Future Prospects for Bioremediation
In preparing this report the National Research Council's Committee on In Situ Bioremediation sought to communicate the scientific and technological bases for bioremediation. As the report has explained, the principle underlying bioremediation is that microorganisms (mainly bacteria) can be used to destroy hazardous contaminants or transform them into less harmful forms. Microorganisms are capable of performing almost any detoxification reaction. Nevertheless, the commercial practice of bioremediation today focuses primarily on cleaning up petroleum hydrocarbons. The full potential of bioremediation to treat a wide range of compounds cannot be realized as long as its use is clouded by controversy over what it does and how well it works. By providing guidance on how to evaluate bioremediation, the committee hopes this report will eliminate the mystery that shrouds this highly multidisciplinary technology and pave the way for further technological advances.
This chapter summarizes new research advances that the committee foresees as expanding the future capabilities of bioremediation. It recommends steps that will improve the ability to evaluate bioremediation technologies objectively, whether the technologies are new or established.
NEW FRONTIERS IN BIOREMEDIATION
Bioremediation integrates the tools of many disciplines. As each of the disciplines advances and as new cleanup needs arise, opportunities for new bioremediation techniques will emerge.
Until now, three types of limitations have restricted the use of bioremediation to cleanup contaminants other than petroleum hydrocarbons: inadequate understanding of how microbes behave in the field, difficulty supplying the microbes with stimulating materials, and problems with ensuring adequate contact between the microbes and the contaminant. Consequently, only a few of the myriad microbial processes that could be used in bioremediation are applied in practice. Recent advances in science and engineering show promise for overcoming these limitations, as illustrated by the following examples:
Understanding microbial processes. As novel biotransformations become better understood at ecological, biochemical, and genetic levels, new strategies will become available for bioremediation. A recent example is microbial dechlorination of polychlorinated biphenyls (PCBs), which is being investigated by a group of researchers from academia and industry. The researchers, studying PCBs in Hudson River sediments, have documented that anaerobic microbes in the sediments can transform highly chlorinated PCBs to lightly chlorinated PCBs, which can be degraded completely by aerobic microbes (see Box 4-3). This research may become the basis for commercial bioremediation of PCBs—compounds once thought to be undegradable. Similar advances are being made for the dechlorination of chlorinated solvents, also once believed to resist biodegradation.
Advances in understanding microorganisms may also improve bioremediation's effectiveness in meeting cleanup standards. As explained in Chapter 2, uptake and metabolism of organic compounds sometimes stop at concentrations above cleanup standards. Research on bioaugmentation and direct control of the cell's genetic capability and/or regulation is very active today and may lead to methods to overcome such microbiological limitations.
Supplying stimulating materials. Innovative engineering techniques for supplying materials that stimulate microorganisms are pushing the boundaries of bioremediation. For instance, the recent innovation of gas sparging has substantially expanded capabilities for aerobically degrading petroleum hydrocarbons. Research is ongoing into optimizing ways to supply materials other than oxygen. Such re-
search will pave the way for emerging bioremediation applications, such as degradation of PCBs and chlorinated solvents and demobilization of metals, which are not necessarily controlled by oxygen.
Promoting contact between contaminants and microbes. Research is under way into engineering advances to increase the availability of contaminants to microbes—advances that, if successfully applied, would increase bioremediation's efficiency. New techniques for promoting contaminant transport to the organisms include high-pressure fracturing of the subsurface matrix, solubilization of the contaminants by injecting heat (via steam, hot water, or hot air), and, perhaps, addition of surfactants. Discovery of improved methods for dispersing the microorganisms may also enhance microbial contact with the contaminants and lead to more effective bioremediation.
THE INCREASING IMPORTANCE OF EVALUATING BIOREMEDIATION
As new bioremediation techniques are brought from the lab into commercial practice, the importance of sound methods for evaluating bioremediation will increase. The Committee on In Situ Bioremediation has recommended a three-part strategy for ''proving" that bioremediation has worked in the field. As explained in Chapter 4, the three central parts of this strategy are (1) documented loss of contaminants from the site, (2) laboratory assays showing that microorganisms from site samples have the potential to transform the contaminants, and (3) one or more pieces of information showing that the biodegradation potential is actually realized in the field. The main goal of this strategy is to show that biodegradation reactions that are theoretically possible are actually occurring in the field, at fast enough rates and in appropriate locations to ensure that cleanup goals are met.
