Bioremediation
Research Needs
"Predictability of bioremediation process
performance cannot be made with a high level of confidence."
(Gibson and Sayler, 1992).
The foundation of bioremediation has been the natural ability of
microorganisms to degrade organic compounds (Atlas, 1995).
Bioremediation is defined by the American Academy of Microbiology
as "the use of living organisms to reduce or eliminate environmental
hazards resulting from accumulations of toxic chemicals and other
hazardous wastes" (Gibson and Sayler, 1992). "This technology is
based on the use of naturally occurring or genetically engineered
microorganisms (GEMS) to restore contaminated sites and protect the
environment" (Miller and Poindexter, 1994).
Bioremediation is not a panacea but rather a "natural process"
alternative to such methods as incineration, catalytic destruction, the use
of adsorbents, and the physical removal and subsequent destruction of
pollutants (AgBiotech, 1991). The cost of moving and incinerating
pollutants is at least ten times that of in situ biological
treatment (Atlas, 1995). By integrating proper utilization of natural or
modified microbial capabilities with appropriate engineering designs to provide
suitable growth environments, bioremediation can be successful in the field
(Skladany and Metting, 1992). However, a gap exists between advances in
laboratory research and commercial field applications (Sayler, 1995). Two major
factors responsible for this gap are the lack of (1) a sufficient knowledge base to
accurately predict pollutant degradation rates and fates and (2) sites designated as
field research centers for bioremediation research and technology demonstrations.
Laboratory and microcosm studies have documented the potential use of
microorganisms for bioremediation. However, the physiologic potential of microbial
populations to remediate environments of relevant size, heterogeneity, and
variability has not been adequately tested (Miller and Poindexter, 1994; Skladany
and Metting, 1992). Successful application of bioremediation techniques must
address both the heterogeneous nature of many contaminated waste sites and the
complexity of using living organisms.
There has been progress in overcoming some of the barriers that have impeded
bioremediation from being successfully applied in the field (Atlas, 1995). Scientists
have begun to search for organisms with better biodegradation kinetics for a variety
of contaminants within broad environmental habitats. Studies examining
extremophiles could result in using organisms in situ that have a high
tolerance for organic solvents and alkaline soils or waters, and that function at high
temperatures for more efficient ex situ activity in bioreactors.
Nine assessments have been conducted over the past four years to
identify bioremediation R&D needs.
In addition, more often scientists are using molecular biology to optimize
bioremediation. For example, molecular engineers are constructing starvation
promoters to express heterologous genes needed in the field for survival and adding
additional bioremediation genes that code for enzymes able to degrade a broader
range of compounds present in perturbed and stressed environments (Atlas, 1995).
Genes such as the lux gene are being inserted as monitors for specific enzyme
activities such as naphthalene degradation. Much of this work is being
accomplished through transposon vectors, multimer resolution system vectors, and
alternative nonantibiotic selection determinants such as resistance to herbicides and
heavy metals, and nutritional markers (growth on lactose only) (de Lorenzo, 1992,
1993). All are efforts to make genetically modified organisms with potential for
field bioremediation more acceptable to the general public and more predictable in
terms of performance and ecological behavior. However, most of this work is still
conducted at the bench level, and technical gaps remain with respect to designing
recombinant bacteria that are stable enough for use by process engineers in both
in situ and ex situ applications.
Summarized below are the R&D needs expressed by academic, industrial, and
government participants at workshops on bioremediation sponsored by universities,
federal granting agencies, and institutions since 1991. Essentially, the take-home
message of all these reports is the need to better understand how bioremediation
works in the field. Unless there is a concerted effort to apply bench technologies for
evaluation, implementation, and assessment in field sites containing contaminants,
bioremediation will not reach its full potential as a proven technology. And unless
field engineers and bioengineers get involved in the early stages of R&D for field
applications, bioremediation will not advance as rapidly as it should as an alternative
to other, more costly methods.
At a workshop organized by Rutgers University in 1991, scientists and engineers
reviewed the current state of bioremediation, addressed the difficulties in making
bioremediation a successful applied technology, and suggested approaches for
resolving these difficulties (AgBiotech, 1991). Eight major problems were
identified:
In 1992, the Environmental Protection Agency (EPA) issued a report recommending
five key research program areas that were to provide a balanced transition of
bioremediation research from the laboratory bench to technology applications in the
field (BTDP, 1992). These research program areas, listed below, could lead to a
fundamental understanding of how to apply bioremediation technologies to
engineering activities.
"There is a need to orient aspects of bioremediation research to
modern biotechnologically integrated science and engineering efforts using field site
demonstrations as vehicles for integration." (Gibson and Sayler,
1992)
The American Academy of Microbiology in 1992 evaluated the scientific
foundations of bioremediation, its current status, and its future needs (Gibson and
Sayler, 1992). The workshop panelists concluded that solid biochemical, molecular,
and ecological foundations of bioremediation existed and that environmental
biotechnology has expanded the knowledge base of on-going processes with
bioremediation. The report lists examples of bioremediation process performance
and credibility, such as land farming and soil slurry reactors. It addresses concerns
regarding information needs to advance current technology, parameters for
successful bioremediation (site characterization, biodegradability of the
contaminants, microbiology, and environmental and nutritional factors) that should
be included in the engineering framework, and limitations of current knowledge.
