Biological
reactors and biofilters are proven remedial alternatives to traditional
remediation technologies such as thermal and catalytic units. However, biodegradation of chlorinated VOCs
while possible requires greater attention to design. Biofilters have been used extensively in Europe and Japan, but
only recently have received attention in the United States. The EPA Site website has more information
about specific biological reactor and biofilter vendors.
The
two-stage reactor consisted of a fixed-film biofilter followed by a biological,
suspended-growth, completely mixed, continuously stirred tank reactor
(CSTR). The fixed film bioreactor, the
first stage of the system, was designed to remove VOCs from the process-gas
stream. The VOCs are used by the
bacteria as sources of carbon and energy.
The biofilm was designed to remove contaminants from the gas streams
that do not require the addition of a cosubstrate for cometabolic degradation. The first stage biofilter was a vertically
oriented reactor that contained previously composted wood chips and compost to
support biomass growth. The gas stream
was designed to flow counter current to the liquid stream that served to
facilitate contaminant mass transfer from the gas phase to the aqueous phase
and to support biological grown on the medium.
All liquid discharged from the biofilter was pumped to a liquid waste
storage container for disposal. The
biofilter consisted of a bioreactor vessel, support medium for the biofilm,
control panel and a vapor feed blower.
The
second stage of the biological treatment system consisted of a CSTR that was
used to degrade any residual contaminants in the gas stream following treatment
in the first-stage reactor. This CSTR
was a cometabolic reactor, where phenol was used as a co-substrate to remove
the principal contaminants that passed through the first stage, such as the
chlorinated solvents. The CSTR was
composed of a bioreactor vessel with automatic pH, foam, and temperature
control, nutrient feeds and discharge tank.
The contaminated vapor was delivered to the bottom of the CSTR by a
blower and the vapor was then mixed with the vessel’s liquid contents to
facilitate biological growth, contaminant mass transfer and VOC
degradation. Chlorinated contaminants
were transported via molecular diffusion from the vapor to the liquid in the
CSTR where they could be cometabolized by the inoculated bacterium. Treated vapor exited the liquid at the top
of the water column while the wastewater was discharged from the reactor after
passing over an overflow weir. The
wastewater contained some residual biomass and salts.
Envirogen
has previously demonstrated greater than 90% TCE removal in a pilot-scale CSTR
when treating a TCE/benzene stream, when treating a mixed chlorinated waste
stream and when treating a pure TCE waste stream. In a separate field test, a 4,000 L pilot-scale bioreactor was
demonstrated at Robin AFB, Georgia, and at F.E. Warren AFB, Wyoming for the
treatment of TCE. Some 85 to 90% total
TCE removal was achieved over a 70-day period at F.E. Warren AFB. The physical system consisted of a two-stage
skid-mounted unit including the biofilter and CSTR, an off-skid, 55-gal caustic
drum, a 55-gal phenol drum, and two storage tanks. A field trailer housed the analytical instruments for on-site
testing, and a telephone and fax communications system for daily communications
between on-site and off-site personnel.
Several
200-pound carbon canisters polished the vapor effluent from the biotreatment
system prior to discharge of the vapor to the atmosphere. The following process chemicals were stored
on site until use: ammonium chloride and potassium phosphate, sodium hydroxide,
phenol, ethanol, and compressed gases such as hydrogen, nitrogen, air and
different calibration gases. The
maximum biofilter system water discharge was 24 gallons per day (gpd). The biofilter and packing were inoculated
with a general, nonspecific, mixed inoculum previously used by Envirogen for
biofilter applications. It took 10 days
for the biomass to grow to a 1 g/L density after the first inoculation in the
CSTR. After a second inoculation, the
desired biomass levels were reached in approximately 8 days.
The
Air Force Research Laboratory, Airbase and Environmental Technology Division
demonstrated a two-stage biological reactor system for the treatment of off-gas
from the SVE system at McClellan AFB.
This demonstration ran for approximately 18 weeks. Primary contaminants of concern are listed
in table 1 below.
|
Table
1. Contaminants of concern at McClellan for the bioremediation demonstration.
|
|
|
Compounds |
Influent concentrations
(average) (ppmv) |
|
Trichloroethene |
6.40 - 18.00 (14.90 ± 4.38) |
|
Tetrachloroethene |
3.50 - 22.00 (17.75 ± 7.06) |
|
1,1,1-Trichloroethane |
6.60 - 34.00 (26.30 ± 9.97) |
|
1,2-cis-Dichloroethene |
1.00-3.80 (2.07 ± 1.12) |
|
1,1-Diehloroethane |
1.60- 3.80 (3.02 ± 0.82) |
|
Vinyl
chloride |
0.23 - 1.20 (0.63 ± 0.42) |
|
Toluene |
4.10- 9.30(7.57 ± 1.83) |
Vapor
sample ports were located at the biofilter influent, the influent to the CSTR, the
effluent from the CSTR, between the two carbon canisters and at the effluent
line from the second carbon canister.
