PROCEEDINGS OF THE EXPERT PANEL WORKSHOP
TO EVALUATE THE PUBLIC HEALTH IMPLICATIONS
FOR THE TREATMENT AND DISPOSAL OF POLYCHLORINATED BIPHENYLS-
CONTAMINATED WASTE
Chapter 4 - Expert Panel Report
Non-Incineration Remedial Technologies
Frederick G. Pohland, Ph.D., P.E., DEE - CHAIR
James J. Cudahy, M.S., M.B.A., P.E. - RAPPORTEUR
Joseph C. Carpenter, P.E. - CO-CHAIR
Lynn C. Wilder, M.S.Hyg - CO-CHAIR
DISCLAIMER
The use of company or product name(s) is for identification only and does not imply endorsement by the Agency for Toxic Substances and Disease Registry.
Executive Summary
Panel Report Outline
I. Introduction
As part of the Agency for Toxic Substances and Disease Registry (ATSDR) Bloomington Polychlorinated Biphenyls (PCBs) Project, a panel of experts was convened to address issues surrounding available non-incineration PCB-treatment technologies. Members of the panel were asked not to evaluate issues associated with specific sites in the Bloomington area, but rather to use their complementary backgrounds and areas of expertise to provide the most current information available on viable non-incineration technologies.
The following seven non-incineration technology categories were examined for their applicability to and limitations for treating PCB-contaminated material: bioremediation, dechlorination/destruction, soil washing, solvent extraction, thermal treatment, solidification/stabilization, and landfilling. If they were known, wastestreams, byproducts, and end products associated with each technology were identified. After each technology was reviewed, limitations and data gaps were identified. That information is necessary for determining possible human exposure routes to site- and process-related contaminants. Throughout the technology review and discussion process, it became very apparent that whatever technology was selected, site- and matrix-specific characteristics were the principal factors in determining the applicability of the technology; the nature of the wastestreams, byproducts, and end products; and the likelihood of the technology's impact on human health.
Although all seven technology categories may be considered for treatment or disposal of PCBs, no single non-incineration remedial technology is ubiquitously applicable to all PCB-contaminated media. Combining two or more technologies in series may offer advantages over the use of a single technology for achieving required treatment or clean-up levels. The stage of development of most of the technologies for treating PCB-contaminated material is such that significant data gaps exist for possible air emissions and end-product contaminants. Performance histories of the technologies are limited for treatment of PCB-contaminated matrices and very limited for the contamination matrix of municipal solid waste. The evaluation of the effectiveness of a specific technology must be based on site-specific circumstances and the availability of treatability and pilot-scale test data. For this reason, it is not possible to make health risk comparisons of the technologies or technology categories.
Possible routes of human exposure for all of the technology categories will vary according to the
process used, the effectiveness of the technology, and site-specific contamination and
contaminant location issues (e.g., proximity to residential areas, sensitive populations, etc.).
Without site- and technology-specific information, along with treatability and pilot-scale test
data, it is not possible to discuss specific exposure scenarios in this report. After determining the
contaminants of health concern, the potential for human exposure to the contaminants, and the
routes of exposure, engineering controls or monitors, or both, and safeguards should be applied
as feasible to the various processes to ensure against adverse health impacts on workers and the
community.
NON-INCINERATION REMEDIAL TECHNOLOGIES
PANEL REPORT
OUTLINE
I. Introduction
II. Technology Description
As part of the Agency for Toxic Substances and Disease Registry (ATSDR) Bloomington Polychlorinated Biphenyls (PCBs) Project, a panel of experts was convened to address issues surrounding available non-incineration PCB-treatment technologies. Members of the panel were asked not to evaluate issues associated with specific sites in the Bloomington area, but rather to use their complementary backgrounds and areas of expertise to provide the most current information available on viable non-incineration technologies.
Potential candidates for this Panel were identified through nominations from the public or literature searches for authors of technology-related PCB publications. The members were selected so that Panel composition would be representative with respect to affiliations, research, and field experience. The 10 members of this panel included engineers, chemists, geochemists, biologists, and environmental scientists. The panelists were selected for their scientific diversity and complementary expertise in the treatment of PCB-contaminated material. Biographical sketches of the panelists are provided in Appendix A. To enhance the clarity of the proceedings, a list of acronyms and abbreviations are presented in Appendix B.
Before convening the Panel, ATSDR prepared a background discussion paper for the panelists to review. This background discussion paper identified questions and issues (see Section I.B.) for the panelists to contemplate during their deliberations. These questions and issues, as presented in Appendix C, were used to develop an agenda for the Panel meeting, which was held September 13 and 14, 1993, in Bloomington, Indiana.
During the 2-day meeting, the panelists attempted to address as many of these questions and issues as possible during their deliberations. The panel also discussed other PCB treatment-related issues as they arose. The discussions held by the Panel did not attempt to reach a consensus of opinion, but rather a broad spectrum of viewpoints. This report summarizes the highlights of those discussions and salient information from the background discussion paper.
The Non-Incineration Remedial Technologies (NIRT) panel divided available technologies into broad groups or categories to discuss the general features that these groups of technologies have in common. The following technology categories were evaluated:
For each of the seven categories, an expert panel member presented an overview of the category, followed by a "state-of-the-art" description of various technologies (if available) within the category and available information regarding treatment of PCB-contaminated media.
A set of issues and questions (See Appendix C) were developed by the Panel Chair, Co-chairs, and the Rapporteur before the expert panel meeting in Bloomington. These questions and issues were developed to ensure that each category and technology would be reviewed as similarly as possible, and to ensure that the health agencies would have as complete information as possible to determine possible data gaps, human exposure routes, etc.
This section focuses on information provided by the expert panel presentations and discussions, the details of the specific technologies, and the discussions surrounding the issues and questions. Technology discussions were constrained by available panel time and the volume of technical information at hand. Each panel session attempted to discuss the technology category using the following criteria:
1. Technology Classification
To provide a consistent technology classification, the status of development of each technology category with respect to treatment of PCB-contaminated material is described. Moreover, the overall technology categories are broadly defined according to the Environmental Protection Agency (EPA) Superfund Innovative Technology Evaluation (SITE) classification:
The EPA SITE categories are intended for individual technologies, not technology categories. However, this report has used EPA's definitions for each technology group to indicate each category's relative stage of development for treatment of PCB-contaminated material.
2. Process Description
Each technology category is introduced with a general description of the physical and chemical processes, residuals, and wastestreams common to all technologies within the category. Where applicable, a process diagram is included. Limitations and advantages of the technology category are outlined, along with applicable PCB-contaminated media that can be treated (i.e., soil, sludge, sediments, municipal solid waste, etc.).
