Removal of Carbonyl Sulfide Using Biofiltration
Paper # 475
Chandraprakash S. Nawal, Graduate Engineer
The University of Texas at Arlington, 416 Yates Street, 425 Nedderman Hall, P.O. Box 19308,
Arlington, TX 76019-0308
Melanie L. Sattler, Ph.D., P.E.
The University of Texas at Arlington, 416 Yates Street, 425 Nedderman Hall, P.O. Box 19308,
Arlington, TX 76019-0308
ABSTRACT
At wastewater treatment plants (WWTP), odorous compounds such as hydrogen sulfide (H2S),
methyl mercaptan, methyl disulfide, dimethyl disulfide, and carbonyl sulfide (COS) are
generated by biological metabolism under anaerobic conditions. Such odorous compounds may
also exist in industrial discharges. Although many studies of H2S and a number of studies of the
other reduced sulfides have been conducted, carbonyl sulfide has not been studied. Thus, the
objective of this research was to determine the removal efficiency of carbonyl sulfide using
biofiltration for various biofilter media.
The lab experimental set-up consisted of 3 PVC columns, 4-1/4” in diameter with a media depth
of 28”. Four biofilter media were tested: aged compost (primarily post oak leaves), fresh
compost, wood chips, and a compost/wood chips mixture. Inlet COS concentrations of 5 - 30
ppm (loadings of 0.17–1.05 g/m3-hr), which are higher than typical WWTP concentrations but
lower than industrial concentrations, were tested.
Compost and the compost/wood chips mixture (20 to 80% ratio by volume) produced higher
COS removal efficiencies than wood chips alone (100% versus 93% for the loadings tested). The
compost and compost/wood chips mixture also had a shorter stabilization time compared with
wood chips alone (3 days vs. 5-6 days). Thus, in regions of the US where wood chips are
commonly used as a biofilter media, it is advisable to add compost to increase removal efficiency
of COS, and perhaps other sulfides as well.
Using fresh versus aged compost did not impact COS removal efficiency. Relative humidity of
50 – 80 % was found to be sufficient for 100% COS removal at loadings tested. Compost pH
remained constant through 45 days of biofilter operation. The microbes found in the compost
media following COS treatment were different from those following H2S treatment, indicating
that microbe community adapted to the pollutant being treated.
INTRODUCTION
At a wastewater treatment plant, odorous compounds arise from biological metabolism under
anaerobic conditions in collection and treatment systems and can also exist in industrial
discharges. Most sewers flow partially full; odorous compounds can accumulate in the gas space
of the sewer and escape wherever the sewer is open to the atmosphere, such as at unsealed
manholes, pumping station wet wells, treatment plant head works, or vent stacks on buildings.1
Generally at a wastewater treatment plant, the odor producing compounds are hydrogen sulfide
(H2S), methyl mercaptan, methyl disulfide, dimethyl disulfide, carbonyl sulfide (COS) and other
reduced sulfides. The main compound that causes most odor problems is hydrogen sulfide (H2S),
but the other reduced sulfides have been found in field testing to also be important.2,3 Many
studies have been conducted for the removal or degradation of hydrogen sulfide and other
reduced sulfides 4-12, but not for carbonyl sulfide, which is thus our main concern here. This
research focuses on removal of COS via biofiltration, which is frequently used for odor removal
at wastewater treatment plants.
Biofiltration uses microorganisms fixed to a porous medium to break down pollutants present in
an air stream. The microorganisms grow in a biofilm on the surface of a medium or are
suspended in the water phase surrounding the medium particles. The filter-bed medium consists
of relatively inert substances (compost, wood chips, etc.) which ensure large surface attachment
areas and additional nutrient supply. As the air passes through the bed, the contaminants in the
air phases sorb into the biofilm and onto the filter medium, where they are biodegraded.
Several field installations have shown that COS removal in biofilters is significantly lower (20 –
75%) than for most other reduced sulfides (80 – 100%).13 Finding an appropriate media may
improve COS removal efficiency. In general, biofilters use organic materials such as peat,
compost and wood products as biofilm growth support media. These materials are selected since
they are not only inexpensive and readily available, but also because they provide a large surface
area for mass transfer and microbial growth.14
A variety of studies have shown that compost media is better for removing reduced organic
sulfides than the other medias.15-17 Compost media possesses a large diversity and density of
microorganisms, good water retention properties, neutral pH and a suitable organic content.
In addition, it is commonly used as a biofilter bed material in practice. Thus, compost was
chosen as a media to test in this research.