Additionally a liquid sampling port was located on the effluent line
from the CSTR. The primary contaminant
of concern was TCE followed by 1,2-cis-DCE and vinyl chloride. The target of 95% DRE was not achieved
during the optimization period.
Table
2 and 3 below present the compound gas phase concentrations and overall DRE’s
in the Biofilter on six different sampling days. Table 4 below shows the concentration of the compounds of
interest in the liquid effluent from the CSTR.
Table 5 below shows the gas phase removal efficiency in both the
biofilter and the CSTR.
|
Table
2. Compound gas phase concentrations and overall DRE’s in the Biofilter on
12/03/97, 12/23/97, and 1/13/98. |
|||||||||||||
|
|
Concentration (ppbv) |
||||||||||||
|
|
Day 0 |
Day 20 |
Day 41 |
||||||||||
|
Compound |
BFI |
CSTRI |
CSTRE |
DRE |
BFI |
CSTRI |
CSTRE |
DRE |
BFI |
CSTRI |
CSTRE |
DRE |
|
|
Vinyl
Chloride |
880 |
950 |
950 |
-8.0 |
900 |
140 |
1.8 |
99.8 |
230 |
150 |
130 |
43.5 |
|
|
1,1-DCA |
1,600 |
1,700 |
1,700 |
-6.3 |
2,700 |
420 |
5.4 |
99.8 |
3,000 |
1,800 |
1,600 |
46.7 |
|
|
1,2-cis-DCE |
2,400 |
2,500 |
2,600 |
-8.3 |
2,800 |
440 |
2.2 |
99.9 |
1,000 |
610 |
240 |
76.0 |
|
|
1,1,1-TCA |
6,600 |
6,900 |
7,000 |
-6.1 |
30,000 |
4,700 |
68 |
99.8 |
32,000 |
19,000 |
18,000 |
43.8 |
|
|
TCE |
6,400 |
6,700 |
6,800 |
-6.3 |
18,000 |
2,800 |
43 |
99.8 |
17,000 |
9,800 |
8,400 |
50.6 |
|
|
PCE |
3,500 |
3,700 |
3,800 |
-8.6 |
20,000 |
3,900 |
59 |
99.7 |
21,000 |
12,000 |
11,000 |
47.6 |
|
|
Toluene |
4,100 |
4,200 |
4,300 |
-4.9 |
7,605 |
1,100 |
5.9 |
99.9 |
8,800 |
4,400 |
1,300 |
85.2 |
|
|
Ppbv
= parts per billion by volume |
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|
The
small negative DRE indicates that a larger amount left the biological system
than came in at the inlet or is an indication of the accuracy of the
measurements since they were not drawn continuously. |
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|
Table
3. Compound gas phase concentrations and overall DRE’s in the Biofilter on
1/30/98, 3/2/98 and 4/6/98. |
||||||||||||
|
|
Concentration (ppbv) |
|||||||||||
|
|
Day 58 |
Day 89 |
Day 126 |
|||||||||
|
Compound |
BFI |
CSTRI |
CSTRE |
DRE |
BFI |
CSTRI |
CSTRE |
DRE |
BFI |
CSTRI |
CSTRE |
DRE |
|
Vinyl
Chloride |
240 |
83 |
91 |
62.1 |
310 |
160 |
130 |
58.1 |
1,200 |
420 |
500 |
58.3 |
|
1,1-DCA |
3,800 |
1,200 |
1,400 |
63.2 |
3,200 |
1,400 |
1,400 |
56.3 |
3,800 |
1,100 |
740 |
80.5 |
|
1,2-cis-DCE |
1,300 |
420 |
480 |
63.1 |
1,100 |
500 |
180 |
83.6 |
3,800 |
970 |
530 |
86.1 |
|
1,1,1-TCA |
34,000 |
11,000 |
12,000 |
64.7 |
28,000 |
13,000 |
13,000 |
53.6 |
27,000 |
8,900 |
9,600 |
64.4 |
|
TCE |
17,000 |
6,000 |
6,200 |
63.5 |
14,000 |
6,200 |
5,800 |
58.6 |
17,000 |
4,400 |
3,800 |
77.6 |
|
PCE |
22,000 |
7,400 |
7,700 |
65.0 |
21,000 |
8,800 |
9,500 |
54.8 |
19,000 |
4,600 |
4,700 |
|