3. Material Handling/Treatment Requirements
Discussions include material handling and treatment requirements, such as excavation, sizing, or other pre- or post-processing of the contaminated material.
4. Technology Performance
Specific technologies discussed under each category during the panel sessions are described, as well as available treatment data, treatment rates, and performance data.
5. Data Gaps
As discussed in the panel sessions, information not available (i.e., air sampling data, performance history, etc.) for a technology process or category is identified as data gaps.
Biological treatment technology is an "emerging technology" in treating PCB-contaminated matrices. The process involves the use of microorganisms to degrade chlorinated organic chemicals. This technique can be enhanced by increasing the bioavailability of the contaminants at the site, reducing the toxicity to bacteria, and delivering adequate amendments, such as moisture and nutrients, to the bacteria.
One of the two main categories of biological treatment is in situ treatment, where no excavation is required, and nutrients are often added to the soil and water at the site of contamination. A second and more usual type of bioremediation, called ex situ, involves excavation, pumping, and introduction into an external reactor system. The construction and operation of a separate reactor, in which microbes and PCB-contaminated material are mixed together, offers the potential for faster degradation rates. Such reactor facilities require operational control, including temperature, pH, time for contact, nutrient concentrations, and concentration of waste feed. Monitoring for air emissions and emission controls may be required.
The in situ processes are expected to be less expensive because they do not require excavation of contaminated soils and sediments. In contrast to the ex situ process, it is generally more difficult to manage an in situ process to achieve maximal degradation rates. This does not necessarily mean that the treatment process will take longer. The greater degradation rate expected of a reactor system must be balanced against the limited capacity of the reactor.
Air emission data for in situ PCB treatment were collected during the General Electric Hudson River Research Study. A passive vapor trap/resin adsorbant collection air sampling system was used to capture PCBs from the gas outlet of each reactor-like system. Of the 16 test reactor trapping sampling events, 14 were nondetect (PCBs trapped were below the detection limit of 0.05 µg/day). The other two traps collected 0.5 and 0.6 µg/day of PCBs. Eight control reactor trapping samples were collected. Analyses of four of the control samples were below detection; four detected PCBs from 0.1 to 2.1 µg/day.
Two classes of bacteria have been shown to degrade PCBs. Aerobic bacteria live in the presence of oxygen and break open the carbon ring in destroying the PCB compounds. Anaerobic bacteria, which live in oxygen-free environments, leave the biphenyl rings intact, but remove the chlorine atoms. Aerobic biodegradation is usually limited to biphenyl and lower chlorinated PCB congeners, primarily mono-dechlorinated biphenyls. Aerobic degradation generally becomes less effective as the degree of chlorination increases. In contrast, anaerobic bacteria are more efficient in transforming the more highly chlorinated PCB congeners into lower chlorinated products, which, in turn, can be degraded more easily by aerobic bacteria. Therefore, sequential anaerobic-aerobic biological treatment processes may permit a more complete biological treatment of PCB-contaminated material than is possible with use of only one of these approaches.
Using only anaerobic dechlorination has two important consequences. Experience with PCB-contaminated media from all sites examined to date has shown that bacteria primarily remove the meta- and para-chlorines. This sharply reduces the tendency of PCBs to persist and accumulate in higher animals, and also reduces the toxicity of the PCB mixture. That toxicity is attributable primarily to the co-planar congeners (e.g., those lacking ortho-chlorine substitutions). The second important consequence is that microbial dechlorination increases the number of PCB congeners that can be aerobically biodegraded. Heavily chlorinated congeners, lacking adjacent unsubstituted carbons, cannot be aerobically degraded, but they can be dechlorinated. For example, Aroclor 1260 can be dechlorinated, but has never been shown to degrade aerobically.
The use of an anaerobic/aerobic sequence has two additional consequences, both based on the fact that the aerobic degradation of PCBs is a co-metabolic process. PCBs do not provide carbon or energy to the aerobic microorganisms degrading them and, with the exception of monochlorinated biphenyls, they do not serve as substrates to induce their own aerobic degradation. However, the dechlorination of a PCB mixture (Aroclor) can produce enough monochlorinated biphenyl (or perhaps even biphenyl in some systems) to induce the aerobic pathway. Even then, most PCB congeners with four or more chlorines, and some with three, are only partially metabolized aerobically. The initial dioxygenase step occurs, but chlorines block some subsequent step in the degradation pathway. If such congeners are dechlorinated first (anaerobically), then the dechlorinated products can be mineralized in a subsequent aerobic step.
Treatability studies on actual waste samples are necessary for two reasons. First, it is necessary to determine if microorganisms capable of dechlorinating and/or degrading the PCBs already exist in the waste (contaminated soil or sediment). If such microorganisms are not present or are limited in number, successful biotreatment will require inoculation with more competent strains. Second, it is necessary to determine if anaerobic dechlorination and/or aerobic degradation can occur (or is already occurring) in the waste matrix. This is simpler than trying to predict the impact of all matrix characteristics on the bioremediation process.
Matrix characteristics that may adversely affect bioremediation in general include these:
Matrix limitations that may adversely impact the microbial dechlorination process include these:
Factors that limit the aerobic degradation of PCBs include these:
Most of these limitations may be overcome in some way, but doing so adds to the total cost and problems associated with managing the biotreatment system.
Unlike other technology categories, biological treatment is not an "off-the-shelf" technology for PCB contamination remediation. Studies are required for every PCB-contaminated site to determine if biological treatment is feasible. Therefore, it is not possible to state that this technology is applicable, or not applicable, for treatment of PCB-contaminated media (i.e., soil, sediment, etc.).
The microbial dechlorination process alone results in a reduction of toxicity or a "risk reduction" of the PCB mixture, rather than in a significant reduction of the contaminant concentration. If a significant decrease in the total PCB concentration is required to reach a clean-up level, a subsequent aerobic biodegradation step will be required. The feasibility of this approach must be assessed on a site-specific basis and may necessitate its use in conjunction with other technologies.
Data gaps for bioremediation typically include the need for identification of microorganisms capable of dechlorinating PCBs; limited volatile emissions data; a lack of data on the ranges of PCB concentrations that can be dechlorinated (although concentrations up to 5,000 milligrams per kilogram (mg/kg) of PCBs in sediment samples have been treated at bench scale); estimation methods for final endproduct concentrations based on performance history; established procedures to control the process; and estimates of treatment rates and costs. The bioremediation of PCBs has been extensively researched over the past 6 to 8 years; however, data from site-specific field applications are quite limited.
