TABLE
I
B
IOGAS CHARACTERISTICS
Component of biogas
Mean±SD
Methane (%)
79.9±2.7
Carbon dioxide (%)
14.3±3.6
Oxygen (%)
0.3±0.2
Others (%)
5.4±4.6
Hydrogen sulfide(ppm)
6,272±1,729
B. Analytical Method
The gaseous sample was taken from the reactorsby
syringe at time 0.5, 3 and 24 h for H
2
S and CH
4
measuring by Cadmium Sulfide Method [15] and
portable biogas analyzer (Geotech; Biogas Check model),
respectively. Noted that by observation, pH ofthe
recirculating liquid reached 0.5 within 25 h.The liquid
was taken at time24 h for pH and sulfate content
measuring by a pH meter (EUTECH instruments;
Cyberscan pH 300 model) and a colorimeter (HACH;
DR/890 model), respectively.
The efficiency of the reactor is represented by the
removal efficiency (RE) (Eq.3). The equation was as
follows.
Where
C
in
is the inletH
2
S concentration (ppm),
C
out
is
the outletH
2
S concentration (ppm).
(3)
(
C
in
- C
out
)
C
in
100
RE
=
