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Applied and Environmental Microbiology, November 1998, p. 4168-4173, Vol. 64, No. 11
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Ethylene Removal by a Biofilter with
Immobilized Bacteria
Lars
Elsgaard*
Department of Soil Science and Crop
Physiology, Danish Institute of Agricultural Sciences, Research
Center Foulum, DK-8830 Tjele, Denmark
Received 14 November 1997/Accepted 13 August 1998
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ABSTRACT |
A biofilter which eliminated ethylene
(C2H4) from the high parts-per-million range to
levels near the limit for plant hormonal activity (0.01 to 0.1 ppm) was
developed. Isolated ethylene-oxidizing bacteria were immobilized on
peat-soil in a biofilter (687 cm3) and subjected to an
atmospheric gas flow (73.3 ml min
1) with 2 or 117 ppm of
C2H4. Ethylene was eliminated to a minimum level of 0.017 ppm after operation with 2.05 ppm of
C2H4 for 16 days. Also, the inlet
C2H4 concentration of 117 ppm was reduced to
<0.04 ppm. During operation with 2 and 117 ppm of
C2H4, an increase in the
C2H4 removal rate was observed, which was
attributed to proliferation of the immobilized bacteria, notably in the
first 0- to 5-cm segment of the biofilter. The maximal
C2H4 elimination capacity of the biofilter was
21 g of C2H4 m3
day1 during operation with 117 ppm of
C2H4 in the inlet gas. However, for the first
0- to 5-cm segment of the biofilter, an elimination capacity of
146 g of C2H4 m3
day1 was calculated. Transition of the biofilter
temperature from 21 to 10°C caused a 1.6-fold reduction in the
C2H4 removal rate, which was reversed during
operation for 18 days. Batch experiments with inoculated peat-soil
demonstrated that C2H4 removal still occurred
after storage at 2, 8, and 20°C for 2, 3, and 4 weeks. However, the
C2H4 removal rate decreased with increasing
storage time and was reduced by ca. 50% after storage for 2 weeks at
all three temperatures. The biofilter could be a suitable tool for C2H4 removal in, e.g., horticultural storage
facilities, since it (i) removed C2H4 to 0.017 ppm, (ii) had a good operational stability, and (iii) operated
efficiently at 10°C.
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INTRODUCTION |
Ethylene
(C2H4) is a gaseous plant hormone and air
pollutant which has a strong effect on plant physiological processes
such as ripening, senescence, and aging (2). The threshold
limit for C2H4 sensitivity varies among plant
species (22), but a general dose response is no effect below
0.01 ppm, half-maximal effect at 0.1 ppm, and maximal effect at 1 to 10 ppm (1). The lower threshold limit of 0.01 ppm is only
slightly higher than the atmospheric background concentration (0.003 to
0.005 ppm) and may be exceeded in polluted areas (1, 2, 13, 17, 18). Also, significant accumulation of
C2H4 may occur in horticultural storage
facilities due to endogenous production by the plant material (2,
12). To reduce the detrimental effects of such
C2H4 (14), chemical
C2H4 scrubbers are widely used in storage
facilities for horticultural produce (16, 19). A drawback of
such scrubbers is the cost of operation and the need for replenishment
of the C2H4-removing agents (2, 7).
Therefore, the use of biological catalysts (3) for
C2H4 removal is an interesting alternative in
horticulture (11, 19). Likewise, biological catalysts could be used to reduce high ethylene concentrations at point sources, such
as petrochemical facilities, where ethylene and polyethylene are
produced (13).
Previously, biological waste gas purification has been reported for
several air pollutants (see, e.g., references 6 and 11), including C2H4 (4,
10, 20, 21). So far, however, no biological system for
C2H4 removal which has a satisfactory operational stability and a sufficient efficiency to reduce the C2H4 concentration to levels near the lower
threshold limit for the plant hormonal response (0.01 ppm) has been described.
Here I report the outcome of a study with a biofilter based on isolated
ethylene-oxidizing bacteria that were immobilized on peat-soil. During
stable operation with inlet levels of 2 and 117 ppm of
C2H4, it was possible to reduce the outlet
levels to less than 0.04 ppm with a minimum level of 0.017 ppm.
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MATERIALS AND METHODS |
Preparation of cell suspension.