Chemical dechlorination technologies involve the destruction or transformation of PCBs by removing chlorine atoms from the molecule (addition-elimination reaction). The dechlorination processes used to treat PCB-contaminated oils represent a proven and commercial technology. In addition, a few dechlorination processes for treating contaminated soils and sludges have been developed to the stage of "applied technology." Several field-scale demonstrations have recently been completed using either alkali metal polyethylene glycolate (APEG) or base-catalyzed dechlorination (BCD). These demonstrations are described later in this section.
Figures 1 and 2 (all figures are included in Appendix D) display the general flow diagrams applicable to PCB-dechlorination processes. This technology requires excavation and preparation of the PCB-contaminated media. Preparation in most cases involves sizing (reduction) requirements and suspending the contaminated media in a liquid phase. The PCB-contaminated material is then fed into a reactor vessel along with the dechlorinating reagent. In many cases, the reaction must be heated. In some cases, the reaction occurs under a nitrogen atmosphere. The treated material and the dechlorination byproducts are outputs of the process.
Depending on the dechlorination agent(s) used, further treatment of the PCB-free material may be required (e.g., excess reagent removal, pH adjustment). The dechlorination byproducts are separated out as much as possible (condensation, separation, etc.) to reduce the volume of material requiring disposal. Air emissions from the process vary according to the dechlorination reagent(s) used, the matrix being treated, and other contaminants that may be present in the PCB-contaminated material. Emission controls usually include carbon adsorption.
Depending on the specific dechlorination process, this technology category can be used to treat PCB-contaminated liquid, oil, soil, and sludge. This process may be less applicable for municipal solid wastes; significant pretreatment would be required to accommodate the maximum size requirements of the PCB-contaminated material (usually about ¼- to ½-inch).
Alkali metal polyethylene glycolate (APEG) reagents have been shown to dechlorinate PCBs in soils and liquids. Potassium polyethylene glycolate (KPEG) and sodium polyethylene glycolate (NaPEG) are the most common reagents used for the APEG process. Limitations of conventional APEG technology include these:
APEG technology has been shown to be more effective for treatment of liquids than for soils. If used to treat PCB-contaminated soils, glycols must be removed from treated soils. A chloride material balance could not be reached in pilot-scale tests. In addition, high organic concentrations react with the caustic to form a saponified, gelatinous, semi-liquid material. The APEG process reacts with aluminum-containing waste to produce hydrogen gas. It also reacts with ammonium ions to produce ammonia gas.
Treatability and some pilot-scale tests have shown the EPA's base-catalyzed dechlorination (BCD) process to be effective on PCB-contaminated sediments, soils, oils, and sludges. On the same scale, it has been shown to be effective on pentachlorophenol, halogenated herbicides and pesticides, and dioxins/furans.
The BCD process involves replacement of the PCB chlorine atoms with hydrogen. It requires pretreatment of the PCB-contaminated media to approximately ½-inch in size, and placement of the contaminated soil and a base (sodium bicarbonate) into a reactor that is heated to approximately 300-330o C (572-626o F) under a nitrogen atmosphere. In the reactor, the PCBs or other halogenated compounds are vaporized from the soil (some dechlorination occurs). The vaporized organic compounds are condensed. A homogeneous, non-poisoning catalyst (patented), a base (sodium hydroxide), and a hydrogen donor (typically fuel oil) are added to the condensed material for dechlorination. The treated matrix contains sodium chloride and sodium bicarbonate; the treated condensate contains olefins and paraffinic hydrocarbons. The treated matrix can be returned without further treatment to its place of origin. The treated condensate is usually separated; the water is treated with activated carbon and the oil is combusted. The air pollution control system consists of carbon adsorption.
The advantages of this treatment process are the low cost of the chemicals used for dechlorination, reduced treatment time (1-3 hours), and the achievement of complete dechlorination (less than 1 part per million [ppm]). This process has been shown to treat up to 300,000 ppm of chlorinated organic compounds. A material balance for chloride has been achieved for this process. However, high concentrations of metals in the contaminated matrix will interact with the base and interfere with the reaction unless additional base is added.
Two commercial BCD systems for treatment of PCB-contaminated oil are in operation in Australia. Technosafe Waste Disposal Pty. Ltd., Dandenong South, VIC, Australia, is treating 20,000 ppm of PCBs within 3 hours using a 3,000-liter reactor. BCD Technologies Rayswaters-Melbourne, Australia, is treating similar PCB concentrations to less than 1 ppm. A water-cooled condenser is employed to return condensate to the reaction medium (Reflux system).
A panel member reported that S.D. Myers (Tallmadge, Ohio) has demonstrated PCB destruction with initial concentrations in excess of 100,000 ppm on a pilot-scale treatment system. In other pilot-scale tests of the BCD process for contaminated soils, treated water contained approximately 0.5 parts per billion (ppb) PCBs, treated soil contained less than 1 ppm PCBs, and the treated condensate contained no detectable levels of PCBs.
An EPA SITE demonstration of the BCD technology was conducted from mid-August through September 1993 at the Koppers Company Superfund Site in Morrisville, North Carolina. Pentachlorophenols (PCPs) and dioxins/furans were the contaminants present. Bench-scale testing was conducted using contaminated site soils. Input PCP levels of 35,000 ppm were reduced to 0.021 ppm (or 0.0006 ppm at higher temperatures), equating to a removal efficiency of greater than 99.99%. In general, the removal efficiency for dioxins and furans was greater than 99.99%. The SITE demonstration results are reported to be similar to those found during bench-scale tests. The results will be published in 1994 in EPA's SITE Demonstration Summary Report.
EPA's Risk Reduction Engineering Laboratory anticipates that the process will be engineered to full field-scale operations of 5-10 tons per hour in 1994. To date, only the contaminants treated (i.e., PCBs, PCPs, etc.) have been targeted as chemicals for analyses in the waste effluent/residuals.
Data gaps for the dechlorination technology include air emissions, performance history, toxicity of any remaining chlorinated compounds, and cost information. Issues not discussed included monitoring and control measures necessary to limit material releases and equipment failure, and worker health and safety parameters.
Soil washing technologies involve a water-based ex situ process for mechanically scrubbing soils to remove contaminants. One process removes contaminants from soils and sediments by either dissolving or suspending them in the wash solution. Another process concentrates them by particle size using physical separation techniques. This latter technology is designed on the premise that most of the organic contamination is concentrated or partitioned in the organic fraction (leaves, roots, etc.) and the fine particles of the sediment or soil, and that minimal contamination is present in the larger particles (sand size and larger). A combination of the two processes is often used. Because the process does not destroy the contaminants, but concentrates them into a small volume, further treatment or disposal of the concentrated end product is required.