Peat media has shown good removal efficiency for hydrogen sulfide, but peat is naturally acidic
and hydrophobic. Because of its hydrophobicity, moisture control for peat beds can be difficult.
Peat does not naturally contain a large population of microorganisms and will require
inoculation, e.g., with activated sludge. It has much lower nutrient levels than compost and thus
may require nutrient supply, e.g., either through initial addition of slow release nutrients or
trickling of a nutrient solution during operation. Peat was widely used as a medium in the 1980s
because it offered a very low pressure drop. It has been slowly replaced by compost, mostly
because of the better performance of the latter medium in the long run and because of the relative
difficulty of controlling moisture in peat beds.18 Because of these reasons, peat was not chosen
for use in this research.
Previous studies have mentioned that the removal efficiency of COS with soil media is only
about 57%.19 Accordingly, soil was not chosen as a media to test in this research.
Wood chips are commonly used in various proportions mostly as bulking agents, but can also be
used alone as medium.20-23 In addition to preventing bed compaction and allowing for
homogeneous air flow, wood chips constitute a reservoir of water that may in some case
attenuate fluctuations in packing moisture content due to poor reactor control or excessive heat
generation. Regular nutrient supply may be needed.24 So far, there is no report indicating that a
particular species of tree would be more suited for biofiltration purposes; however, it is
reasonable to assume that some may be better than others.25 In the Dallas-Fort Worth Metroplex
area, hard wood chips are easily available and are a commonly used biofilter media at
wastewater treatment plants. Thus, wood chips were chosen as a second media to test in this
research.
The primary objective of this research was thus to compare carbonyl sulfide removal efficiency
with biofiltration using compost and hard wood chips media. Removal efficiency for H2S, which
is commonly present in wastewater treatment unit off gases along with COS, was also measured.
RESEARCH METHODOLOGY
The experimental set up is shown in Figure 1 below. The biofilter columns, made of transparent
PVC, were 3’ long and 4-1/4” ID. The columns were closed by a 8” × 8” square plate with an Oring groove to fit the column and had a ½” National Pipe Thread (NPT) threaded hole at the
center. O-rings were used to make the system air tight, as well as for proper fitting of columns.
For uniform spreading of the mixture of pollutant gas and air, a perforated PVC plate was used
(½” thick, 4-1/4” in diameter with holes 3/32” in diameter). The perforated plate was kept at
about 5” distance from the bottom end of the column, so that before entering into the column the
air and carbonyl sulfide were mixed properly. Sampling ports were fixed at the inlet and outlet of
each column. During all experiments the temperature was maintained at room temperature at
around 70-80F.
Compost and wood chips were used as biofilter media. The compost media was from backyard
waste (80-90% post oak leaves, with the rest grass trimmings and other leaves) and the wood
chips were from Good Times Wood Products, Inc. The bed depth was about 28” (2’ 4”).
The COS cylinder contained 3% COS, with the balance nitrogen. Air was provided via the fume
hood nozzle. A combined flow rate (air and COS) of around 1 liter per minute was used. Flow
regulators of stainless steel with interior Teflon coating were used to regulate the combined flow.
Experiments were also conducted using H2S, using a cylinder containing 3% H2S and balance
nitrogen.
Biofilter inlet COS concentrations of 5 to 30 ppm were used. Previous field data collected from
wastewater treatment plants has shown COS concentrations of up to 2 ppm.26,27 Industrial
processes, however, can produce concentrations of up to 400-600 ppm. For this research, due to
experimental limits associated with the COS sensor and the COS cylinder regulator, an
intermediate range was chosen for testing.
Inlet H2S concentrations of 3 to 46 ppm were used. Previous field data collected from wastewater
treatment plants has shown H2S concentrations ranging from to 0.2 to 2240 ppm.28,29
Figure 1. Experimental Set Up
The COS/air (or H2S/air) mixture was passed through a humidification chamber to achieve 5080% relative humidity. The humidification chamber was of the same size as of biofilter columns.
The COS/air mixture traveled through a long glass tube to the bottom of the chamber, where it
was released. The mixture was humidified as it bubbled upward through the water.
To measure carbonyl sulfide concentration, a carbonyl sulfide IQ-350 sensor from International
Sensor Technology, Irvine, CA was used. The sensor had the capability of measuring up to 50
ppm, with an accuracy of 0.1 ppm. Multiple readings (2-3) were taken at the inlet as well as at
the outlet.
