2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 52 -
Abstract
-- This research aims tostudy the effectof empty
bed retention time (EBRT) (100, 140 and180 s) and liquid
recirculation rate(2.4, 4.7 and 7.1m
3
/m
2
/h) on efficiency of
hydrogen sulfide (H
2
S) removal and sulfuric acidproduction
in biofilter. Theoperational pH range of recirculation liquid
was set at 4 to 0.5. The biogas used was taken from a
digester
of
rubber
latex
factorywithCH
4
and
H
2
Sof80.0percent and 6,300 ppm, respectively. The
highestH
2
S removal efficiency achievedwas 96.8 percent
when the EBRT was 180 s and the liquid recirculation rate
was
4.7m
3
/m
2
/h.
Increased
oxygen
(O
2
)
supplybyhigherliquid recirculation rate at 7.1 m
3
/m
2
/h was
found to enhance complete H
2
S oxidationmarked by the
highest sulfuric acid production at 0.39 gH
2
SO
4
-S/gH
2
S-
S.The cleaned biogas was able to substitute for anindustrial
conventional LPG.
Index Terms
—
biofiltration, biogas, hydrogen sulfide,
sulfuric acid
I.
I
NTRODUCTION
Para rubber is amajor economic cropofThailand. In
2010, theoverall exported value of rubber, consisting
ofribbed smoked sheet, block rubber, concentrated rubber
latex and compound rubber, totaled for 296,380 million
baht. Among these products,concentrated rubber latex
shared
approximately
17
percent[1].
Conventionally,sulfuric acid was used inconcentrated
latex production for rubber particle coagulation, hence,
dischargingsulfate-rich wastewaterat 1,819 mg/L[2].
Anaerobic treatment is a promising technology due to
its low operating cost and energy recovery as biogas. In
treatment of sulfate-rich wastewaterinan oxygen void
environment, sulfate is converted into H
2
S. A H
2
S
concentration as high as 13,189 ppm was reported in the
case of concentrated latex factory [3], which is caused by
an intense biochemical sulfate reduction using the
abundant sulfate as terminal electron acceptor[4, 5].
In our survey, the biogas from the digester of a rubber
latex factorywas found to have approximately 80
percentCH
4
, which can readily substitute for conventional
fuels.But,in biogas combustion, H
2
S can be oxidized and
combined with moistureto form sulfuric acid leading to
corrosion which willreduce alife time of the combustion
engine.A level below 500 ppm is a general recommended
upper limit to avoid severe corrosion. The low H
2
S
biogas to combust could also reduce environment effect
as acid rain. Furthermore, human exposure to H
2
S could
cause instantaneous death at 1,000-3,000 ppm[6]. Thus,
H
2
S removal is required for economic,environment and
health improvement.
H
2
S removaltechnologiescould be one or a
combination of physical, chemical and biological means.
Biological treatment as abiofiltration is widely used
sinceit is effective, economical and generateslittle or no
waste to be disposed of. Inbiofiltration, H
2
S and O
2
in gas
phase are dissolved into liquid phaseand then diffused
into a biofilm on porous media. Its metabolic products
areelemental sulfur and sulfuric acid depending
onwhetherpartial oxidation and complete oxidation are
taking place(Eq.1- Eq.2)[7, 8]. Degorce-Dumas et al. [9]
studied H
2
S removal by biofiltrationand found thatat
biogas to air ratio of 2:1, only 40 percent of H
2
S removed
was converted into sulfuric acid and the rest to elemental
sulfur(measured as sulfate).Since both reactions are
carried out by sulfide oxidizing bacteria (SOB), at what
degree each reaction will proceed relates to the biofilter
operation and the micro environment within the biofilter
bed.
H
2
S + ½ O
2
Æ
S
0
+ H
2
O (1)
S
0
+ ½ O
2
+ H
2
O
Æ
H
2
SO
4
(2)
The biofiltration is suitable for H
2
S removal in biogas
due to that H
2
S has a higher solubility in water than
CH
4
as indicated by a
Henry’s constant
of 483 atmfor H
2
S
and 37,600 atm for CH
4
at 20°C [4] and there are many
groups of bacteria able to use it. In addition, H
2
S has
abetter biodegradability compared toCH
4
[8] and could go
throughbiological
oxidationwithCH
4
with
selective
degradation.
Empty bed retention time (EBRT)represents a
relationship between a gas flow rate to an empty
bedvolume of a biofilter. It is one of the important
factorsthat affect efficiency of abiofilter[8].Chung et al.
[10] has reported the efficiency of H
2
S removalat 98
percent when a 60ppm H
2
S gas was introduced to a
biofilterunder EBRT of 28 s.At elevated H
2
S
concentrations, increased contact time of H
2
S to the
biofilm is required, thus higher EBRT was needed.Aroca
et al. [11]has reported that an inlet H
2
S of 4,600 ppm
resulted in successfulremoval (100 percent) for EBRT of
120 s.Chaiprapat et al. [3] studied the removal of 13,189
ppm H
2
Sin biogas under inlet biogas to air of 1: 4 (inlet
mixed gas of H
2
S 2,327 ppm). The efficiency of H
2
S
removal was increased from85.6to 94.7 percent with
increasing EBRT form 78 to 313 s. At EBRT 313 s, its
The Effect of Retention Time and Liquid
Recirculation on H
2
S Removal by Biofiltration
B. Charnnok*, S. Chaiprapat**, P. Boonsawang***, and T. Suksaroj*
* Faculty of Environmental Management, Prince of Songkla University,HatYai, (
Thailand
)
**Green Technology Research Unit, Department of Civil Engineering, Faculty of Engineering,Prince of Songkla University,
(
Thailand
)
***Department of Industrial Biotechnology, Faculty of Agro-Industry, Prince of Songkla University, (
Thailand
)