Soil washing is a commercial process used for many years by the mining industry to recover valuable minerals. However, for treatment of PCB-contaminated soil and sediment, the soil washing technology category is classified as both an "applied" and an "emerging" technology. Full-scale soil washing technology is in use in Europe; however, it was reported that the removal efficiency for PCBs in European systems has yielded "mixed" results. The first U.S. application of soil washing technology is for non-PCB contaminated soils and is being conducted at the King of Prussia Superfund Site in New Jersey. This site contains volatile organic compounds (VOCs) and pesticides in sludges and coarse-grained sediments.
Figure 3 displays the general process flow diagrams for froth flotation and counter- current PCB soil washing technologies. This technology requires excavation and preparation of the PCB-contaminated media before treatment. Pretreatment involves sizing requirements (less than 1/8-inch) and directing the waste material through a soil washer where it is sprayed or agitated with a washing fluid containing surfactants and, possibly, pH modifiers. Soil particles greater than 0.08-inch (2 mm) in diameter are sorted and rinsed, removed from the washer, and dewatered. Additional washing fluid is added to the smaller particles, which are then directed through either a froth flotation or a countercurrent extraction process to remove the contaminants. The soils are separated from the liquid froth or extract. The PCBs are concentrated in froth or extract material and must undergo final disposal. The surfactant material and the wash water are typically recycled, if possible. Possible air emissions from the froth or extraction stage are generally captured with activated carbon.
The soil washing technology would be most applicable to PCB-contaminated soils and sediments. PCB-contaminated sludges are generally not treatable using this technology because of the typically high organic content of the matrix. Similarly, because municipal solid waste from landfills has high amounts of organic material, as well as significant pretreatment requirements, it is not a good candidate for washing.
Limitations of soil washing technology for PCB-contaminated media include these:
Advantages of soil washing include these:
Several soil washing technologies have undergone pilot-scale or field-scale testing or both. During the panel discussion, two specific processes were discussed: the Waste-Tech Services Inc. soil washing system and the GHEA Associates process, which was developed at the New Jersey Institute of Technology. The former involves froth flotation; the latter involves countercurrent extraction and solvent/surfactant recovery and reuse.
The Waste-Tech process has been used for remediation of soils and sediments containing solvents, metals, and petroleum hydrocarbons. In treatability tests, soils with greater than 10,000 ppm PCBs have been treated to residual concentrations of 25 to 50 ppm. If PCB scavengers (kerosene, co-adsorbents) are added, soils containing 5,000 to 10,000 ppm PCBs can be reduced to 5 to 50 ppm. Analyses of air samples collected during these tests did not detect PCBs (detection limit not specified).
A pilot-scale test of the GHEA Associates process was conducted in 1992 as part of the EPA's SITE Emerging Technology Program. The final report of this test should be available in 1994. Treatability test results indicated that soil contaminated with 380 ppm PCBs was treated to 0.57 ppm, and that water containing 6,000 ppb was treated to less than 0.1 ppb. It was reported that no PCB air emissions were detected.
The Bergmann system uses both physical and chemical methods and is applicable for use on soil and sediments containing no more than 40% silt and clay material; the solids content should not exceed 20% by volume. Inorganic contamination (i.e., metals) does not interfere with the process. After physical separation, surfactants, pH modifiers, and polymer flocculents are used for further separation.
A SITE demonstration of this technology was performed in May and June 1991 at the Army Corps of Engineers' Confined Disposal Facility (Saginaw Bay, Michigan). A 5-day pilot test was performed on river sediments contaminated with PCBs and heavy metals. During the test, the system operated continuously 8 hours per day, 5 tons per hour. Reported results showed a greater than 90% removal of PCBs from the matrix. The distributions of PCBs in the input sediment and output sand, organic compounds, and fines were as follows:
Input sediment: 1.6 mg/kg PCBs
Output sand (treated material): 0.2 mg/kg
Output organic compounds (requires subsequent treatment/disposal):11.0 mg/kg
Output fines (requires subsequent treatment/disposal): 4.4 mg/kg
Data gaps for the soil washing process include air emissions data, performance history information, the selection criteria for the soil washing scavengers, site-specific treatability studies, and cost data.
Solvent extraction technologies involve the use of solvents to extract contaminants from a matrix by separating the contaminants into their respective phase fractions (i.e., organic, water, particulate solids). This process does not destroy the contaminants, making it necessary to further treat or dispose of the concentrated end products.
The technology for chemical extraction, which has been established for approximately 40 years, has been used to treat oily sludges containing hydrocarbons and other high molecular weight organic compounds. This ex situ process has also been applied to contaminated soils. For the treatment of PCB-contaminated soils and sediments, the solvent extraction technology category is classified as an "applied" and a "proven" technology. Although anticipated within the next few years, long-term performance data on solvent extraction for PCB-contaminated soils from commercial systems are not yet available. Without such performance history information, extensive site-specific testing is required to determine the feasibility of applying this technology to a given situation. The selection of the most appropriate solvent to be used in an extraction process is more of a treatability study outcome rather than an "off-the-shelf" technology.
Figure 4 displays the basic process for PCB solvent extraction technologies. This technology requires excavation and pretreatment of the PCB-contaminated material. Pretreatment involves sizing requirements for solids and sludge material (less than ¼-inch). Depending on the waste matrix, slurrying of the waste may be necessary as part of the treatment process. The contaminated material then undergoes one or several extraction processes, with temperatures usually ranging from less than 15o C (60o F) to approximately 20o C (70o F), depending on the technology. These extractions allow the solvent (either liquid or liquid gases/supercritical fluids) to be simultaneously miscible with the oil and water, and to extract the organic contaminants adsorbed onto the particles. After completion, the solvated oil, water, and solvent are decanted; the decanted solution undergoes evaporation and condensation to separate the solvated oil from the water and solvent. The solid material is dried to recover the solvent for recycling through the system. The process results in three end products: product solids, water, and concentrated oil containing the PCB contaminants. After further treatment to remove process additives, the solids can be placed back onto the site or land disposed; water is further treated (activated carbon or stripping) to remove any remaining solvent; and recovered oil can either be dechlorinated or incinerated to destroy the PCBs.
Solvent extraction technology is applicable for PCB-contaminated soils, sediments, and some sludges. It is reported to be ineffective with municipal solid waste (landfills), but is applicable for use in cleaning transformer shells. It was suggested that pretreatment of the solid waste by composting might be one method of modifying the organic material to levels that would render solvent extraction of PCBs from municipal solid waste more feasible. The EPA currently (1993) has three records of decision (RODs) that name solvent extraction as the remedy selection for PCB-contaminated sites.
