The Current Practice of Bioremediation
Increasingly, in situ bioremediation is being heralded as a promising ''new" alternative ground water cleanup technology. In fact, however, bioremediation is not new. It has been used commercially for more than 20 years. The first commercial in situ bioremediation system was installed in 1972 to cleanup a Sun Oil pipeline spill in Ambler, Pennsylvania.
Since 1972, bioremediation has become well developed as a means of cleaning up spills of gasoline, diesel, and other easily degraded petroleum products. In general, in situ bioremediation has not developed to the point where it can be used on a commercial scale to treat compounds other than easily degraded petroleum products. However, although in situ bioremediation of petroleum-based fuels is the only common use of the technology now, in the future bioremediation will likely be used to treat a broad range of contaminants. Recently, research has intensified on bioremediation of less easily degraded compounds, such as chlorinated solvents, pesticides, and polychlorinated biphenyls (PCBs). Bioremediation of many such recalcitrant compounds has been successfully field tested (see the case examples in Boxes 4-1 and 4-3, in Chapter 4).
This chapter describes the state of the practice of in situ bioremediation as used today. Although the current uses of bioremediation apply primarily to petroleum-based fuels, the principles of practice
outlined here extend to a much broader range of uses for the technology in the future.
BIOREMEDIATION VERSUS OTHER TECHNOLOGIES
For the past decade, the method of choice for ground water cleanup has been pump-and-treat systems. These systems consist of a series of wells used to pump water to the surface and a surface treatment facility used to clean the extracted water. This method can control contaminant migration, and, if recovery wells are located in the heart of the contaminant plume, it can remove contaminant mass. However, recent studies have shown that because many common contaminants become trapped in the subsurface, completely flushing them out may require the pumping of extremely large volumes of water over very long time periods. In situ bioremediation, because it treats contaminants in place instead of requiring their extraction, may speed the cleanup process. Consequently, bioremediation is likely to take a few to several years compared to a few to several decades for pump-and-treat technology. Thus, while capital and annual operating costs may be higher for bioremediation, its shorter operating time usually results in a reduction of total costs. Factors contributing to cost reductions in bioremediation compared to pump-and-treat systems include reduced time required for site monitoring, reporting, and management, as well as reduced need for maintenance, labor, and supplies. Furthermore, the surface treatment methods that are part of pump-and-treat systems typically use air stripping and/or carbon treatment to remove contaminants from the water—processes that transfer the contaminant to another medium (the air or the land) instead of destroying it. Bioremediation, on the other hand, can completely destroy contaminants, converting them to carbon dioxide, water, and new cell mass.
For cleaning up contaminated soils, in situ bioremediation is only one of several possible technologies. Alternatives include (1) excavation followed by safe disposal or incineration, (2) on-site bioremediation using land-farming or fully enclosed soil cell techniques, (3) low-temperature desorption, (4) in situ vapor recovery, and (5) containment using slurry walls and caps. In situ methods (desorption, vapor recovery, containment, and bioremediation) have the advantages of being minimally disruptive to the site and potentially less expensive. Because ex situ methods require excavation, they disrupt the landscape, expose contaminants, and require replacement of soils. For these reasons, ex situ methods sometimes are impractical. Potential advantages of bioremediation compared to other in situ methods
include destruction rather than transfer of the contaminant to another medium, minimal exposure of on-site workers to the contaminant, long-term protection of public health, and possible reduction in the duration of the remediation process.
BASICS OF BIOREMEDIATION PROCESS DESIGN
Biological and nonbiological measures to remedy environmental pollution are used the same way. All remediation techniques seek first to prevent contaminants from spreading. In the subsurface, contaminants spread primarily as a result of partitioning into ground water. As the ground water advances, soluble components from a concentrated contaminant pool dissolve, moving forward with the ground water to form a contaminant plume. Because the plume is mobile, it could be a financial, health, or legal liability if allowed to migrate off site. The concentrated source of contamination, on the other hand, often has settled into a fixed position and in this regard is stable. However, until the source can be removed (by whatever cleanup technology), the plume will always threaten to advance off site.
Depending on the nature of the site, the types of contaminants, and the needs of the parties responsible for the contaminated site, the treatment technologies administered may vary. The source area and the ground water plume may be treated by engineered bioremediation, intrinsic bioremediation, a combination of the two, or a mixture of bioremediation with nonbiological treatment strategies. Contaminant concentrations in ground water plumes are typically much lower than in the source area. Because of this concentration difference, management procedures for the source area and the plume may be quite different. When the source area is highly contaminated, aggressive containment and treatment are often required to bring the site under control.
