Ruokojärvi, Ruuskanen, Martikainen, and Olkkonen
feeding with MeSH (38 ± 36 mg/m3) was begun 7 days
later. The removal efficiencies of both H2S and MeSH im-
mediately exceeded 99% (see Figure 2), though the re-
moval efficiency of Me2S dropped to 72% soon after
beginning the feeding with H2S and MeSH but increased
to 95% again within a few days. Almost all the H2S was
removed by the first biofilter, in contrast to MeSH and
Me2S, which were removed predominantly by the sec-
ond biofilter.
DISCUSSION
The accumulation of H2SO4 as a result of microbial oxida-
tion of H2S lowers the pH of the medium and thus re-
duces the activity of microbes degrading organosulfur
compounds. Furthermore, the presence of H2S and MeSH
inhibits the ability of some microbes (e.g., Hyphomicrobium
I55)5 to degrade Me2S. The experiments with peat biofilters
showed that Me2S was oxidized only in Me2S-acclimated
peat and that the presence of MeSH reduced the maxi-
mum removal rate (Vm) for Me2S-S from 0.38 to 0.1-
0.2 g/kg dry peat/day.18
Long-Term Filtration Experiments
The maximum RC for H2S-S in the first biofilter was
1008 g/m3/day (removal efficiency 99.8%), and for Me2S-S
it was 521 g/m3/day (removal efficiency 43.5%) (see Fig-
ure 3a). The removal capacity of MeSH-S in the first
biofilter varied greatly, from 0 to 40 g/m3/day (removal
efficiency 0–100%), with the correlation coefficient of
linear fitting remaining low compared with the two
other gases. The maximum RC for H2S-S in the second
biofilter was 255 g/m3/day (removal efficiency 99.6%),
for Me2S-S it was 521 g/m3/day (removal efficiency
98.9%), and for MeSH-S it was 38 g/m3/day (removal
efficiency 98.4%) (see Figure 3b). The added removal
efficiency of the whole two-stage system (biofilter 1 +
biofilter 2) for total sulfur was greater than 99%, with a
total sulfur load of up to 1390 g/m3/day. The maximum
loads yielding total removal efficiencies of at least 99%
were 1150 g/m3/day for H2S-S, 66 g/m3/day for MeSH-S,
and 879 g/m3/day for Me2S-S.
In our two-stage biotrickling filter system, it was
mainly H2S and some of the MeSH that was oxidized in
the first biofilter using a low pH. The removal efficiency
of MeSH varied greatly. Approximately half of the Me2S
and almost all of the H2S was purified already in the first
stage. The rest of the MeSH and Me2S was oxidized in
the second biofilter with neutral pH. The results show
that this two-stage biotrickling filter system eliminates
disturbance to the oxidation of organosulfur compounds
caused by H2S. This technique would be useful for re-
duced sulfur gases, especially in the pulp industry and
in wastewater treatment plants, where these gases ap-
pear in various mixtures and unpredictable high peaks
are common.
It has been reported that the permissible loads when
aiming at a removal efficiency of 95% or more are 2.0,
0.07, 0.03, and 0.004 g/kg dry peat/day for H2S-S, MeSH-S,
Me2S-S, and Me2S2-S, respectively, when using a biofilter
inoculated with Thiobacillus thioparus DW44.2 The Vmax
for H2S was 147 g/m3/day with simultaneous feeding of
Me2S (10.8 ppmv; removal efficiency 30–35%) in a com-
post/hog fuel/perlite biofilter.14 Our two-stage
biotrickling filter was able to purify mixed gases at loads
of H2S-S up to 1150 g/m3/day, Me2S-S up to 879 g/m3/day,
and MeSH-S up to 66 g/m3/day. Because the limits for
sulfur removal capacity by this biotrickling filter system
did not yet meet with those of the studied loads, it is
possible that even higher loads than those presented here
could be treated with this system. The considerable
Removal Efficiency and Gas Retention Time
Empty bed retention times (i.e., filter bed volume [L] di-
vided by air flow [L/sec]) varying from 61 to 132 sec were
studied in the first biofilter. A decrease in the retention
time from 2 to 1 min affected the removal efficiency of
H2S in the first biofilter by only 2.9% (see Table 1), and
the decrease in the removal efficiency of Me2S in the sec-
ond biofilter was also almost negligible, 1.2% (see Table 2)
when the empty bed retention time was reduced from 4
to 2 min.
Table 1. Removal efficiencies of H2S in the first stage of the biotrickling filter with
different gas retention times. The inlet concentration of MeSH was 48 ±14 mg/m3 and
that of Me2S was 685 ±326 mg/m3.
Table 2. Removal efficiencies of Me2S in the second stage of the biotrickling filter
with different gas retention times. The inlet concentration of H2S was 6 ±6 mg/m3 and
that of MeSH was 34 ±20 mg/m3.
Number of
Empty Bed Removal Efficiencies Inlet Concentration
Number of
Empty Bed Removal Efficiencies Inlet Concentration
Experiments
Retention
of H2S
of H2S
Experiments
Retention
of Me2S
of Me2S
Time (sec)
Mean ± S.D. (%) Mean ± S.D. (mg/m3)
Time (sec)
Mean ± S.D. (%) Mean ± S.D. (mg/m3)
4
4
5
61–67
77–82
96.7 ±3.9
98.7 ±1.6
99.6 ±0.7
187 ±101
224 ±69
231 ±45
5
5
4
118
135–157
236
98.7 ±1.0
98.8 ±0.9
99.9 ±0.1
742 ±388
830 ±398
750 ±41
122–132
14 Journal of the Air & Waste Management Association
Volume 51 January 2001