Limitations of solvent extraction technology for PCB-contaminated material include these:
The Basic Extractive Sludge Treatment (B.E.S.T.TM) uses triethylamine as the extraction agent to remove or separate contaminants from sludges, soils, and sediments. This technology, developed by the Resources Conservation Company, is currently in its third generation of commercial design. The process begins with one or more "cold" extractions, which involve adding the prescreened material to a refrigerated tank containing 50% sodium hydroxide. After the tank is sealed and purged with nitrogen, chilled (less than 35o C [60o F]) triethylamine is added and the mixture is agitated and then allowed to settle. The solvated oil, water, and solvent are then decanted. The decanted solution undergoes evaporation and condensation to separate the solvated oil from the water and solvent.
The solid material then undergoes one or more "warm" and "hot" (54-80o C [130-176o F]) extractions using a steam-jacketed extractor/dryer. Warm solvent is added to the solids; the mixture is then heated, agitated, settled, and decanted. This process removes organic compounds remaining from the "cold" extraction. During this process, caustic is added for pH control.
This process was demonstrated on a pilot scale in July 1992, at a location adjacent to the Grand Calumet River in Gary, Indiana, for river bottom sediment contaminated with PCBs (12 and 430 ppm) and polyaromatic hydrocarbons (PAHs) (550 and 73,000 ppm) from two separate locations. Results of two separate tests indicated a PCB removal efficiency of equal to or greater than 99%. The second test indicated that the residual solvent remained in the product solids (103 ppm), water (less than 1 ppm), and the oil (730 ppm).
Currently, a pilot-scale, skid-mounted 100 pound per day (lb/day) system and a full-scale 100 ton per day (ton/day) transportable system are available. The pilot system can treat pumpable (e.g., oily sludges) and non-pumpable waste material; the full-scale system treats pumpable waste only. During clean-up activities at one site, the full-scale system throughput was 72 ton/day. Feed rates depend on the characteristics of the waste material. The need for multiple extractions depends on the clean-up criteria and would also impact throughput rates.
The CF Systems Organics Extraction System uses liquified gas (carbon dioxide, propane, or propane/butane) as the extraction solvent to remove organic compounds from wastewater, sludges, sediments, and soils. Carbon dioxide is generally used for contaminant extraction from liquids; propane is used for contaminant extraction from sludges, sediments, and soils. Currently, commercial systems are available for sludges/solids and for wastewater. A mobile pilot-scale system is available for soils, sludges, and semisolids (100 to 200 lb/day). A less than 1/8-inch sizing pretreatment process is necessary for the pilot system. The pilot-scale system has the ability to reprocess the contaminated matrix back through the system; the commercial system is designed with multiple extraction units. It was recommended that bench-scale tests be conducted to better approximate the number of extraction cycles necessary to reach the required contaminant clean-up levels for a particular site before field-scale work.
The contaminated material is fed into the extractor from the top of the system. Condensed solvent gas (compressed at 21o C [70o F]) flows up through the extractor, counterflow to the waste material, and dissolves the organic contaminants from the waste matrix. Clean water or water with solids, or both, are removed from the bottom of the extractor; the mixture of the solvent gas and organic compounds exit the top of the extractor and into a separator where the extraction gas is vaporized and recycled. The organic contaminants are removed and require further treatment or disposal.
Results of the CF systems pilot-scale technology demonstration (EPA SITE program) indicated that PCB extraction efficiencies of 90% were achieved for sediments containing PCBs up to 2,575 ppm. A mass balance of 96% was achieved for the total mass and solids passing through the system; a mass balance was not established for PCBs. In one test, sediments containing 350 ppm PCBs were reduced to 40 ppm after 10 extractions. In another test, 288 ppm PCBs were reduced to 82 ppm after three extractions. In the final test, 2,575 ppm PCBs were reduced to 200 ppm after six extractions. Fifty to 150 gallons per day were treated using this system. The system capacity is designed to treat up to 2,160 gallons per day. The pilot test indicated that the treatment process did not adversely impact the leachability of metals present in the waste matrix.
Air monitoring (combustible gas meters) during the CF Systems pilot test did not detect "significant" leaks of propane from the system. No sudden releases of any of the materials used or treated in the process occurred. One "minor" leak was reported. It was also reported that results of background air sampling and personnel monitoring for organic vapors and PCBs were below the detection limits for the analytical method. In the literature reviewed, no information was found regarding the sampling and analytical methods used for that sampling.
Because of the flammable and explosive properties of propane, all electrical equipment used in the process must be explosion proof, all equipment must be spark proof, and all potential ignition sources must be eliminated. Because the process operates under high pressure, frequent system monitoring for leaks is required for the propane or butane as well as for the contaminant(s). During the pilot test, health and safety monitoring indicated that OSHA Level B protection was required for personnel handling material input or output. OSHA Level C was required for personnel operating the extraction process.
The Dehydro-Tech Corporation (Carver-Greenfield) extraction process uses a food-grade carrier oil to extract oil-soluble contaminants from sludges and soils. This process has been commercially applied to municipal wastewater sludge, paper mill sludge, rendering waste, and pharmaceutical plant sludge to remove oil-soluble contaminants. Sizing pretreatment of the waste matrix is necessary (less than ¼-inch). The carrier oil is mixed with the contaminated matrix and then heated to evaporate water from the system. The oil-soluble contaminants are extracted from the matrix by the carrier oil. Any VOCs that volatilize during the evaporation step are condensed. After the water is evaporated, the dried mixture is centrifuged for removal of the carrier oil (and the extracted contaminants) from the matrix. Following the removal of the carrier oil, it is recovered by evaporation and steam stripping. The contaminants are then removed from the oil by distillation and the carrier oil is recycled. Further treatment of the distilled material and of the wastewater stream may be required.
This process underwent an EPA SITE demonstration in August 1991 (at EPA's research facility in Edison, New Jersey) using spent petroleum drilling fluids from the PAB Oil Site in Abbeville, Louisiana. The results of the demonstration indicated that this process successfully separates a petroleum-contaminated sludge into its solid, oil, and water phases. This process has been used on a limited bench-scale evaluation where PCB concentrations were reduced from 2,000 ppb to less than 1 ppb.
Data gaps for the solvent extraction process include air sampling data, performance history information, the solvent selection criteria, and site-specific treatability studies.
Thermal desorption or thermal separation is a process that uses temperatures high enough to volatilize or vaporize, but not to destroy, organic compounds from contaminated media. Because this process is not intended to destroy the contaminants, further treatment or disposal of the organic compound wastestream is required. Thermal desorption has been used to treat PCB-contaminated soil and sediment. This process has not yet been proven for treatment of PCB-contaminated biological sludges, municipal solid waste, or PCB capacitors. (Some panel members believe that with proper treatability studies and pretreatment, thermal desorption could be applied to waste from municipal landfills.)