Selection and application of a bioremediation process for the source or the plume require the consideration of several factors. The first factor is the goals for managing the site, which may vary from simple containment to meeting specific regulatory standards for contaminant concentrations in the ground water and soil. The second factor is the extent of contamination. Understanding the types of contaminants, their concentrations, and their locations is critical in designing in situ bioremediation procedures. The third factor is the types of biological processes that are effective for transforming the contaminant. By matching established metabolic capabilities with the contaminants found, a strategy for encouraging growth of the proper
organisms can be developed. The final consideration is the site's transport dynamics, which control contaminant spreading and influence the selection of appropriate methods for stimulating microbial growth.
Once site characteristics have been discerned, strategies for gaining hydrologic control and for supplying the requisite nutrients and electron acceptors for the microorganisms can be developed. If there is a sufficient natural supply of these substances, intrinsic bioremediation may be effective. On the other hand, if these biochemical or environmental requirements must be artificially supplied to maintain a desired level of activity, engineered bioremediation is the desired course. The ultimate consideration is if and when the targeted cleanup goal can be achieved.
Engineered bioremediation may be chosen over intrinsic bioremediation because of time and liability. Because engineered bioremediation accelerates biodegradation reaction rates, this technology is appropriate for situations where time constraints for contaminant elimination are short or where transport processes are causing the contaminant plume to advance rapidly. The need for rapid pollutant removal may be driven by an impending property transfer or by the impact of contamination on the local community. A shortened cleanup time means a correspondingly lower cost of maintaining the site, as more rapid remediation reduces the long-term sampling and analysis costs. Actions implicit in engineered bioremediation also address the political and psychological needs of a client or community that has been affected by the contamination. The construction and operation of engineered bioremediation systems can demonstrate to the local community that the party responsible for the contamination has a responsive "good neighbor" attitude.
Engineered bioremediation can take a number of forms. The different applications vary according to both the context of the contamination (site geology, hydrology, and chemistry) and the biochemical processes to be harnessed. Regardless of site conditions, however, certain principal parameters guide the design, depending on whether the system is to treat soil or water.
Bioremediation Systems for Unsaturated Soils
When subsurface contamination exists substantially or entirely above the water table (in what is known as the unsaturated, or va-
dose, zone), the treatment system relies on transport of materials through the gas phase. Thus, engineered bioremediation is effected primarily through the use of an aeration system, oxygen being the electron acceptor of choice for the systems used so far to treat petroleum contamination. If the contamination is shallow, simple tilling of the soil may accelerate oxygen delivery sufficiently to promote bioremediation. For deeper contamination, aeration is most commonly provided by applying a vacuum, but it may also be supplied by injecting air. In either case the three primary control parameters are, in order of importance, oxygen supply, moisture maintenance, and the supply of nutrients and other reactants. Figure 3-1 shows an
engineered bioremediation system for unsaturated soils. It indicates the types of systems used to supply oxygen and nutrients and to maintain moisture.
The design and implementation of an effective vacuum or injection system for oxygen delivery require knowledge of the vertical and horizontal location of the contaminants and the geological characteristics of the contaminated zones. Because air flow is proportional to the permeability characteristics of each geological stratum, aeration points must be separately installed at depths that correspond to every contaminated geological unit. For effective oxygen delivery, the spacing of the aeration points within a geological unit is a function of the soil permeability and the applied vacuum (or pressure). Determination of spacing should be based on field data and/or computer models. In some clay-rich soils the circulation of sufficient oxygen to promote bioremediation is extremely difficult because such soils are relatively impermeable. In these soils hydraulic fracturing or another engineered approach may be required to facilitate air flow.
The passage of air through the subsurface will remove moisture.
This can cause drying that, if severe enough, may impede biological processes. Therefore, maintaining a proper moisture balance is critical to the system's success. Moisture is added to the treatment area by spraying or flooding the surface (if the surface is relatively permeable) or by injecting water through infiltration galleries, trenches, or dry wells. Care must be taken that excess water is not added, because it can leach contaminants into the ground water or decrease the amount of air in the subsurface pores.
If inorganic nutrients or other stimulants are required to maintain the effectiveness of the bioremediation system, they may be added in soluble form through the system used for moisture maintenance, as shown in Figure 3-1. In some cases, nutrients and stimulants could be added as gases. At some sites, nitrogen has been added in the form of gaseous ammonia. Future applications of bioremediation could add methane gas to stimulate the cometabolism of chlorinated ethenes. Gaseous additives can be administered through wells or trenches constructed parallel to the aeration system.
Bioremediation Systems for Ground Water
Bioremediation systems for treating ground water below the water table fit two categories: water circulation systems and air injection systems. Most aquifer bioremediation systems have used the former approach, but in the last few years air injection systems have become increasingly common.
