SEPA
United States
Environmental Protection
Agency
EPA/540/SR-93/521
September 1993
SUPERFUND INNOVATIVE
TECHNOLOGY EVALUATION
Emerging Technology
Summary
Bioscrubber for Removing
Hazardous Organic
Emissions from Soil, Water, and
Air Decontamination Processes
An advanced biofiltration system has
been developed for the removal of trace
organic contaminants in air. This
bioscrubber uses activated carbon as
a support for biogrowth. An advanced
engineering design was incorporated
into the bioscrubber to allow biomass
removal and nutrient supplement if nec-
essary. In a bench-scale study, >95%
removal efficiency has been consis-
tently demonstrated in an air stream
containing 5 to 40 ppm of toluene for
>11 mo. It shows a much higher degra-
dation efficiency than the existing prac-
tice of using compost and other natu-
rally occurring media under the same
operating conditions. This bioscrubber
also provides several operational ad-
vantages over conventional activated
carbon adsorbers for soil, water, and
air decontamination processes.
This Summary was developed by
EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see ordering
information at back).
Introduction
Biofiltration is now a well-established
air pollution control technology in several
European countries. As many as 500
biofilters are currently believed to be ac-
tive in Germany and the Netherlands. Con-
trol efficiencies of more than 90% have
been achieved for many common air pol-
lutants. Bio-filtration when applied to sys-
tems can provide significant economic ad-
vantages over other air pollution control
devices. Environmental benefits include
low energy requirements for operation and
complete degradation of the pollutants.
This bioscrubber^was developed under
the Superfund Innovative Technology
Evaluation (SITE) Emerging Technology
(ET) Program. The SITE ET Program is
part of the U.S. Environmental Protection
Agency's (EPA's) research into cleanup
methods for hazardous waste sites
throughout the nation. Through coopera-
tive agreements with developers, alterna-
tive or innovative technologies are refined
at the bench-scale and pilot-scale level
and then demonstrated at actual sites.
EPA collects and evaluates extensive per-
formance data on each technology for use
in remediation decision making for haz-
ardous waste sites.
Most biofilters have been built as open
single-bed systems. Open, multiple story
systems are also built if space constraints
exist. Some European firms have devel-
oped enclosed systems usually with
Printed on Recycled Paper
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stacked bsds. Media used include com-
post, mineral soil, and peats. Microscopi-
cally, concentration profiles exist from the
bulk gas stream through the biofilm and
then to the solid surface.
Btofittration, in its most general sense,
is the removal and decomposition of con-
taminants from gases into nonhazardous
substances through the use of microor-
ganisms. Biofilters are believed to be the
most economical way to treat low level
contaminants (up to several thousand ppm)
in gas streams. The advanced biofiltration
system developed here under the SITE
ET program uses a selected activated car-
bon as a microbial support and incorpo-
rates unique engineering features to main-
tain a steady environment for biogrowth.
The report summarized here documents
the results of bench-scale tests of the
btoscrubber to remove trace organic con-
taminants from air.
For efficient operation, the filter media
must meet several requirements:
• provide optimum environmental condi-
tions for the resident microbes,
« exhibit uniform pore size and particle
structure (for low-bed pressure drop,
minimizing gas channeling, high re-
active surfaces), and
• exhibit minimal bed compaction (mini-
mize maintenance, media replace-
ment).
The composition of existing, commer-
cially available biofilters utilizing compost
and other naturally occurring media gen-
erally satisfies the first requirement by pro-
viding sufficient nutrients for the microor-
ganisms (typically bacteria). Some prob-
lems with composting, however, are the
huge space requirement, continual loss of
effective surface area during biomass
build-up (slothing), and inefficient biodeg-
radation of particularly refractory contami-
nants (i.e., chlorinated compounds).
The carbon-based biofiltration module
developed here addresses the current de-
ficiencies of composting and other natu-
rally occurring media-based biofilters by:
• having minimal pressure drop because
of no slothing,
• having much smaller bed require-
ments (allowing the use of compact
filters),
• accommodating removal of biomass
as necessary, thus no replacement of
disposal requirement of spent media,
and
• retaining high water in the mic-
roporosity (long shelf life while not in
use during start up/shut down, minimal
requirements for adding water).
Additionally, activated carbon media
beds provide another key separation
mechanism for biofilters—the desorption
of gases onto the carbon. This increases
surface concentration of contaminants and
removes hydrophobia gases that would
not typically be absorbed into the aque-
ous phase.