H2S concentrations were measured using an Interscan Corp. Series 1000 portable analyzer, with
the capability of measuring up to 300 ppm. Humidity was measured using a Testo 605-H1
humidity stick.
pH was measured using an Accumet AR 50 Fisher Scientific. Duplicate samples of compost or
wood chips were taken from the top, middle, and/or bottom of the column. The media was
mixed with a very small amount of water just to make slurry, and the probe of the pH meter was
inserted in the slurry to obtain a reading.
To identify microbes in the compost media, a sample was sent to Microcheck, Inc. laboratory in
Northfield, VT. The compost media tested had been in the operating biofilter column for 7
weeks. The method used to identify the microbes is described elsewhere.30
RESULTS
Carbonyl Sulfide
The removal efficiency of carbonyl sulfide was measured for various inlet concentrations using 4
biofilter media: aged compost, fresh compost, wood chips, and a wood chips/compost mixture.
Aged Compost Media
The first set of experiments was conducted with aged compost media (experiments were started
almost 6 months after the compost was brought to the lab). Due to delay in shipment of the COS
sensor, the first COS readings were taken after more than 3 weeks of operation. At that time
according to the results shown in Table 1, all the microbes appear to have become completely
acclimated and were removing or degrading 100% of the COS. (Outlet concentrations were
measured as 0 ppm.) The removal efficiency of 100% was sustained through Day 46 of
operation, over inlet concentrations that ranged from 6.5 ppm to 23.3 ppm, and inlet mass
volume loadings that ranged from 0.23 to 0.77 g/m3-hr.
Table 1. COS Loadings and Elimination Capacity for Aged Compost Media
Day
of Inlet COS
Operation
Conc.
Mass Volume
(ppm)
Loading
(g/m3-hr)
24
17
0.48
25
18.3
0.56
26
17.7
0.54
27
6.5
0.23
28
9.4
0.32
29
10.7
0.34
30
14.3
0.45
37
17
0.52
46
23.3
0.77
Outlet COS
Conc.
(ppm)
0
0
0
0
0
0
0
0
0
Removal
Efficiency
Mass Volume (%)
Loading
(g/m3-hr)
0
100
0
100
0
100
0
100
0
100
0
100
0
100
0
100
0
100
Elimination
Capacity
(g/m3-hr)
0.48
0.56
0.54
0.23
0.32
0.34
0.45
0.52
0.77
Elimination capacity as a function of inlet loading for aged compost media is shown in Figure 2.
The average pH for aged compost media was 6.1 after the experiments; the pH remained
constant between Days 34 and 46.
0.9
0.8
Elimination Capacity, g/m3-hr
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
3
Inlet COS Loading, g/m -hr
Figure 2. Elimination Capacity vs. Inlet COS Loading for Aged Compost Media
Fresh Compost Media
A test was run to determine whether use of fresh versus aged compost would impact COS
removal efficiency. Results are given in Table 2 and Figure 3. Inlet COS concentrations ranged
from 15.2 to 25 ppm, and inlet COS mass volume loadings ranged from 0.35 to 0.64 g/m3-hr.
Removal efficiency was found to be 100% for fresh compost, as it had been for aged compost;
thus, the age of the compost did not affect removal efficiency. Since removal efficiency was
already at 100% on the day that the first concentration was measured (Day 4), the startup/acclimation time for the biofilter was less than 3 days (on Day 1, the system had been
operating 0 days, so that on Day 4, the system had been operating 3 days).
Table 2. COS Loadings and Elimination Capacity for Fresh Compost Media
Day
of Inlet COS
Operation Conc.
Mass Volume
(ppm)
Loading
(g/m3-hr)
4
25
0.53
5
15.2
0.35
6
16.3
0.37
7
22.9
0.64
Outlet COS
Conc.
Mass Volume
(ppm)
Loading
(g/m3-hr)
0
0
0
0
0
0
0
0
Removal
Efficiency
(%)
Elimination
Capacity
(g/m3-hr)
100
100
100
100
0.53
0.35
0.37
0.64
0.7
0.6
Elimination Capacity, g/m3-hr
0.5
0.4
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
3
Inlet COS Loading, g/m -hr
Figure 3. Elimination Capacity vs. Inlet COS Loading for Fresh Compost Media
Hard Wood Chips Media
For experiments conducted with hard wood chips media, inlet COS concentrations ranged from
5.5 to 30.4 ppm, and inlet COS mass volume loadings ranged from 0.17 to 1.05 g/m 3-hr.