This technology includes processes that use direct or indirect heat exchange and operate with
temperatures between 93o C and 649o C (200-1,200o F). Because of the high temperatures
involved in some of the systems, some degree of thermal degradation typically occurs. The types
of desorbers, their soil-processing rates, operating temperature, and developmental status are
included in Table 1.
| Type | (TPH*) |
(o F) |
for PCBs |
| Rotary Dryers | |||
| Thermal Screw | |||
| Indirect Calciners | |||
| Belt Desorber | |||
| * TPH = tons per hour | |||
Figure 5 displays the general process flow diagram for thermal desorption technologies. Before the contaminated soil can be conveyed to the desorber, it must be excavated and sized/screened (¼- to 3/4-inch). In the desorber, the waste is heated to vaporize the organic contaminants. The volatilized organic compounds are transported (sometimes with a carrier gas such as nitrogen) to the air pollution control equipment.
Waste streams from the thermal desorption process can include treated waste, oversized material rejects (from the screening/sizing process), condensed contaminants and water, particulate control dust, treated off gas, and spent carbon (if used). The treated soil may be suitable for placement back onto the site. The concentrated organic liquids, spent carbon, and other air pollution control condensates are stored for further treatment or disposal or both. Collected particulates can be recycled through the desorber and possibly placed back onto the site.
Air pollution control (APC) equipment on thermal desorption units may consist of one or a combination of the following: baghouse, wet scrubber, thermal or catalytic afterburners, activated carbon, and condensation and recovery equipment.
Limitations of thermal desorption technology for PCB-contaminated media include the following:
Advantages of thermal desorption technology for PCB-contaminated media include these:
There was some discussion concerning the operating parameters that may lead to the formation of dioxins and furans. It was postulated that with temperatures less than 320o C (600o F), indirect fire, and an inert (nitrogen) atmosphere, the formation of dioxins/furans would be reduced. An opposing view was expressed, indicating that an indirect-fired desorption unit with an afterburner for air pollution control may lead to the formation of these compounds. The response to this view was that the afterburner was direct fired, and therefore would lead to the formation of the compounds. Existing data indicate that even with an afterburner, the levels of dioxins and furans are low.
The SoilTech Anaerobic Thermal Processor (ATP) Systems Inc. system is reported to be effective in the treatment of PCB-contaminated soils and sludges. This process uses four separate thermal zones (preheat zone, retort, combustion, and cooling). The system operates between 200o C and 340o C (400-650o F) in the preheat zone, between 480o C and 620o C (900-1,150o F) in the retort zone, between 650o C and 790o C (1,200-1,450o F) in the combustion zone, and between 500o F and 800o F (260-427o C) in the cooling zone.
Water and VOCs are vaporized and drawn off from the preheat zone and transferred to the cooling zone where they are condensed and captured. Light organic vapors that do not condense are blown into the combustion zone. Heated granular solids exit the preheat zone and are carried into the retort zone. Temperatures in this zone cause heavy oils to vaporize, and thermal cracking of hydrocarbons occurs (forming coke and low molecular weight gases). Vapors are conveyed off into a vapor condensing and cooling system by an induced draft fan. The coked solids pass into the combustion zone where they are combusted. Treated coke solids are either transferred to the cooling zone or are rerouted to provide heat to the retort zone. Treated soils exit into the cooling zone, are quenched with water, and are transferred to an outside storage pile. Air emission controls for the flue gas exiting the combustion zone consist of a cyclone and a baghouse to remove particulates, and a carbon adsorption system to remove trace organic compounds.
This system is designed to treat approximately 10 tons of contaminated soil or sediment per hour. The preferred moisture content of the waste is between 5% and 10% by weight. The nominal hydrocarbon content of the waste to be treated is reported at 10%. The ATP system process was demonstrated (EPA SITE program) at two PCB-contaminated sites: Wide Beach Development Site, Brant, New York, and Waukegan Harbor Superfund Site, Waukegan, Illinois. At the Wide Beach demonstration, the ATP process was used in conjunction with a dechlorination process to chemically treat more than 42,000 tons of PCB-contaminated soils. APC equipment included a cyclone separator, a baghouse, an acid gas scrubber, and an activated carbon bed. Wastewater treatment included an oil/water separator, a carbon/clay mixture oil and grease remover, chemical oxidation, and carbon polishing. Treated wastewater was then sent for off-site treatment and discharge.
PCB concentrations in contaminated soils for this demonstration averaged 28.2 ppm. Treated soils were below the required clean-up standard of 2 ppm for that site (average of 0.043 ppm). Dioxins and furans measured at the air pollution equipment emission point were less than 0.19 nanogram per dry standard cubic meter (ng/dscm) toxic equivalents at 7% oxygen. Other air emissions tested were PCBs (6 x 10-6 lb/hr), polyethylene glycol (4.0 x 10-5 lb/hr), particulate (0.02 grain per dry standard cubic feet [gr/dscf], corrected), carbon monoxide (3.6 lb/hr), sulfur dioxide (0.02 lb/hr), and nitrogen dioxide (1.35 lb/hr). It was reported that no VOC or semi-VOC degradation products were detected in the treated soil.
At the Waukegan Harbor demonstration, a total of 224 tons of PCB-contaminated soil and sediment were treated. During the demonstration tests, the unit treated a total of about 12,000 tons of contaminated sediment. PCB concentrations were reduced from an average of 9,761 ppm in pretreated soil and sediment to an average of less than 2 ppm. Three replicate tests of the system were conducted. Each test consisted of 8.5 hours of solids and liquid sampling, and 8 hours of stack sampling. Air pollution control equipment consisted of a cyclone separator, a baghouse, and an activated carbon bed. Wastewater treatment consisted of an oil/water separator, polymer additive clarifier, carbon/clay oil and grease removal, chemical and ultraviolet oxidation, pH adjustment, and carbon polishing. The treated wastewater was discharged to a publicly owned treatment works (POTW).
Total dioxins and furans measured at the air pollution equipment emission point were 13.74 ng/dscm at 7% oxygen. The 13.74 value was total mass emissions of dioxins and furans, not the toxic equivalent factor (TEF) method as was used during the Wide Beach test. Other air emissions tested were particulate (0.08 lb/hr), hydrogen chloride (0.20 lb/hr), not detectable sulfur dioxide (0 ppm), and total hydrocarbons (< 0.1 lb/hr). A PCB Destruction Removal Efficiency (DRE) was calculated at 99.99999%. Approximately 0.12 mg of PCBs were discharged from the stack per 1.0 kg of PCBs fed through the system. It was reported that no leachable VOCs or semi-VOCs were detected in the treated soil.
