United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens, GA, 30613-7799
Research and Development
EPA 600 /S3-90/031 August 1990
&EPA Project Summary
Available Models for
Estimating Emissions
Resulting from Bioremediation
A Review
Susan Sharp-Hansen
The use of bioremediation
processes to treat hazardous waste
has increased in the last 10 years.
Biological treatment offers the
advantage of contaminant
destruction rather than transfer to
other media. There is concern,
however, that significant amounts of
organic pollutants are emitted to the
air from biological treatment facilities
before they can be degraded. To
estimate the magnitude of emissions
from these facilities, overall fate
models that incorporate the effects of
several competing removal
mechanisms are needed.
This report focuses on the state-of-
the-art in modeling air emissions
from bioremediation processes. The
biological treatment systems are
described, as are the important
pathways that affect the fate of
organic pollutants in those systems.
Currently available models are
identified, described, and evaluated
for each bioremediation process.
Finally, some limitations of the
models and the need for further
research are discussed.
This Project Summary was
developed by EPA's Environmental
Research Laboratory, Athens, GA, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Biological treatment of organic
pollutants offers the advantage of
contaminant destruction rather than
transfer to other media. Thus,
biotreatment systems may provide the
potential to reduce or eliminate the
human health risks associated with the
treatment and/or disposal of hazardous
wastes. However, concern has grown in
recent years that many volatile organic
pollutants are emitted to the atmosphere
from biotreatment facilities before they
can be degraded. In response to the
increased attention, a number of models
have been developed that predict the fate
of organic pollutants during specific
bioremediation processes.
The full report focuses on the potential
for bioremediation processes to transfer
contaminants between the soil or water
and the air. The intent is to identify,
describe, and evaluate available methods
and models for estimating atmospheric
emissions from bioremediation processes
used to treat hazardous waste. Models
that consider only volatilization will tend
to overpredict the magnitude of air
emissions from bioremediation
processes. Therefore, this report
concentrates on models that incorporate
the competing effects of biological
activity, volatilization, and possibly other
mechanisms, such as adsorption and
transport, on the fate of organic
pollutants. Models of some non-biological
components of the treatment processes
also are included. Bioremediation
processes that are currently in use or in
advanced stages of research are listed in
the final report, where each process is
briefly described in terms of its operation
and component parts, the types of
contaminants treated, advantages and
disadvantages associated with its use,
and the likelihood of air emissions.
An extensive literature search was
conducted to locate models that simulate
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the fate of organic pollutants for each
biotreatment process. Summaries of the
models located during that search make
up the bulk of this report. Models are
summarized in terms of their theoretical
basis, assumptions, and the mechanisms
of transport and fate that are considered.
Input requirements, output, previous
model applications, resource
requirements, ease of use, and model
availability also are discussed briefly. Air
emissions from some biotreatment
processes have not been modeled. For
those processes, the current status of
research is discussed. In addition, the
status of modeling two physical remedial
processes (i.e., soil washing and soil
venting) often used in conjunction with
bioremediation efforts is briefly
addressed.
A chapter in the full report describes
the important pathways by which
pollutants are transferred from a waste
treatment site. Generally accepted
theories associated with each pathway
are briefly summarized. In addition, when
possible, methods of estimating
equiibrium and rate constants are
presented.
Current efforts to model the fate of
hazardous compounds at biological
treatment facilities suffer from a number
of limitations that should be understood
by users of the models described in the
full report. These limitations and the
related need for further research are
discussed in the final section of the full
report.
Conclusions and Research
Needs
In order to estimate the magnitude of
emissions from biological treatment
facilities, overall fate models that
incorporate the effects of several
competing removal mechanisms are
needed. Although such models have
been developed for some bioremedial
processes, no models exist for other
processes. The models that are available
are of limited utility both because of
limited laboratory and field data and
because of the assumptions made in
developing the models.
Conclusions drawn from this report
are:
1) Models of bioremediation processes
are empirical. Data are unknown or
Incomplete and the interrelation-
ships of components and pathways
are complex. Moreover, at
wastewater treatment facilities, the
concentrations of hazardous organics
can vary by several orders of
magnitude on a daily basis. Thus,
the release of organics to the air or
other media from bioremediation
sites is difficult to estimate.