These qualities enhance biodegradation
of typical organic contaminants, as well
as substances (i.e., refractory compounds,
low concentration, operating concentration
fluctuations) that would not be efficiently
degraded in commercially-available
biofilters. This study focused on the con-
ventional degradation of a dilute hydro-
phobic contaminant, 10 to 20 ppm of tolu-
ene in air, with a biofilter using activated
carbon as the medium. Bench-scale units
designed and operated for more than 11
mo in the' laboratory are discussed. — -
Bench-Scale Apparatus
The bench-scale bioscrubber testing unit
assembled in the laboratory consisted of
five parallel glass columns (2.5 x 61 cm)
packed with a selected activated carbon.
The columns were inoculated with acti-
vated sludge in a synthetic media contain-
ing benzoic acid and other inorganic nutri-
ents. After inoculation, an air stream con-
taining 10 ppm of toluene was fed to each
column and their degradation efficiencies
were monitored through sampling ports,
A, B, and C, located at 13, 25, and 61 cm,
respectively, from the inlet of the stream.
In addition, an inorganic nutrient solution
was supplied to the columns at 1 cc/hr for
the dual purposes of inorganic require-
ments and additional humidification to the
filter. The effluents were measured by gas
chromatography with a method detection
limit of 0.86 ppm of toluene.
Process Description
Before 1992, the bioscrubbers operated
steadily for 3 mo before the removal effi-
ciency declined. Channeling'the air flow,
drying the filter media, and a poor inocula-
tion procedure were considered possible
sources of this activity decline. An im-
proved inoculation and maintenance pro-
cedure led to a steady operation for more
than 11 mos.
To improve inoculation, activated sludge
was collected from a local sewage author-
ity and 100 ml of supernate was added to
the benzoic acid media in batch mode.
The municipal microbes were grown in
this solution for 5 days and then put into
columns. A dilute benzoic acid solution
supported the biomass and allowed im-
pregnation without clogging the bed. A
portion of the benzoic acid solution was
removed from each column after 24 hr
and analyzed. The initial feed concentra-
tions of inoculation were more than 1500
mg/L of COD; by the fourth day, Column
B had a COD concentration range of 20
to 50 mg/L. After 96 hr, the columns were
drained of excess solution and placed on-
line. The biofilters were fed on influent in
a down flow mode. All influent variations
were corrected immediately without caus-
ing any alteration' in column performance.
Biodegradation
The filters were operated from 3/23/92
to 2/28/93 with varied flow rates, (0.5,1,2,4
L/min). From 3/21/92 to 6/30/92, all filters
were fed air with ~10 ppm of toluene at
0.5 L/min. During this period, no toluene
breakthrough was observed at Port A for
any column. More importantly, the mass
transfer- zone~(MiZ).,remained .stationary
for the entire period. Biodegradation of
toluene evidently was effective and com-
plete, showing no signs of accumulation
of contaminants or metabolic byproducts.
Bioregeneration ;of, activated carbon has
been discussed in the literature as a
means to prolong its service life in water
and waste water! treatment. This filter ex-
tends the concept to air pollution control,
which offers a suitable environment for
biogrowth. j
To ensure thai biodegradation, instead
of carbon adsorption, was responsible for
the removal of toluene, the filter was
presaturated with -10 ppm of toluene in
air before inoculation. The substantial
breakthrough, | or "roll-over," of the
preadsorbed toluene on the carbon ob-
served at Ports IB and C after inoculation
resulted from desorption of the
preadsorbed toluene. The roll-over de-
clined drastically and eventually disap-
peared within 2 to 3 wk. That toluene is
removed by biodegradation is evident
based on the appearance and disappear-
ance of the "roll-lover" observed during the
initial operations. In field start-up ^opera-
tions, no roll-over will be observed since
the contaminants need not be preadsorbed
and biodegradation will take place imme-
diately. ;
Effect of Flow Rate
The flow rates were increased to 1, 2,
and then 4 I/mm, from 0.5 L/min to study
its effect on the degradation efficiency.
During these increased flow rates, the sys-
tem showed some breakthrough ranging
from 0 to 5 ppm at Port A. Nevertheless,
only trace amounts of toluene were de-
tected at Port ,B. The MTZ's were esti-
mated to be 19;cm and 25 cm for the flow
rates of 2 and 4 L/min., respectively. Be-
fore the end of the study, the flow rate
was reduced to 0.5 L/min. on 1/21/93; no
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toluene breakthrough was detected at Port
B as had been observed previously. The
recovery of the column to the original MTZ
indicates that the increase of the MTZ
from 13 to 25 cm is most likely due to
degradation kinetics versus the linear ve-
locity of the contaminant. The MTZ re-
quirement (i.e., the bed-depth requirement)
is extremely shallow and stationary, indi-
cating that the in-situ bioregeneration ef-
fectively restored the carbon capacity.