The ethylene-oxidizing
bacterial strain RD-4 was isolated and cultivated as described by
Elsgaard and Andersen (8). For the present purpose, the
strain was grown in 5-liter vessels containing 2 liters of defined
mineral medium without added C sources. The medium in each bottle was
inoculated with 20 ml of a pregrown culture, and
C2H4 was added to a headspace concentration of
ca. 1%. The cultures were incubated with shaking (120 rpm) at 30°C, and the C2H4 concentration was replenished to
ca. 1% when it dropped below 0.5% (assayed by gas chromatographic
[GC] analysis). After incubation for 5 days, the ethylene-oxidizing
bacteria were harvested by centrifugation (16.000 × g
for 15 min) and resuspended in 1.5 liters of autoclaved tap water. The
density of the cell suspension was estimated to be 2 × 108 cells ml1 by direct microscopic counts in
a Bürker-Türk counting chamber. The cell suspension was
stored at 0 to 2°C until needed for biofilter experiments (1 week
later) and batch experiments (2 weeks later).
Biofilter construction.
A laboratory-scale biofilter was
made from an acrylic core with an inner diameter of 5 cm and a length
of 40 cm (Fig. 1). Six butyl rubber
stoppers (for gas sampling) were inserted in holes at a vertical
distance of 5 cm apart. The core was stoppered with rubber stoppers,
each equipped with a T-connector. The T-connectors served as channels
for the gas flow and had a butyl rubber stopper inserted for gas
sampling (Fig. 1). In operational mode, the biofilter had a volume of
687 cm3 and could be sampled at depths from 0 to 35 cm
(inlet and outlet, respectively) at increments of 5 cm.
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FIG. 1.
Peat-soil biofilter with butyl stoppers (BS) for gas
sampling at the inlet (In), for each 5-cm depth increment, and at the
outlet (Out). A flow of humidified air with ethylene
(C2H4/air) was applied to the inlet from two
mass flow controllers. GP, glass wool plug; RS, rubber stopper; TC,
T-connector.
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A mixture of atmospheric air and C
2H
4 in
N
2 (AGA SpecialGas, Lidingö, Sweden) was supplied to
the biofilter by use of two
mass flow controllers, with a resulting
flow rate (
F) of 73 ml
min
1. The
pseudo-residence time (
p =
V/F) of
the biofilter
was 9.4 min. Before reaching the inlet, the gas was
humidified
by being bubbled through a flask containing distilled water.
To
verify the gas flow was constant during the experiments, a digital
flowmeter (Jour Research, Onsala, Sweden) was connected to the
outlet
of the biofilter and the readings of the flowmeter were
checked on a
daily to weekly
basis.
Biofilter experiments.
Immobilized ethylene-oxidizing
bacteria were prepared by mixing 300 g of peat-soil (Pindstrup
blend 2; Pindstrup Mosebrug) with 200 ml of distilled water and 100 ml
of bacterial cell suspension. The soil sample was thoroughly mixed and
allowed to equilibrate at room temperature (20 to 22°C) for 1 h.
The inoculated peat-soil was loosely packed in the biofilter to a
density of 0.44 g cm3. The final dry-matter content
(9) of the inoculated peat-soil was 20.5%.
(i) Operation with 2 ppm of C2H4.
A
mixture of 10 ppm of C2H4 and atmospheric air
(1:4) was applied to the biofilter. During an operational period of 16 days at 20 to 22°C, gas samples (0.6 ml) were collected from all
depths of the biofilter and analyzed for C2H4.
Gas samples (0.2 to 0.5 ml) for measurement of O2 and
CO2 at the biofilter inlet and outlet were collected after
1 day of operation.
(ii) Operation with different C2H4
concentrations.
After 16 days of operation with 2 ppm of
C2H4, the biofilter was operated with
atmospheric air for 25 days (i.e., the biofilter was starved for
C2H4). Subsequently, the inlet level was
adjusted to 2 ppm of C2H4 for 8 days and then
to 117 ppm for 80 days. The latter concentration was obtained by mixing
1,000 ppm of C2H4 and atmospheric air (1:8).
Gas samples for C2H4 analysis were withdrawn regularly.
(iii) Transition from 21 to 10°C.
During operation with
117 ppm of C2H4, the influence of temperature
was tested by placing the biofilter at 10°C in a thermostated incubator. Control experiments showed that temperature equilibrium at
10°C was reached in the center of the biofilter within 5 h. Ethylene concentrations were measured at the biofilter inlet and outlet
during operation for 18 days.