According to results shown in Table 3, the removal efficiency increased from 71% on Day 2 as
the microbes acclimated to the COS, and leveled off at a fairly stable maximum range of 91-93%
on Days 5-13. On the day with the maximum inlet COS concentration (30.4 ppm), the removal
efficiency was a near maximum at 93.4%, and the elimination capacity was a maximum at 0.98
g/m3/hr. Thus, the critical load (maximum elimination capacity) had not yet been reached.
Table 3. COS Loadings and Elimination Capacity for Hard Wood Chips Media
Day
of Inlet COS
Operation
Conc.
Mass Volume
(ppm)
Loading
(g/m3-hr)
1
10.5
0. 33
2
5.5
0. 17
3
11.8
0. 40
4
8.0
0. 29
5
30.4
1.05
6
14.8
0 .47
7
14.7
0. 51
9
19.7
0. 68
10
17.4
0. 57
12
20.7
0. 68
13
28.9
0. 95
Outlet COS
Conc.
(ppm)
1.5
1.6
1.3
1.2
2.0
1.2
1.3
1.5
1.2
1.3
2.7
Removal
Efficiency
Mass Volume (%)
Loading
(g/m3-hr)
0.05
85.7
0.05
70.9
0.04
89.0
0.04
85.0
0.07
93.4
0.04
91.9
0.05
91.2
0.05
92.4
0.04
93.1
0.04
93.7
0.09
90.7
Elimination
Capacity
(g/m3-hr)
0.28
0.12
0.35
0.25
0.98
0.43
0.46
0.63
0.53
0.64
0.86
Elimination capacity as a function of inlet loading for hard wood chips is shown in Figure 4. The
average pH for hard wood chips media was 5.1 before as well as after the experiments.
1.2
Elimination Capacity, g/m3-hr
1
0.8
0.6
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1
1.2
Inlet COS Loading, g/m3-hr
Figure 4. Elimination Capacity vs. Inlet COS Loading for Hard Wood Chips Media
Compost/Hard Wood Chips Mixture
Last, a mixture of 20% compost and 80% wood chips on a volume basis was tested. This ratio
was chosen based on previous studies using various wood chip/compost ratios.31-35 Inlet COS
ranged from 15.1 to 25.2 ppm, from 0.40 to 0.83 g/m3-hr. As shown in Table 4 and Fig. 5, the
removal efficiency remained at 100% for Days 4-7, when measurements were taken. Thus,
adding 20% compost by volume improved the removal efficiency over the previous range of 9193% for wood chips alone. Since efficiency was already at 100% on the day of the first
concentration measurement (Day 4), the biofilter start-up/acclimation time was less than 3 days.
Table 4. COS Loadings and Elimination Capacity for Compost/Hard Wood Chips Mixture
Day
of Inlet COS
Operation
Conc.
Mass Volume
(ppm) Loading
(g/m3-hr)
4
15.1
0.40
5
16.5
0.42
6
20.7
0.56
7
25.2
0.83
Outlet COS
Conc.
(ppm)
0
0
0
0
Removal
Efficiency
Mass Volume (%)
Loading
(g/m3-hr)
0
100
0
100
0
100
0
100
Elimination
Capacity
(g/m3-hr)
0.40
0.42
0.56
0.83
0.9
0.8
Elimination Capacity, g/m3-hr
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
3
Inlet COS Loading, g/m -hr
Figure 5. Elimination Capacity vs. Inlet COS Loading for Compost/Hard Wood Chips
Mixture
Hydrogen Sulfide
The removal efficiency of hydrogen sulfide was measured for various inlet concentrations using
compost media and a compost/hard wood chips mixture.
Compost Media
A gas stream containing H2S was fed through compost media that had been used for COS
degradation (the fresh compost). Results for compost media alone are shown in Table 5 and Fig.
6. The removal efficiency was 100% over inlet concentrations ranging from 3 ppm to 42 ppm,
and inlet mass volume loadings ranging from 0.05 to 0.71 g/m3-hr. Since removal efficiency was
already at 100% on the day that the first concentration was measured (Day 3), the startup/acclimation time for the biofilter was less than 2 days (on Day 1, the system had been
operating 0 days, so that on Day 3, the system had been operating 2 days).
Table 5. H2S Loadings and Elimination Capacity for Compost Media
Day
of Inlet COS
Operation Conc.
Mass Volume
(ppm)
Loading
(g/m3-hr)
3
3.0
0.053
4
30
0.50
5
42
0.79
Outlet COS
Conc.