Under the highest flow rate tested
(about 80 g/m3/hr of toluene), the degra-
dation rate was estimated with an empty
bed contact time of about 1 sec. This
degradation efficiency was about 40 to 80
times higher than those reported in the
literature for existing biofilters using com-
- post rand,, other- naturally- occurring -media.-
The degradation efficiencies observed here
were very likely enhanced by the adsorp-
tion of activated carbon. It also offered an
advantage as a sink to adequately cush-
ion any feed fluctuations. During the 11
mo of operation, a consistent removal effi-
ciency was observed although the feed
fluctuated from 5 to 40 ppm.
Biomass Removal
The biomass generated and accumu-
lated in the filter as a result of the degra-
dation of contaminants was expected. Bio-
mass was visually detected occupying the
interparticle space. This buildup would
eventually result in a pressure drop in-
crease. Biomass was occasionally re-
moved manually to maintain a minimal
pressure drop throughout the operation
period. Although the excess biomass was
removed from the column, sufficient
amounts of biomass were retained on the
carbon to maintain effective biodegrada-
tion when the bed was replaced. The
biofilter efficiency was not reduced as a
result of the biomass removal The biom-
ass generated from the filter is expected
to be similar to the sludge generated from
the fixed-film biological treatment practiced
routinely in water and wastewater treat-
ment. The biomass should be disposed
according to the current practice in water
and wastewater treatment.
Pilot Unit
The pilot bioscrubber developed in this
program is as simple as a carbon adsorber
system incorporating a nutrient delivery
system and a biomass removal capability.
Because of-the simple configuration, it
can be integrated into existing production
processes or added downstream from ex-
isting remediation processes, such as air
stripping towers, soil vacuum vents, bio-
logical wastewater treatment, etc. The sys-
tem consists of four major components:
(1) a gas delivery system, (2) the biofilter,
(3) a nutrient delivery system, and (4) a
biomass removal system. Through our ex-
tended operating experience, an advanced
engineered filtration technology has been
incorporated into the pilot testing unit to
become a reliable and usable biological
treatment system.
Applications
The proposed technology will have wide
application to clean up Superfund sites.
Potential areas include: (1) organic emis-
sion control for groundwater decontami-
nation using air strippers, (2) emission
control for biological treatment of ground
and surface water, and (3) emission con-
trol for soil decontamination. These pri-
mary treatment processes currently under
development or practice, have not b.een
designed to prevent volatile organic com-
pound emissions from discharging into the
atmosphere. The requirement to treat
these airborne pollutants may, however,
cause these treatment processes, 'to be-
come expensive or economically prohibi-
tive. The proposed technology is an ideal
posttreatment for these processes because
of its effectiveness in handling trace or-
ganic volatiles economically and effectively.
This bioscrubber, which uses activated
carbon as a medium, provides several
operational advantages over conventional
activated carbon adsorbers for the above-
listed applications. The bioregeneration
keeps the maximum adsorption capacity
available constantly; thus, the MTZ re-
mains stationary and relatively short. No
expensive, off-site carbon regeneration is
required, and the bed length is greatly
reduced. These features translate into re-
duced capital and operational costs. The
bioscrubber's advantages would be fully
used when off-gas contains weakly
adsorbed contaminants, such as methyl-
ene chloride or adsorbates competing with
moisture in the stream. Finally, the chro-
matographic effect (or premature desorp-
tion) commonly experienced in an adsorber
would not exist because the maximum
capacity is available constantly. The
bioscrubber is expected to replace some
existing biofilters that currently use acti-
vated carbon.
&U.S. GOVERNMENT PRINTING OFFICE: 1993 - 750-071/80094
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PaulK.T. Liu Is with the Aluminum Company of America, Pittsburgh, PA 15238;
and Trade Williams is a USEPA/University of Cincinnati Research Apprentice,
Cincinnati, OH 45221
Naomi P. Barkley is the EPA Project Officer (see below). . ,
The complete report, entitled "SITE Emerging Technologies: Bioscrubber for
Removing Hazardous Organic Emission from Soil, Water, arid Air Decon-
tamination Process," (Order No. PB93-227205; Cost: $19.50, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
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$300
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