Batch experiments.
Batch experiments were done to test the
effect of storage on the C2H4 removal rate of
the inoculated peat-soil. A sample of immobilized bacteria was prepared
as described above, while a control sample was prepared by mixing
100 g of peat-soil with 100 ml of distilled water. Each of the
soil samples was divided into three portions that were placed at 2, 8, and 20°C. To test the C2H4 removal rate
before storage, triplicate 10-g soil samples were transferred to 120-ml
serum bottles that were purged with atmospheric air and closed with
butyl rubber stoppers. Ethylene was added to a headspace concentration
of ca. 20 ppm, and the time course of C2H4
removal (20 to 22°C) was monitored by GC analysis of withdrawn gas
samples (0.4 ml). Similar assays were done with the stored soil samples
after 2, 3, and 4 weeks of storage. Before these assays, the 10-g
subsamples were allowed to equilibrate at room temperature for 1 h.
Gas analysis.
Gas samples for analysis of
C2H4, CO2, and O2 were
collected with 1-ml gas-tight syringes (Pressure-Lok). Generally,
C2H4 was quantified with a Hewlett-Packard
5840A gas chromatograph equipped with a flame ionization detector
(8). For injection of a 0.6-ml gas sample, the
C2H4 detection limit was 0.04 ppm for a
signal-to-noise ratio of 3.
Occasionally, samples from the biofilter were collected by passing the
outlet flow through a stoppered 120-ml serum bottle
for 1 to 2 h
with two hypodermic needles. The bottle was then
removed from the gas
stream, and subsamples of 1.0 ml were analyzed
for
C
2H
4 with a Photovac 10Splus gas chromatograph
equipped with
a photoionization detector (
8). The ethylene
detection limit
was 0.002 ppm for a sample volume of 1.0
ml.
Oxygen (0.5-ml samples) was measured on a Varian 3700 gas chromatograph
with a thermal conductivity detector (TCD) and a molecular
sieve 5A
column. The oven temperature was 30°C, and the carrier
gas was He
(flow rate, 76 ml min
1). Carbon dioxide (0.2-ml samples)
was measured on a GC 82 (Mikrolab,
Højbjerg, Denmark) with a TCD. The
column (Porapak N) was operated
at 60°C with He as the carrier gas
(flow rate, 43 ml min
1).
Statistics.
Data from replicate samples are presented as
mean ± standard error (SE) with the number of samples
(n) indicated. The significance of differences between data
from the biofilter inlet and outlet was tested by use of a two-tailed
Student t test (23).
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RESULTS |
Biofilter experiments.
During the experiments, the flow rate
measured at the outlet of the biofilter ranged from 71.4 to 75.9 ml
min1 with a mean and SE of 73.3 ± 0.3 ml
min1 (n = 23). In a control experiment,
it was demonstrated that the flow rates at the inlet and the outlet
were identical (76.5 to 76.8 ml min1) and equaled the sum
of the flow rates for the two gaseous components that were mixed (67.9 and 8.7 ml min1). Furthermore, it was demonstrated that
with an empty biofilter, the outlet C2H4
concentration (2.19 ± 0.03 ppm; n = 3) was equal to the inlet concentration (2.18 ± 0.03 ppm; n = 3). This showed that no significant leakage or adsorption occurred.
(i) Operation with 2 ppm of C2H4.
During the first experiment, a stable inlet concentration of 2.05 ± 0.01 ppm (n = 57) was applied to the biofilter (Fig.
2). Measurements of the outlet
C2H4 concentration after 1 h of operation (0.23 ± 0.01 ppm; n = 4) demonstrated that 89%
of the incoming C2H4 had already been removed
by the biofilter at this early stage (Fig. 2). During the experiment,
the efficiency of C2H4 removal gradually
increased to 99%. Thus, at the end of the experiment the biofilter had
a stable performance with an outlet concentration of only 0.017 to
0.020 ppm of C2H4 (Fig. 2).
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FIG. 2.
Ethylene concentration in the gas flow at the inlet and
at the outlet of the biofilter during operation for 16 days with 2.05 ppm of C2H4 (73.3 ml min1). Data
represent the mean ± SE of three or four samples. Note the scale
break on the y axis.
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Gas analysis at different biofilter soil depths showed that after
1 h of operation all the soil layers were exposed to
C
2H
4 at concentrations ranging from 2.05 to
0.23 ppm (Fig.