Water Circulation Systems. Water circulation systems work by circulating water amended with nutrients and other substances required to stimulate microbial growth between injection and recovery wells. This approach is sometimes referred to as the Raymond Method, named after the scientist who designed the system for the 1972 Sun Oil spill. The method has typically incorporated an optional above-ground water treatment facility into the ground water circulation system. Figure 3-2 shows a diagram of a water circulation system, with oxygen supplied by hydrogen peroxide (H2O2) and the recovered water treated with an air stripper to remove any remaining volatile contaminants.
Under normal operations, all of the ground water is recovered, and all or a portion of the treated ground water is reinjected after being amended with nutrients and a final electron acceptor. Recovery systems most frequently use wells, although trenches can be used in some situations. Injection is commonly achieved with wells, but several systems have used injection galleries. In some systems all of the recovered water is discharged to an alternate reservoir, and ei-
ther drinking water or uncontaminated ground water is used for injection. The injected ground water moves through the saturated sediments toward the ground water capture system. As the amended water moves through the contaminated portions of the site, it increases microbial activity by providing the elements that limit intrinsic biodegradation.
Nutrients typically added are nitrogen and phosphorus, although other minerals are occasionally used. Ammonium and nitrate salts are the most common nitrogen sources. Orthophosphate and tripoly-phosphate salts are the most common phosphorous sources. The electron acceptor is most commonly oxygen in the form of air, pure oxygen, or hydrogen peroxide. Nitrate has also been used as an electron acceptor in some commercial scale engineered bioremediation systems.
Air Injection Systems. Historically, one of the major limitations of water circulation systems has been the effective supply of an electron acceptor. Delivery of oxygen, the most common electron acceptor, is difficult because oxygen gas has limited water solubility and other oxygen vehicles (such as hydrogen peroxide and liquid oxygen) are costly and have had limited effectiveness. The difficulties associated with oxygen delivery have hampered the performance of bioremediation technology.
In the past few years, U.S. contractors have adopted the European practice of air sparging—the injection of air directly into ground water (see Figure 3-3). Air sparging serves two purposes. First, it is an efficient method of delivering oxygen to promote microbial growth. The injected air displaces water in the subsurface, creating pores tem-
porarily filed with air that is easily available to the microbes. Second, air sparging can help remove volatile contaminants. As the injected air sweeps upward through the contaminated zone, it can carry volatile contaminants to the soil above the water table for capture by a vapor recovery system.
Essential to the proper use of air sparging is delineation of the extent of contamination and the subsurface geological profile. These requirements must be met to ensure that air can move readily and uniformly through the area to be treated. If there are geological barriers that can trap or channel the air flow, the use of air sparging may be precluded. The spacing of wells and the injection pressure for the sparging system need to be determined by field testing and/or modeling.
Addition of nutrients or other amendments, if necessary, can be accomplished through the use of injection wells or infiltration galleries (as shown in Figure 3-3). The movement of air through the subsurface provides a mixing function that helps disperse nutrients through the water column. Gaseous reactants, such as methane, that may be required for cometabolic bioremediation strategies could be added through the sparge wells.
In most situations, sparging is conducted with a ground water capture system to prevent migration of dissolved contaminants. When volatile compounds are present, air recovery systems are used to prevent contamination of the air above the system and contaminant transfer to adjacent areas.
Because intrinsic bioremediation relies on the innate capabilities of naturally occurring microbial communities, the capacity of the native microbes to carry out intrinsic bioremediation must be documented in laboratory tests performed on site-specific samples. These tests must be carried out before intrinsic bioremediation can be proposed as a legitimate cleanup strategy. In addition, the effectiveness of intrinsic bioremediation must be proven with a site-monitoring regime that includes chemical analysis of contaminants, final electron acceptors, and/or other reactants or products indicative of biodegradation processes (as explained in Chapter 4).
Intrinsic bioremediation may be used alone or in conjunction with other remediation techniques. For instance, soils may be excavated for disposal or treatment, with intrinsic bioremediation used to eliminate residual contamination. Similarly, this process may be implemented after a pump-and-treat or engineered bioremediation system has reduced the potential for migration of contaminants off site.
For intrinsic bioremediation to be effective, the rate of contaminant biodegradation must be faster than the rate of contaminant migration. These relative rates depend on the type of contaminant, the microbial community, and the subsurface hydrogeochemical conditions. Frequently, the rate-controlling step is the influx of oxygen. Lack of a sufficiently large microbial population can also limit the cleanup rate; this can be caused by a lack of nutrients or an inhibitory condition, such as low pH or the presence of a toxic material.