Mass Volume
(ppm)
Loading
(g/m3-hr)
0
0
0
0
0
0
Removal
Efficiency
(%)
Elimination
Capacity
(g/m3-hr)
100
100
100
0.053
0.50
0.79
0.8
Elimination Capacity, g/m3-hr
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
3
Inlet H2S Loading, g/m -hr
Figure 6. Elimination Capacity vs. Inlet H2S Loading for Compost Media
0.7
0.8
Compost/Hard Wood Chips Mixture
The results for H2S removal using a mixture of compost and wood chips (20 to 80% ratio by
volume) are shown in Table 6 and Figure 7. The removal efficiency was 100% over inlet
concentrations that ranged from 3.3 ppm to 46 ppm, and inlet mass volume loadings that ranged
from 0.065 to 0.82 g/m3-hr. Since removal efficiency was already at 100% on the day that the
first concentration was measured (Day 3), the start-up/acclimation time for the biofilter was less
than 2 days.
Table 6. H2S Loading and Elimination Capacity for Compost/Wood Chips Mixture
Day
of Inlet COS
Operation Conc.
Mass Volume
(ppm) Loading
(g/m3-hr)
3
3.3
0.065
4
30
0.56
5
46
0.82
Outlet COS
Conc.
Mass Volume
(ppm) Loading
(g/m3-hr)
0
0
0
0
0
0
Removal Elimination
Efficiency Capacity
(%)
(g/m3-hr)
100
100
100
0.065
0.56
0.82
0.9
0.8
Elimination Capacity, g/m3-hr
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
3
Inlet H2S Loading, g/m -hr
Figure 7. Elimination Capacity vs. Inlet H2S Loading for Compost/Hard Wood Chips
Mixture
DISCUSSION
Impact of Media on Removal Efficiency
Table 7 summarizes the maximum removal efficiency observed for each of the 4 media tested.
For COS, use of aged versus fresh compost did not impact removal efficiency. Compost alone
gave a higher COS removal efficiency than hard wood chips alone (100% versus 93%,
respectively). Adding a small amount of compost to the hard wood chips (20% compost by
volume) increased the COS removal efficiency to 100%, similar to compost alone. This
increased removal efficiency over hard wood chips alone may have been due to nutrients in the
compost. Use of the hard wood chips/compost mixture may have advantages over use of
compost alone in that wood chips can reduce bed compaction, allow for more homogeneous air
flow, and provide a reservoir of water.
For H2S, the removal efficiency of compost alone, as well as the compost plus hard wood chips
mixture, was 100%. This is to be expected, since previous field studies have indicated that H2S
is easier to biodegrade than COS. Hard wood chips alone were not tested for H2S, since the
compost/hard wood chips mixture had performed better for COS. Similarly, aged compost was
not tested for H2S, since fresh and aged compost had given similar results for COS.
Table 7. Summary of Maximum Removal Efficiency for Various Media
Media
Aged Compost
Fresh Compost
Hard Wood Chips
Compost/Hard Wood
Mixture
Maximum Removal Efficiency
for COS
(%)
100
100
93
Chips 100
Maximum
Removal
Efficiency for H2S
(%)
-100
-100
Impact of Media pH on Removal Efficiency
Conventional wisdom indicates that biofilters treating sulfides operate more effectively at near
neutral pH, where bacteria that degrade sulfides thrive. As shown in Table 8, the fresh and aged
compost, with pHs of 5.7 and 6.1, respectively, both achieved removal efficiencies of 100%.
Wood chips, with a lower pH of 5.1, gave a lower removal efficiency for COS (93%); however,
this was likely due to factors other than pH, such as differing amounts of available moisture and
nutrients.
Table 8. Average pH value for Different Biofilter Media
Media
Aged Compost
Fresh Compost
Hard Wood Chips
Compost/Hard
Wood
Mixture
Media Sample Location
Bottom of the column
Middle of the column
Top of the column
Middle of the column
Middle of the column
Chips Middle of the column
Average pH
6.1
6
6.2
5.7
5.1
5.7
Impact of Gas Stream Humidity on Removal Efficiency
To achieve high removal efficiencies, conventional wisdom states that the relative humidity of
the biofilter should be near 100%. In our experiments, the gas stream relative humidity was
observed in the range of 50 – 80%, but the removal efficiency for COS and H2S still reached
100% for the loadings tested, which as discussed above were below critical load. Relative
humidity may have been slightly underestimated due to inability to insert the humidity probe into
the center of the gas stream. It is likely, however, that 100% relative humidity of the gas stream
was not necessary for this experimental set-up (which did involve low concentrations and long
residence times).