The X*TRAXTM (Chemical Waste Management) thermal desorption process uses an indirect-fired rotary dryer where temperatures range from 400o C to 510o C (750-950oF). Evaporated contaminants are removed by a recirculating nitrogen carrier gas (less than 4% oxygen to prevent combustion). Solids leaving the dryer are sprayed with treated cooling water to minimize fugitive dust emissions. The carrier gas is routed through a liquid scrubber (eductor) and a series of two condensers (to reduce its temperature and remove any remaining organic compounds). From 5% to 10% of the gas exits the system (into the atmosphere) after passing through a particulate filter and a carbon adsorption system. The remaining carrier gas is recycled back through the system.
Organic liquid phases and water phases of the waste stream are routed through a series of scrubbers, condensers, filters, or carbon adsorption systems. The concentrated organic liquid must undergo further treatment or disposal. Filter cake material is recycled back through the desorption process.
This system is reported to process solids at feed rates up to 7.5 tons per hour and has been used to treat PCB-contaminated solids. An EPA SITE demonstration using this system has been conducted (May 1992, Re-Solve Superfund Site, Massachusetts), and the system is now being used to treat approximately 35,000 tons of soil and sediment contaminated with PCBs at the site. The demonstration soil contained PCBs in concentrations ranging from 181 to 515 ppm in three separate tests; each test lasted 6 hours. PCB concentrations in all treated soil samples were less than 1 ppm; the average concentration was 0.25 ppm. Pentachlorodibenzo-dioxins (PCDDs) and pentachlorodibenzo-furans (PCDFs) were not found in the stack gas at detectable levels. Organic emissions from the process vent were reported at 0.4 gram/day. PCBs were not detected in vent gases. Other organic contaminants present in the waste material (tetrachloroethene, petroleum hydrocarbons, and oil and grease) were all reduced to below detectable levels in the treated soil. Metals and the physical properties of the soil were reported to be unaltered by the process.
The data gap identified for thermal desorption technologies was air emissions. Panel discussions focused on the need for a comparison of air emissions from thermal desorption with those of incineration. This comparison would assist in evaluating the relative risks of the respective technologies.
F. Solidification/Stabilization
Solidification/stabilization is the process by which a hazardous waste is converted to a less-soluble, less-mobile state. Solidification is a process that involves encapsulating waste material into a solid material of high structural integrity. This process does not necessarily form a chemical bond between the solidification additive and the contaminant; a mechanical bond may be formed. Stabilization is a process that chemically converts the contaminants to a less mobile, soluble, or toxic form. Cement, lime, and binders are examples of materials used as solidification/stabilization additives.
This technology has been used to treat contaminants before their acceptance into a landfill or for placement back onto the site. The process results in a 25% to 30% volume increase. Historically, solidification/stabilization has been used to treat metals and other inorganic compounds. With currently available technology, inorganic compounds are generally easier to successfully solidify and stabilize than organic compounds.
Pretreatment processes required for the use of solidification/stabilization technologies include removal of large debris, segregating incompatible waste types, and conditioning or removal of waste material containing high concentrations of oil and grease. The process may be suitable for liquids, soils, sludges, and sediments, but is considered unsuitable for contaminants in municipal landfills. A high degree of homogeneity in the waste material will reduce mixing times and increase the likelihood of meeting leachate toxicity testing requirements. Mixing during stabilization could promote the release of volatile air emissions. Therefore, it may be necessary to cover the mixers and maintain a negative pressure during mixing operations, and to filter the exhaust through activated carbon.
It is not possible to stabilize an organic waste with only inorganic material such as cement and fly ash. The organic compounds retard the cement polymerization process, resulting in a waste form that has a consistency similar to dough. That waste product is unlikely to pass the TCLP testing. Binders can be added to overcome these problems. However, if the organic content of the waste is greater than approximately 3%, stabilization is not a viable option. When using binders, stabilization of organic wastes may require 1 ton of binder per 4 tons of waste material. The selection of the appropriate binder depends on the presence of other compounds in the waste material to be stabilized.
The TCLP test requires grinding the solidified waste material before determining if the leachate formed during the test will meet the required performance standards. The test specifies the maximum allowable concentrations for 36 chemicals that may be present in the extract.
For ex situ stabilization, the treated waste material must be deposited on site for long-term storage or transported off site for ultimate disposal. Volume and weight increases of the treated material make off-site disposal more costly because of increased transportation and landfill costs.
In situ stabilization of PCB-contaminated material has been attempted at one Superfund site in Hialeah, Florida. In this case, immobilization of PCBs was accomplished by solidifying volumes of soil at the site. The long-term viability of in situ stabilization is unknown.
EPA investigated the use of quicklime (CaO) for solidification/stabilization of PCB-contaminated material. The studies indicated that most of the PCBs were stripped or volatilized during the heat-producing reactions after the lime addition. A very small portion of PCBs was decomposed during the process. EPA does not consider the use of quicklime a viable solidification/stabilization technology for the treatment of PCBs.
Other attempts to use solidification/stabilization technology on PCB-contaminated wastes have generally yielded inconclusive results. Two companies have tested solidification technologies under the EPA SITE program. HAZCON's process involves blending PCB-contaminated soils with cement and a proprietary additive called Chloranan. The International Waste Technologies (IWT) process also involves using a proprietary additive, known as HWT-20, with water and PCB-contaminated soil. As was the case with the HAZCON process, leachate analysis did not detect PCBs. However, the EPA concluded that the demonstration could not confirm the ability of the processes to solidify and immobilize the PCBs.
Data gaps include the uncertainty of the effectiveness of the technology for organic waste, including PCBs. The risks associated with contaminant leaching at very low rates into the environment still exist. The potential for leaching should be determined in a treatability study before pilot or full-scale implementation of this technology. Additional bench- and pilot-scale investigations are needed to determine the viability of this technology for treatment of PCB-contaminated material.
Vitrification is the process of converting materials into a glass or glassy substance. When accomplished, vitrification may destroy organic contaminants via pyrolysis or combustion. Organic compounds treated by the vitrification process are primarily destroyed thermally. Therefore, the potential fates of organic compounds include these: 1) destruction via pyrolysis or combustion; 2) removal in the off gas system; and 3) migration to adjacent soil during the vitrification process. This remedial process must meet or exceed the Toxic Substances Control Act-required 99.9999% destruction and removal efficiency for the contaminant. The vitrification process will immobilize many inorganic compounds by incorporating them into the product glass. Depending on the contaminated soil characteristics, it may be necessary to add materials (i.e., silicates) to create the special characteristics of glass. This technology decreases the volume of soil by 20%-40%.