2) There is a correlation between
operational experience with a
biotreatment process and the state-
of-the-art in modeling that proces. No
models exist for some innovative
new processes, such as the
composting of hazardous materials.
For some wastewater treatment
processes, only models of substrate
removal exist. On the other hand,
many models have been developed
to describe the fate of pollutants in
some commonly used treatment
processes, such as landfarming.
Most of the models identified for this
project are analytieal-models-based
on a mass balance approach. Some
more complex numerical models
also are reviewed.
3) Among the suspended-growth
systems, a number of models have
been developed to describe
activated-sludge aeration tanks and
surface impoundments. Biodegra-
dation, adsorption, and volatilization
and/or stripping are the removal
mechanisms considered. Most of the
models describe steady-state,
completely mixed conditions. Few
models of sequencing batch reactors
or of powdered activated carbon
treatment are available.
4) No models have been produced to
simulate the emission of specific
organic pollutants from fixed-film
biologiical treatment systems. Most
available models reflect a concern
for substrate removal, in terms of
biologocal oxygen demand (BOD).
Under some conditions, estimates or
measurements related to the mass
transfer of oxygen can be used to
approximate air emissions" fom these
systems.
5) Among soil-incorporation systems,
land treatment facilities have been
modeled most. The fate mechanisms
included in the models include bio-
degradation, diffusion and
volatilization, adsorption and solute
transport. The models are non-
steady-state and ouput is generated
for specified times. No mathematical
models exist to describe the
composting of hazardous materials.
One model simulates biodegradation
of organic vapors in a soil treatment
bed.
6) A number of models predict the fate
of organic pollutants in the
subsurface under natural conditions.
The unsaturated-zone models are not
designed to simulate in situ
bioremediation, which often includes
air sparging to stimulate microbial
growth. While some of the saturated-
zone models can be applied to in
situ bioremedial efforts, they cannot
be used to dirctly estimate air
emisions. Pollutant vapors escaping
from a contaminated aquifer must
pass through the unsaturated zone.
Thus, the likelihood and magnitude
of air emissions from the saturated
zone must be modeled using an
unsaturated zone model with a
pollution source term supplied at the
water table boundary.
7) The biodegradation of organics in
biological treatment facilities is
poorly understood and limited-data
are available to estimate the
biodegradation rate constants for
compounds of interest. In addition,
data generated in laboratories may
vary greatly from field study data.
Thus, reported values should be
used with caution and experimentally
verified when possible.
8) Suspended-growth system modeling
results have been shown to be very
sensitive to the values of the
biodegradation parameters. Thus,
when accurate information about
biological activity is not available,
modeling studies should use a range
of values, including zero, for the
biodegradation rate constant. By
ignoring biodegradation, estimates of
air emissions under "worst-case"
conditions (e.g., periods of
acclimation or of inactivity due to
toxic shock) are obtained.
9) Most of the models summarized in
this report have not been validated in
the field. When predicted model
results are compared to laboratory or
field data, many authors report
agreement within an order-of-
magnitude. That level of agreement
is generally considered adequate for
screening purposes. Because of the
many assumptions incorporated into
the models, they should be used with
caution for detailed site-specific
studies.
10) To be able to adequately predict air
emissions from biological treatment
facilities, further research is needed
in several areas. More collection of
field data is needed, both in order to
understand the severity of the
problem and in order to aid in model
development and validation. An
improved understanding of the
kinetics of biodegradation in multi-
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component heterogneous systems is
needed. For some processes, first-
generation fate models have yet to
be developed. For other processes,
the capabilities of the current models
could be expanded.
U. S. GOVERNMENT PRINTING OFFICE: 1990/748-012/20061
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Susan Sharp-Hansen is with AQUA TERRA Consultants, Mountain View, CA
94043
Gerard F. Laniak is the EPA Project Officer (see below).
The complete report, entitled "Available Models for Estimating Emissions
Resulting from Bioremediation Processes:/* Review," (Order No. 90-228 6101
AS;Cost: $31.00 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:
Environmental Research Laboratory Research Laboratory
U.S. Environmental Protection Agency
College Station Road
Athens, GA 30613-7799
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-90/031
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