Stabilization Time
As noted previously, the compost media as well as the compost/wood chips mixture took less
than 3 days to stabilize for COS removal; on the other hand, hard wood chips alone took almost
5-6 days to stabilize. An advantage of adding compost to wood chips then, in addition to
increased removal efficiency, appears to be decreased stabilization time.
Microbes in the Compost Media
To identify the microbes in the compost media, a sample was sent to Microcheck, Inc.
Northfield, VT after 7 weeks of operating the biofilter with COS. In the literature review, the
bacteria or microbes found for degrading reduced organic sulfides were Thiobacillus Strain TJ
330 and Hyphomicrobium MS3. However, in our research the microbes found in the compost
media were Streptomyces, Bacillus subtilis, and Staphylococcus hominis hominis. The difference
in the microbe community is likely due to the difference of the pollutant gas. Thiobacillus Strain
TJ 330 was found while treating a mixture of hydrogen sulfide and carbon disulfide, and
Hyphomicrobium MS3 was found when the pollutant gas was dimethyl sulfide.
After running four days of experiments with hydrogen sulfide using compost, a sample was sent
for microbial check to the same laboratory. The microbes found in that media were Bacillus
sphaericus, Cellulosimicrobium cellulans and Clostridium mangenotii Clostridium cadaveris.
These are different species from those found in the compost when COS was treated. This
indicates that different microbes are adept at degrading hydrogen sulfide versus COS.
CONCLUSIONS AND RECOMMENDATIONS
The following are conclusions based on this research, along with recommendations for further
research:

Compost and a compost/wood chips mixture (20 to 80% ratio by volume) produced higher
carbonyl sulfide (COS) removal efficiencies than wood chips alone (100% versus 93% for
the COS loading rates tested). In regions of the US where hard wood chips are commonly
used as a biofilter media, it may be advisable to add compost to increase removal efficiency
of carbonyl sulfide, and perhaps other sulfides as well.

The compost media as well as the mixture of compost and wood chips took less than 3 days
to stabilize for COS removal; on the other hand, the hard wood chips alone took almost 5-6
days to stabilize. An advantage of adding compost to wood chips then, in addition to
increased removal efficiency, appears to be decreased stabilization time.

At the COS concentrations and loading rates tested (5-30 ppm and 0.17–1.05 g/m3-hr),
biofilters are capable of removing 100% COS. The 5-30 ppm concentration range is higher
than would be observed at a wastewater treatment plant, but lower than would typically be
observed in an industrial setting. Additional biofilter testing should be conducted at higher
COS concentrations, more representative of industrial settings. The critical load should be
determined for COS using compost media and a compost/hard wood chips mixture. If the
load is high enough, industrial applications of COS removal via biofiltration may be viable.

Using fresh versus aged compost did not impact COS removal efficiency.

The pH of the compost biofilter treating COS remained constant through Day 46. Previous
studies have indicated that sharp pH decreases which degrade biofilter performance typically
occur within the first 6 weeks of biofilter operation. Since COS was treated for a little more
than 6 weeks with no change in pH, pH decreases would likely not be a problem.

In our experiments, the gas stream relative humidity was observed in the range of 50 – 80%,
but the removal efficiency for COS and H2S still reached 100% for the loadings tested. This
indicates that 100% relative humidity of a gas stream is not always necessary to achieve high
biofilter removal efficiencies.

With compost media and with the mixture of compost and hard wood chips media the
hydrogen sulfide removal efficiency was also 100% for the concentrations/loadings tested.

The microbes found in the compost media while treating carbonyl sulfide were Streptomyces,
Bacillus subtilis, and Staphylococcus hominis hominis. While treating hydrogen sulfide, the
microbes were Bacillus sphaericus and Clostridium mangenotii Clostridium cadaveris. This
indicates that the microbes most adept at treating H2S and COS are different. Future research
should test a mixture of COS and H2S in proportions typical of those found at a wastewater
treatment plant, where H2S dominates, to see if the removal efficiency of COS remains high.
The microbes may adapt to H2S degradation, resulting in decreased COS removal.
ACKNOWLEDGMENTS
Funding for this research was provided by the University of Texas at Arlington Research
Enhancement Program. The authors gratefully acknowledge Paul Shover for his assistance in
assembling the laboratory scale biofilter system. We also acknowledge Alan Plummer
Associates, Inc. for use of the Interscan H2S analyzer.
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Key words: Biofiltration, carbonyl sulfide, odor, hydrogen sulfide, compost, wood chips.