Vitrification is generally divided into two methods: electric process heating and thermal process heating using fossil fuels. This technology can be either in situ (in situ vitrification or ISV) or ex situ. An advantage of ISV is that it proceeds with the material in place (no material excavation or handling). The depth to which in situ vitrification has been achieved is 5 meters. Because this is an in situ process, air releases from material handling (excavation/transportation) are not of concern. An off gas collection hood is placed over the process area to control the flow of air and to maintain a negative hood pressure. An induced draft system draws the gases from the hood through an off gas treatment system. Another advantage may be that the treated material remains in place and underground. The advantages of ex situ processes are that vitrification is not limited to the immediate area of electrode placement, and there is better process control.
In situ vitrification involves placing electrodes into the excavated soil and generating an electric current between the electrodes, which produces temperatures high enough to cause PCBs to volatilize or pyrolize, and the soil to become molten (1,600-2,000o C [2,900-3630o F]). When the soil cools, it becomes glass-like and integrates the contaminants into the glass complex. The products of pyrolysis, combustion, and volatilized inorganic compounds are collected in an off gas collection hood placed over the treatment area and must be treated before emission into the atmosphere. As with all thermal treatment systems, the collection of mercury emissions is difficult, but can be achieved.
Limitations of this technology include these:
Overall advantages of this technology include these:
The in situ technology is currently being used for the first time on a full-scale basis at the Parson Superfund Site in Michigan. The vendor indicates that the process exceeds PCB DREs of 99.9999%. The vendor also indicates that the method is applicable to a variety of organic, inorganic, and radioactive materials.
Data gaps for vitrification include air emissions data, performance history data, and worker health and safety information.
The following factors influence site selection for a landfill: 1) waste characteristics; 2) topography; 3) hydrogeology; 4) site access; 5) land use; 6) environmental sensitivity; and 7) cost. Emissions in the form of leachate and gas create the potential for environmental contamination (e.g., air, surface soil, subsurface soil, groundwater) if engineering controls are not provided. Therefore, some primary long-term considerations involve the sufficiency and longevity of containment systems (flexible membrane liners, clay liners, etc.), leachate and gas collection and management systems, and monitoring. Figure 6 displays the basic landfilling operations and processes.
Liners are used to control and prevent seepage into and out of landfill facilities. Flexible membrane liners are typically made of plastic or rubber. Plastic and rubber in thin sheets can also be used to line areas for disposal of hazardous waste on land. In either case, double and triple liners may be used for control and are selected on the basis of material properties and performance. Certain flexible membrane liners can be weakened by oxygen and ultraviolet degradation, pH, chemical (acidic and alkaline) degradation upon exposure to leachate, freeze-thaw cycling, and creep (deformation of the liner over a prolonged period of time under conditions of constant stress). Concerns have also been expressed about stress cracking of high-density polyethylene liners. Cracking has been observed in exposed field seams where the liner was exposed to localized stresses and wide temperature variations.
Clay has a long history of use as a liner material. Clays do not experience degradation or stress cracking. However, rapid freezing and thawing can affect the integrity of clay liners. High or low moisture content, high concentrations of certain organic solvents, and acid/alkaline extremes have also created problems for clay liners. Various types of salts affect conductivity and can lead to problems with clay liners.
Leachate collection and removal systems are affected by clogging of underdrains or drainage lines. Particulates can clog geotextiles, sand, or drainage gravel. There is also a potential for biological clogging. A final possible problem with leachate collection systems is extrusion and intrusion of construction materials into the drainage lines.
Most landfills incorporate intermediate and final covers to contain the landfill waste and to facilitate leachate and gas management. Final covers often are constructed from materials similar to liners, but with features facilitating drainage and vegetation of the surface. Gas collection and recovery systems (wells, trenches, etc.) are also currently integral to the proper installation and operation of landfills. Figure 7 shows the development and completion of a solid waste land disposal unit.
Environmental monitoring and long-term maintenance are important considerations for landfilling disposal technologies. Monitoring systems are used to confirm that the facility is performing in accordance with design requirements. Groundwater monitoring is used to track changes in the potentiometric surface and to detect possible leachate releases. Air monitoring is required to protect facility workers and surrounding populations. Both organic and inorganic compounds can be released into the liquid or gas phases during operations and after closure. Mechanisms of release include direct discharge or volatilization of exposed waste, diffusion through the cap, cracks, gas venting, and leachate collection systems. Conventional landfill practice employs regulated containment and treatment systems for both the gas and leachate produced during landfill disposal.
Aboveground disposal options have received attention for the containment and storage of hazardous and solid wastes. Types of aboveground storage include warehouse storage, vaulting, and aboveground storage mounds. The main advantage of these storage systems is the ability to keep the waste dry and place it in a more certain environmental setting. Very small amounts of leachate are generated compared with some belowground systems. Because the waste is on the surface, the possibility of detecting releases and preventing groundwater contamination is increased. One of the selection criteria for aboveground storage is the volume of waste material. Significant volumes of waste material may not lend themselves to this technology.
Warehouse storage is typically used for temporary storage periods of 10 to 20 years. Historically, aboveground warehouse storage is used when a means of treatment or disposal of a waste is being developed or will require several years for completion. Material is stored in a warehouse (rather than left on site) to reduce the potential for contaminant migration and human exposure.
Vaulting or entombment storage time frames are estimated to range from 50 to more than 100 years. Although longer lasting than warehouse storage, these structures, typically built with concrete, require maintenance and monitoring to ensure the continued integrity of the system.
Aboveground storage mounds are similar in design to belowground landfills and are suitable for areas with high water tables. For groundwater considerations, site selection is less critical for aboveground mounds than for belowground landfills. Because the leachate is continuously removed by gravity drainage, there is less potential for higher volumes of leachate accumulation and less dependence on sustaining liner integrity. The aboveground mounds are designed to increase runoff and may have a greater potential for erosion of the surface layer than conventional landfills. Therefore, surface maintenance is an important management requirement.
Landfilling of PCB-contaminated material is regulated under the Toxic Substances Control Act (TSCA). Landfills must be authorized by EPA to receive PCB waste. One of the permitted facilities, located in Emelle, Alabama, is owned and operated by Chemical Waste Management Inc. Another landfill that is approved to receive PCB-contaminated waste is in Model City, New York.
Landfilling disposal technologies can be used to store and allow in situ processes to affect reduction in the volume or toxicity of PCB-contaminated materials. With appropriate containment systems, unwanted materials are isolated from the environment while natural biodegradation proceeds. The PCB-contaminated wastes that are particularly attractive candidates for landfilling are those in which PCB dechlorination and detoxification processes are already slowly underway (such as sludges and soils).
Data gaps for landfilling and other land disposal or storage techniques include accessibility and geologic suitability of a proposed site, congener-specific analytical data on the PCBs present to indicate whether biodegradation or conversion, or both, are underway, and long-term performance data.