3). However,
as the rate
of C
2H
4 removal increased in the soil layers
close
to the inlet, removal of the incoming
C
2H
4 ultimately occurred
within the first 15 cm
of the biofilter (Fig.
3). Segment-specific
C
2H
4 removal rates were calculated as follows:
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(1)
|
where SR(s) is the specific C
2H
4 removal
rate (micrograms of C
2H
4 per gram [dry
weight] of soil per hour) by a given 5-cm
segment (s),
C
2H
4(s) is the difference between the inlet
and
outlet C
2H
4 concentration of the segment
(microliters per liter),
F is the flow rate (4.4 liters
h
1),
(C
2H
4) is the density of
C
2H
4 at 21°C (1.16 µg µl
1),
and
M(s) is the soil dry-matter content in a 5-cm segment
(8.8
g). These calculations showed that the initial
C
2H
4 removal rate
in each segment (after 1 h of operation) depended linearly on
the C
2H
4
concentration (data not shown). In the 0- to 5-cm segment,
the
C
2H
4 removal rate increased from 0.35 to 0.89 µg of C
2H
4 g
(dry weight) of
soil
1 h
1 during the experiment (Fig.
4). In the subsequent segments, a
relatively stable or decreasing rate was observed (Fig.
4).
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FIG. 3.
Ethylene concentration at different soil depths of the
biofilter after operation with 2.05 ppm of C2H4
for 1 h, 5 days, and 16 days. Soil depths of 0 and 35 cm represent
the biofilter inlet and outlet, respectively.
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FIG. 4.
Ethylene removal rates by individual 5-cm segments of
the biofilter during operation for 16 days with 2.05 ppm of
C2H4. Data are shown for the segments from 0 to
5 cm, 5 to 10 cm, and 30 to 35 cm.
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Oxygen measurements after 1 day of operation showed that the
O
2 content of the inlet gas and the outlet gas was 15.6% ± 0.1%
(
n = 3) and 15.2% ± 0.1% (
n = 3), respectively. This demonstrated
that oxic conditions prevailed
throughout the whole soil column.
At the same time, the CO
2
concentrations at the inlet and the
outlet were 0.018% ± 0.001%
(
n = 4) and 0.024% ± 0.001% (
n = 3),
respectively. Thus, the outlet CO
2 concentration was
significantly
higher than the inlet CO
2 concentration
(
P < 10
4), demonstrating that a net
mineralization of organic C occurred
in the
biofilter.
(ii) Operation with different C2H4
concentrations.
Operation for 25 days without
C2H4 in the inlet gas caused a decrease in the
biofilter efficiency of C2H4 removal. When 2 ppm C2H4 was reapplied, only 11 to 15% of the
incoming C2H4 was removed after 0.5 to 3 h
of operation (Fig. 5); however, after operation for 8 days, the efficiency of C2H4
removal was recovered and >98% of the incoming
C2H4 was removed by the biofilter (Fig. 5). Transition of the inlet C2H4 level
from 2 to 117 ppm similarly caused a transient decrease in the
efficiency of the biofilter, which initially (after 0.5 h) removed
only 10% of the incoming C2H4 (after the
transition, the inlet concentration was 117.2 ± 0.4 ppm
[n = 76] during the subsequent experiments). However, after operation at 117 ppm of C2H4 for 4 days,
the C2H4 concentration at the outlet of the
biofilter was below the detection limit of 0.04 ppm. Thus, more than
99.9% of the incoming C2H4 was removed by the
biofilter (Fig. 5).
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FIG. 5.
Ethylene removal efficiency of the biofilter after
transition of the inlet C2H4 concentration from
0 to 2 ppm and from 2 to 117 ppm. The dotted line indicates the latter
transition. Before time zero, the biofilter was starved for
C2H4 for 25 days. Data are the mean ± SE
of three pairs of analyses of inlet and outlet concentrations.
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The long-term operational stability of the biofilter was tested with
the inlet level of 117 ppm of C
2H
4 (Fig.
6). It was demonstrated
that for more
than 75 days of constant operation, the biofilter
was able to reduce
the outlet C
2H
4 concentration to less than
0.04 ppm. Most of the C
2H
4 was removed during
passage through
the first 0 to 5 cm of the biofilter (Fig.
6). However,
after
70 days of operation, the C
2H
4 removal by
the 0- to 5-cm segment
started to decrease but the removal was then
accomplished within
the 5- to 10-cm segment (Fig.