There are opportunities for enhanced technology if improvements can be made in
process performance, degradative capacity via molecular engineering, predicting the
outcome of multicomponent substrate biodegradation, and understanding the
regulation of microbial enzyme production for simultaneous degradation of several
compounds that are all present at low concentrations. Bioremediation can be
developed as a technology if the exploratory research base is exploited. There is a
need to better understand and define the following: the roles and applications of
microbial biofilms and activities at interfaces, the microbial responses to
multipollutant mixtures, the dynamics of microbial community structure and
function, site characterization, process monitoring and optimization, and integration
of molecular genetics and physiology with environmental needs. The report also
recommends that kinetic models be developed and coupled to existing and emerging
hydrogeochemical transport models to predict the potential success of
bioremediation and that this technology be evaluated and verified in the field.
Finally, the report concludes that there is a lack of well-characterized field sites that
could be used for understanding the natural events that occur and for demonstrating
feasibility of the laboratory-demonstrated technology. Only under environmental
conditions and by using an interdisciplinary approach can bioremediation be applied
and verified.
In 1993, the American Academy for Microbiology held a follow-up workshop to the
1992 workshop. The resulting report documented the critical need for field research
in environmental bioremediation by addressing strategies and mechanisms for field
research in environmental bioremediation (Miller and Poindexter, 1994). Criteria
for field site selection and use are described, as well as use of components of
scientific disciplines in conducting field research in environmental bioremediation.
The report recaps some of the technologies already developed that have applications
for field use, such as physical and chemical analysis and microbiological and genetic
methods. Now, concludes the report, is the time for bioremediation to move into
field sites, where the necessary information can be obtained to predict efficacy and
determine the duration of treatment, costs, and safety.
The National Research Council's 1993 report "In Situ Bioremediation: When Does
It Work?" discusses the future prospects for bioremediation and three limiting
factors that restrict the use of field applications of the variety of microbial processes
well documented in the laboratory (CISB, 1993). These factors are the lack of
understanding of how microorganisms behave in the field, the difficulties associated
with stimulating microbial growth in the field, and making the contaminant
bioavailable to the microbial community. To overcome these limitations, it was
recommended that research focus on the following:
The American Chemical Society sponsored a symposium entitled "Bioremediation
through Rhizosphere Technology" in 1993. The published papers from this
symposium indicate that the rhizosphere provides a complex and dynamic
microenvironment where bacteria and fungi associated with plant roots can degrade,
mineralize, and stabilize toxicants, thus offering a potentially important treatment
strategy for bioremediation of numerous classes of hazardous chemicals in the
environment (Anderson and Coats, 1994). Radionuclides and heavy metals were not
discussed. However, the recommendations of the participants indicate that an
understanding of the fundamental biochemical, physiological, and ecological
interactions in the rhizosphere will facilitate the successful use of vegetation to
remediate chemically contaminated soils.
In 1994, the Department of Energy held a workshop to discuss the current status and
the basic and applied research needs of phytoremediation (DOE, 1994b).
Phytoremediation uses the natural attributes of plants for applications in site
remediation efforts. Useful attributes of plants include their roots, which have
enormous surface area to bioaccumulate and concentrate contaminants such as heavy
metals and other inorganic compounds, and their diverse genetic adaptations to
handle toxic levels of contaminants and mineralize toxic organic compounds. In
addition, there is a positive synergy between microorganisms in the rhizosphere, in
that the numbers and activities of microorganisms are increased due to the nutrient
and energy sources provided by the plant. In many instances these microbial
activities are bioremedial. Overall, plant cultivation and management is cost-effective and in many ways more easily accepted by the general public, since it is a
solar energy driven process. Even though phytoremediation is considered a viable
technology for some contaminants, certain research and development needs
exist:
A workshop held by the European Community and the U.S. government in 1994
focused on the potential of microorganisms to solve environmental problems
(Economidis and Palmisano, 1995). Three areas of research were discussed:
anaerobic biotransformations of pollutants, biodegradability testing using
microcosm and mesocosm approaches, and tailor-made microorganisms for the
treatment of toxic waste. The participants concluded that future research on the use
of microorganisms to solve environmental problems should focus on the need
to:
The take-home message of all these reports is the need to better
understand how bioremediation works in the field.
In 1995, the National Science and Technology Council's Biotechnology
Research Subcommittee reported that several priorities for research in
bioremediation are needed to further advance the field (BRS, 1995).
These priorities are:
Over a ten-year period, NABIR will address most if not all of the research and
development needs compiled over five years by international scientists and engineers
working in the bioremediation field. In reports written since 1991 on the research
needs for bioremediation, all agree that the real question that needs to be answered
is whether or not in situ bioremediation can be an efficient, cost-effective
technology. There is a growing body of evidence that various bioremediation
processes can often provide a clean, natural, and cost-effective method for treatment
of pollutants (Atlas, 1995). Yet bioremediation is described as an innovative but
unproven technology and is often not even considered as an alternative technology
by field engineers (Grimes, 1995; Sayler, 1995). One way to answer this question
of feasibility is to provide a systematic long-term integrated program of field
research focused on testing and optimizing the potential of bioremediation as a
major environmental technology for recovery of contaminated sites, and assessing
its safety. The following pages describe such a program.
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