While the three-part strategy provides a general framework for evaluating bioremediation, the level of detail with which it should be applied depends on the interests of those involved with the bioremediation. Each party involved must realize that "success" may mean different things to the different parties. Regulators are primarily concerned that legislated standards are achieved, clients emphasize attaining cost-effective goals, and vendors have a vested interest in demonstrating that their technology is effective and predictable. Clear communications about everyone's goals and negotiations about specific criteria to meet the different goals are critical to the project's perceived success and must occur in advance of its implementation.
The current knowledge base is sufficient to allow implementation
of the three-part strategy. However, the specific experimental protocols for carrying out the strategy need to be developed. In addition, further research and better education of those involved in bioremediation will improve the ability to implement the strategy as well as understanding of the fundamentals of bioremediation.
Recommended Steps in Research
The committee recommends research in the following areas to improve evaluations of bioremediation:
Evaluation protocols. Protocols need to be developed for putting the three-part evaluation strategy into practice. Consideration should be given to evaluating a range of chemical contaminants (including petroleum hydrocarbons, chlorinated solvents, PCBs, and metals) and site characteristics (such as shallow and deep aquifers and sites with high and low heterogeneity). These protocols should be field tested through co-ordinated efforts involving government, industry, and academia and should be subject to scientific and peer review.
Innovative site characterization techniques. Rapid, reliable, and inexpensive site characterization techniques would have a significant impact on the ease of evaluating bioremediation. Examples of relevant site measurements include distribution of hydraulic conductivities, contaminant concentrations associated with solid or other nonaqueous phases, native biodegradation potential, and abundance of different microbial populations. Techniques to measure physicochemical characteristics in situ are being developed and could revolutionize the capability to do field assessments. Methods adapted from molecular biology seem especially promising for augmenting current techniques for assaying biodegradation potential and microbial populations. Gathering more and better characterization data would diminish uncertainties and reduce the needs for overdesign via safety factors.
Improved models. Improvements in mathematical models are essential because models link understanding of chemical, physical, and biological phenomena. One particularly promising advancement is the use of modeling as a key part or improved means for on-site management, which requires an appreciation of the dynamic interactions among the many phenomena. As field sampling becomes more rapid and accurate, on-site decisions will be limited more by the ability to understand the dynamic interactions than by turnover times between sampling and analysis.
Recommended Steps in Education
Steps need to be taken to educate all components of society about what bioremediation is and what it can and cannot do. Especially important is improved education of the people who are in direct decision-making positions. The committee recommends three types of education:
Training courses that selectively extend the knowledge bases of the technical personnel currently dealing with the uses or potential uses of in situ bioremediation. This step explicitly recognizes that practitioners and regulators who already are dealing with complicated applications of bioremediation need immediate education about technical areas outside their normal expertise.
Formal education programs that integrate the principles and practices for the next generation of technical personnel. This step explicitly recognizes the need to educate a new generation of technical personnel with far more interdisciplinary training than is currently available in most programs.
Means for effective transfer of information among the different stakeholders involved in a project. Effective transfer requires that all types of stakeholders participate, that all are invested in achieving a common product (e.g., a design, a report, or an evaluation procedure), and that sufficient time is allocated for sharing perceptions and achieving the product. This step may involve more time and more intensive interactions than have been the norm in the past.
In summary, in situ bioremediation is a technology whose full potential has not yet been realized. As the limitations of conventional ground water and soil cleanup technologies become more apparent, research into alternative cleanup technologies will intensify. Bioremediation is an especially attractive alternative because it is potentially less costly than conventional cleanup methods, it shows promise for reaching cleanup goals more quickly, and it results in less transfer of contaminants to other media. However, bioremediation presents a unique technological challenge. The combination of the intricacies of microbial processes and the physical challenge of monitoring both microorganisms and contaminants in the subsurface makes bioremediation difficult to understand—and makes some regulators and clients hesitant to trust it as an appropriate cleanup strategy. The inherent complexity involved in performing bioremediation in situ means that special attention must be given to evaluating the success of a project. Whether a bioremediation project is intrinsic—