6).
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FIG. 6.
Ethylene concentration in the gas flow of the biofilter
during operation for 80 days with 117 ppm of
C2H4. Data are shown for the 5-cm depth the
10-cm depth, and the outlet. At time zero, the inlet
C2H4 concentration was changed from 2 to 117 ppm.
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(iii) Transition from 21 to 10°C.
When the biofilter was
transferred from 21 to 10°C (Fig. 7),
the outlet C2H4 concentration gradually
increased from <0.04 to 46.6 ± 0.3 ppm (n = 3).
However, after 2 days of operation at 10°C, the outlet concentration
started to decrease with a linear time course (Fig. 7). Thus, when the
experiment was stopped after 18 days, the outlet concentration was
1.6 ± 0.1 ppm C2H4 (n = 3), which was equivalent to a removal efficiency of 98.6%.
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FIG. 7.
Ethylene concentration in the gas flow at the inlet and
the outlet of the biofilter after transition from 21 to 10°C.
Temperature equilibrium at 10°C was reached after operation for
5 h. Data are mean ± SE (n = 3). Note the
scale break on the y axis.
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Batch experiments.
With freshly inoculated peat-soil, rapid
depletion of the added ~20 ppm of C2H4 was
observed (Fig. 8). During the first
1 h of the experiment (Fig. 8), C2H4 was
removed at rate of 17.1 ppm h1, which was equal to a
specific rate of 1.06 µg of C2H4 g (dry weight) of soil1 h1 (8). After
storage of the inoculated peat-soil for 28 days at 20°C, a
constitutive C2H4 removal still occurred but
took place at a lower rate than for the freshly inoculated soil (Fig.
8), so that the data for the first 1.5 h of the experiment showed a removal rate of 3.3 ppm h1 (0.21 µg of
C2H4 g [dry weight] of soil1
h1). Within the assay time of 5 h, no
C2H4 removal occurred in samples of
uninoculated peat-soil either immediately or after 28 days of storage
at 20°C (Fig. 8).
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FIG. 8.
Time course of C2H4 removal in
batch experiments with inoculated or uninoculated peat-soil. Ethylene
was added to ca. 20 ppm. Results are shown for freshly inoculated
peat-soil ( ), inoculated peat-soil stored at 20°C for 28 days
( ), uninoculated peat-soil prior to storage ( ), and uninoculated
peat-soil after storage at 20°C for 28 days ( ). Data are mean ± SE (n = 3).
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All time courses of C
2H
4 removal by inoculated
peat-soil stored at 2, 8, or 20°C for 0, 2, 3, and 4 weeks fell
within the
range depicted by the data from day 0 and day 28 in Fig.
8.
Thus,
for comparison, the initial C
2H
4 removal
rate for each assay was
calculated from the first two or three data
points obtained during
0 to 1.5 h of incubation. These rates
demonstrated that the C
2H
4 removal rate
decreased as a result of longer storage times (Fig.
9). However, the extent of the decrease
caused by the storage
time was almost unaffected by the storage
temperature (Fig.
9).
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FIG. 9.
Initial C2H4 removal rates in
batch experiments with inoculated peat-soil. Soil samples were assayed
before storage and after storage at 2, 8, or 20°C for 2, 3, and 4 weeks. Data are mean ± SE (n = 3).
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DISCUSSION |
Efficiency and capacity of C2H4
removal.
The biofilter showed an initial removal efficiency
of 89%, which increased to 99% during 2 weeks of operation with
an inlet concentration of 2.05 ppm C2H4.
In a comparable study, van Ginkel et al. (20) reported the
removal of C2H4 by a biofilter operating at
30°C with Mycobacterium strain E3 immobilized on compost.
With an inlet concentration of 2 ppm of C2H4,
an initial removal efficiency of ca. 67% (outlet concentration, ca.
0.66 ppm) was obtained, which increased during the first 2 weeks and
stabilized at ca. 87% (outlet concentration, ca. 0.27 ppm) during
operation for 8 weeks (calculated from Fig. 2 in reference
20). However, because the study by van Ginkel et al.