Prior to implementation of intrinsic bioremediation, the site must be thoroughly investigated. Parameters of concern include the type, mass, and distribution of contaminant; the contaminant's susceptibility to biodegradation by microorganisms at the site; the flow of ground water under nonpumping conditions (including seasonal fluctuations); historical data on plume migration; and the closeness and sensitivity of potential receptors that may be adversely affected if reached by the contaminant. If information on all of these parameters is available, a mathematical model can be used to predict the rates of migration and biodegradation. Thus, prospects for expansion or recession of the contaminated area can be assessed.
For regulators to approve intrinsic bioremediation, the regulatory agency must be provided with supportive data to ensure that the public health will be adequately protected. The implementation plan must include a site-monitoring program to confirm that intrinsic bioremediation is performing as expected. If performance falls short of expectations and the contamination spreads, further corrective action will likely be required.
INTEGRATION OF BIOREMEDIATION WITH OTHER TECHNOLOGIES
As a contaminated site is cleaned up, the biological reactions promoted in bioremediation affect site chemistry in ways that may make the site more amenable to cleanup with nonbiological technologies. For example, biological activity may speed contaminant desorption from solids, making it easier to extract the contaminants with a pump-and-treat system. Similarly, nonbiological cleanup technologies can affect microbial activity at a site, sometimes promoting bioremediation. For example, techniques designed to vent volatile contaminants may increase the oxygen supply, encouraging microbial growth. Such synergistic effects can maximize rates of contaminant loss. Thus, bioremediation is frequently integrated with other technologies, both sequentially and simultaneously. Some examples follow:
When contaminant concentrations are high and affected zones are accessible, bioremediation frequently follows excavation of soils near the contaminant source. Excavated soils may be disposed of off site or treated by a surface (ex situ) bioremediation system or by a thermal method. Removal of heavily contaminated soils reduces the demands on an in situ technology and immediately reduces the potential for impact on the ground water.
Where residual pools of contaminants are present in the subsurface, these pools may be removed prior to implementation of other remediation technologies by a process known as free product recovery. For pools of contaminants that are less dense than water, such as gasoline and diesel fuel, free product recovery is accomplished by pumping the liquids from wells or trenches. This removes the contaminant mass in the most concentrated form and reduces the demand for nutrients and electron acceptors during bioremediation procedures that may follow. No good recovery methods exist for pools of contaminants that are more dense than water, such as chlorinated solvents, because these tend to sink deep into the subsurface, where they are difficult to locate.
Bioremediation may follow the use of a pump-and-treat system. The pump-and-treat system may be used to shrink the contaminant plume and, along with a free product recovery system, help remove contaminant mass. Once a sufficient quantity of the contaminant has been removed, the pump-and-treat system may be augmented with or converted to an in situ bioremediation system.
Frequently, in situ bioremediation for cleaning up ground water is conducted in conjunction with in situ vapor recovery for cleaning up regions above the water table. The in situ vapor recovery system uses a series of recovery wells or trenches to extract air and volatile contaminants from above the water table. In addition to withdrawing volatile contaminants, the wells and trenches provide oxygen for biodegradation. Hence, the process has become commonly known as bioventing. By a combination of volatilization and biodegradation, contaminant levels above the water table are reduced, thus decreasing the potential for the contaminants to leach into the ground water. Further, as the water table drops during dry seasons, more subsurface sediments are exposed to air movement; thus, contamination is reduced within the zone of water table fluctuation. Alternatively, air sparging may be used along with vapor extraction procedures to reduce contamination below the water table.
It is possible to follow engineered bioremediation with intrinsic bioremediation. After removal of free product contaminants, engineered bioremediation may be used to eliminate the majority of residual contaminants. Then, after an asymptotic decline of contaminants in the plume, final polishing and containment may be accomplished using intrinsic bioremediation. In this case, microbial activity will have been stimulated and the biodegradation process at the site will be well understood. (See Box 4-2, in Chapter 4, for an example.)
The general approach required to earn credibility in the bioremediation industry is the same as for any technical business: work only within areas of expertise, be aware of the general limitations of the technology, pay attention to details, and work closely with clients. In general, buyers of bioremediation services can determine whether a bioremediation contractor is competent to do the job by reviewing the contractor's credentials. Competent contracting firms have employees and consulting experts with credentials in the scientific and engineering fields important to bioremediation. And, like any other successful business, a bioremediation firm should have a
track record of reliable performance that can be determined by reviewing the firm's references. Box 3-1 lists standards of practice that all contractors should follow.
BOX 3-1 STANDARDS OF PRACTICE FOR BIOREMEDIATION CONTRACTORS