(20) was performed with different reactor characteristics
(V = 15 cm3, F = 8 ml
min1, p = 1.9 min), the higher
efficiency in the present study was at least partly related to a
longer pseudo-residence time (9.4 min). When the data from the
two biofilters were evaluated in terms of the ethylene elimination
capacity, defined as the amount of ethylene degraded per unit of
reactor volume and time (15), it was found that the capacity
of the biofilter in the present study was 0.4 g of
C2H4 m3 day1 while
the capacity of biofilter of van Ginkel et al. (20) was 1.5 g of C2H4 m3
day1. However, as C2H4 was
removed from 2.05 to 0.49 ppm within the first 5-cm segment of the
biofilter in the present study, the ethylene elimination capacity of
this segment was 1.9 g of C2H4 m3 day1. This capacity was slightly higher
than that calculated for the biofilter of van Ginkel et al.
(20).
De heyder et al. (
4) studied the removal of
C
2H
4 by a packed granular activated carbon
biobed (19 to 23°C) inoculated with
Mycobacterium strain
E3. During operation with 127 ppm of C
2H
4 in
the inlet gas, they obtained a maximal elimination capacity
of 250 g of C
2H
4 m
3 day
1,
which corresponded to a removal efficiency of 84.9% (
4).
However, this was a maximal rate obtained during cycles of wetting
and
drying out of the biobed. Under regular operation (i.e., with
constant
wetting), the typical elimination capacity was 112 g
of
C
2H
4 m
3 day
1
(
4). In comparison, the elimination capacity obtained with
the biofilter in the present study was 21 g of
C
2H
4 m
3 day
1 during
stable operation (75 days) with 117 ppm C
2H
4 in
the inlet
gas. However, since C
2H
4 was
eliminated mainly within the first
5-cm segment of this biofilter (Fig.
6), the maximal elimination
capacity found within this segment was
146 g of C
2H
4 m
3
day
1. Thus, the maximal elimination capacity of the
biofilter material
was somewhat lower than in the studies of De heyder
et al. (
4),
but on the other hand, the
C
2H
4 outlet concentrations (<0.04 ppm)
in the
biofilter in the present study were more than 400-fold
lower than those
reported (ca. 19 ppm) by De heyder et al. (
4).
In summary, the biofilter in the present study appeared to be an
interesting tool for C
2H
4 removal because it
combined a high
removal efficiency, a good elimination capacity, and an
extremely
low outlet C
2H
4 concentration.
Depth dependence and stability of C2H4
removal.
Data from the 0- to 5-cm segment demonstrated that with
an inlet concentration of 2.05 ppm, there was an increase in the
C2H4 removal rate from 0.35 to 0.89 µg of
C2H4 g (dry weight) of soil1
h1 during operation for 16 days. This could be due to (i)
microbial adaptation to the C2H4 concentration,
(ii) an increase in the cell number in the segment, or (iii) a
combination of these effects. If the rate increase were due to an
increase in the number of bacteria (with a constant
C2H4 removal rate per cell), it was calculated
that the population in the 0- to 5-cm segment should increase from the
initial number of 1.6 × 108 to 4.1 × 108 cells g (dry weight) of soil1. This could
be obtained just by slow growth of the RD-4 bacteria, which had a
doubling time of ca. 22 h when cultivated in mineral medium with a
headspace of 1% C2H4 (data not shown). No
increases in the C2H4 removal rate occurred in
the soil layers above the 0- to 5-cm segment, indicating that no growth
of the added bacteria occurred in these layers.
After C
2H
4 starvation for 25 days, the
biofilter lost 89% of its C
2H
4 removal
efficiency. However, recovery of the efficiency
occurred within 4 days
after reapplication of C
2H
4. This demonstrated
that the bacteria were able to survive in the peat-soil without
an
external source of C
2H
4. Bacterial growth after
alleviation
of the C
2H
4 starvation was
indicated by the progressive increase
in C
2H
4
removal after transition from 0 to 2 ppm of
C
2H
4 and from
2 to 117 ppm of
C
2H
4.
The operational stability of the biofilter was demonstrated by the
performance of the 0- to 5-cm segment, which caused almost
complete
removal of 117 ppm of C
2H
4 for more than 75 days of constant
operation. In turn, subsequent reduction of the
removal efficiency,
which may have resulted from depletion of (unknown)
bacterial
growth factors supplied by the peat-soil, was noted.
Correspondingly,
an enhancement of biological ethylene removal by
soluble microbial
products has recently been indicated in studies with
Mycobacterium strain E3 (
5). In the present
study, the decrease in the efficiency
of the 0- to 5-cm segment had no
influence on the overall performance
of the biofilter, because the next
soil layers were able to remove
the incoming
C
2H
4 to less than 0.04
ppm.
Effect of temperature.
After transition of the biofilter
temperature from 21 to 10°C, the difference between the inlet and
outlet C2H4 concentrations decreased from 117 to 71 ppm in 1 day. This corresponded to a decrease in the
specific C2H4 removal rates from 9.7 to 6.1 µg of C2H4 g (dry weight) of
soil1 h1, as calculated (equation 1) for
the whole biofilter, using (C2H4) = 1.20 µg µl1 at 10°C. Previously (21), it was
observed that the activity of immobilized Mycobacterium
strain E3 declined rapidly below 10°C and was almost absent at 4°C.
In the present study, the activity below 10°C was not assayed, and so
it is not clear whether strain RD-4 would produce a similar response to
lower temperatures. However, after 3 days of operation at 10°C, the
biofilter outlet concentration started to decrease, indicating that the
C2H4-removing bacteria proliferated at 10°C.
Such a potential could be important for the application of strain RD-4
in biofilters for use with horticultural produce, which is often kept
at temperatures below 10°C. However, a more detailed study of the
temperature response of the strain RD-4 should be performed before this
suitability can be properly evaluated.
Effect of storage.
During storage of inoculated peat-soil at
2, 8, or 20°C, the bacterial cells lost about half (47 to 58%) of
their C2H4 removal activity within 2 weeks. In
comparison, it was reported that Mycobacterium strain E3
stored as free cell suspensions at 4°C lost about half of its
activity within 2 days (21). Before being used in the present biofilter studies, the RD-4 suspension was stored at 0 to 2°C
for 1 week. The effect of this storage was not evaluated, but probably
an even more efficient C2H4 removal could be
obtained in biofilter experiments if a freshly harvested cell
suspension was used as the inoculum.
It was found that storage at 2, 8, or 20°C had a similar influence on
the initial C
2H
4 removal rates of the
inoculated peat-soil.
Thus, after 4 weeks of storage at all three
temperatures, the
C
2H
4 removal rate represented
12 to 17% of the rate observed before
storage. These data agreed with
the reduction in the C
2H
4 removal
efficiency
(from 99 to 11%) that was observed for the biofilter
after
C
2H
4 starvation for 25 days. The comparable
responses to
storage at 2, 8, and 20°C indicated that the bacteria
did not
proliferate at the expense of organic substrates in the
peat-soil,
because such growth would be expected to be higher at 20 than
at 2°C.
Concerning the application of inoculated peat-soil for purposes of
C
2H
4 removal in biofilters, the results
demonstrated that
cold storage did not improve the keeping quality of
the inoculated
peat-soil. Rather, the results suggested that even at
20°C, the
inoculated peat-soil could be starved for
C
2H
4 for up to 2 weeks
and still retain half of
the original activity. This would allow
a delay between the preparation
of a biofilter and its subsequent
use for C
2H
4 removal.
Conclusions.
Ethylene was removed to 0.017 to 0.020 ppm in a
peat-soil biofilter with immobilized bacteria. Reduction of
C2H4 to such extremely low levels is attractive
at industrial point sources and is an important prerequisite for the
development of a biofilter for use in horticultural storage facilities.
Other characteristics of the biofilter which were favorable for such
use included the observations that (i) the operational stability
extended for more than 75 days, (ii) the biofilter adapted to
C2H4 removal at 10°C, and (iii) storage of
the inoculated peat-soil for 2 weeks at 20°C caused only a halving of
the C2H4 removal rate.
| |
ACKNOWLEDGMENTS |
I thank Gitte Hastrup Andersen for skillful laboratory assistance
and Lise Andersen for analysis of gas samples on the Photovac portable
GC. Also, I thank Hubert de Jonge, B. T. Christensen, and two anonymous
reviewers for helpful comments on the manuscript.
This work was done as a part of the research program
"Prydplantepakken" at the Danish Institute of Agricultural Sciences.
| |
FOOTNOTES |
*
Mailing address: Department of Soil Science and Crop
Physiology, Danish Institute of Agricultural Sciences, Research Center Foulum, P.O. Box 50, DK-8830 Tjele, Denmark. Phone: 45 8999 1873. Fax:
45 8999 1619. E-mail: lars.elsgaard{at}agrsci.dk.
| |
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This article has been cited by other articles:
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