Assessment and Remediation of Contaminated Sediments (ARCS) Program
Table of Content
- Chapter 1
- Chapter 2
- Chapter 3
- Chapter 4
- Chapter 5
- Chapter 6
- Chapter 7
- Chapter 8
- Chapter 9
- Chapter 10
- Chapter 11
- List of Figures
- List of Tables
Remediation Guidance Document
US Environmental Protection Agency. 1994. ARCS Remediation Guidance Document. EPA 905-B94-003. Chicago, Ill.: Great Lakes National Program Office.
Table of ContentsTREATMENT TECHNOLOGIES
DESCRIPTIONS OF TECHNOLOGIES
- Thermal Destruction Technologies
- Thermal Desorption Technologies
- Immobilization Technologies
- Extraction Technologies
- Chemical Treatment Technologies
- Bioremediation Technologies
- SELECTION FACTORS
- FEASIBILITY EVALUATIONS
- ESTIMATING COSTS
- ESTIMATING CONTAMINANT LOSSES
There are numerous treatment technologies for sediments contaminated with hazardous substances. Many of these technologies have been developed for treating contaminated soils at hazardous waste sites, especially those designated under the Superfund Program. This chapter provides an introduction to some of the better-established technologies, particularly those that have been demonstrated on contaminated sediments. However, other sources of information should be consulted for more up-to-date and detailed information on specific applications.
The list of potential remediation technologies is continually changing as new technologies are developed and become available, and other technologies are withdrawn from use. The need for an up-to-date database of treatment technologies has been recognized by governmental agencies in both the United States and Canada. Three of the more useful databases developed to date are described below:
|Sediment Treatment Technologies Database (SEDTEC)|
Wastewater Technology Centre
867 Lakeshore Road
Burlington, Ontario L7R 4L7
Great Lakes Cleanup Fund
Currently in its second edition, SEDTEC provides fact sheets on 168 different technologies submitted to the Wastewater Technology Centre from vendors and technology developers around the world.
|Vendor Information System for Innovative Treatment Technologies (VISITT)|
PRC Environmental Management, Inc.
1505 PRC Drive
McLean, Virginia 22102
U.S. Environmental Protection Agency
Office of Solid Waste and Emergency Response
Technology Innovation Office
Washington, DC 20460
Similar to SEDTEC, except that only innovative technologies are included, and technologies are not specific to sediments. The current Version 1.0 contains 94 technologies for treating sediments. Specific performance data may be included.
|Risk Reduction Engineering Laboratory (RREL) Treatability Database|
U.S. Environmental Protection Agency
Risk Reduction Engineering Laboratory
Cincinnati, Ohio 45268
Provides results of published treatability studies that have passed the USEPA's quality assurance review. Although the most current data are for wastewater treatment, recently available treatment data for soils and sediments will likely be added in future updates.
New technologies must be subjected to a lengthy process of testing and evaluation before they can be applied in a full-scale remediation project. Many innovative technologies have only been demonstrated in bench-scale (i.e., laboratory) tests, while others have undergone pilot-scale testing. In general, both bench- and pilot-scale testing of any treatment technology must be conducted prior to the application of that technology for full-scale remediation.
Sediment that is contaminated to the extent that it requires decontamination or detoxification in order to meet environmental cleanup goals may be treated by using one or more of a number of physical, chemical, or biological treatment technologies. Treatment technologies reduce contaminant concentrations, contaminant mobility, and/or toxicity of the sediments by one or more of four means:
- Destroying the contaminants or converting the contaminants to less toxic forms
- Separating or extracting the contaminants from the sediment solids
- Reducing the volume of contaminated material by separation of cleaner sediment particles from particles with greater affinity for the contaminants
- Physically and/or chemically stabilizing the contaminants in the dredged material so that the contaminants are fixed to the solids and are resistant to losses by leaching, erosion, volatilization, or other environmental pathways
Destruction technologies described in this chapter include thermal destruction, chemical treatment, and bioremediation; separation technologies include extraction and thermal desorption. Volume reduction using particle separation techniques was discussed in Chapter 6, Pretreatment Technologies. Immobilization or stabilization techniques are also described in this chapter. Discussions of the factors for selecting from the available technology types, methods for evaluating their feasibility, and techniques for estimating costs and contaminant losses are also provided.
Descriptions of Technologies
The processes considered in this section are those that heat the sediment several hundreds or thousands of degrees above ambient temperature. These processes are generally the most effective options for destroying organic contaminants, but are also the most expensive. Included in this category are:
- High-pressure oxidation
Most of the thermal technologies are highly effective in destroying a wide variety of organic compounds, including PCBs, PAHs, chlorinated dioxins and furans, petroleum hydrocarbons, and pesticides. They do not destroy metals, although some technologies (e.g., vitrification) immobilize metals in a glassy matrix. Volatile metals, particularly mercury, will tend to be released into the flue gas. Additional equipment for emission control may be needed to remove these contaminants.
These technologies will be briefly summarized here; for a more complete discussion see Averett et al. (in prep.) and USEPA (1985b, 1991e, 1992g).
Incineration is by far the most commonly used process for destroying organic compounds in industrial wastes. Incineration basically involves heating the sediments in the presence of oxygen to burn or oxidize organic materials, including organic compounds. A critical component of the overall treatment process is the emission control system for the gases produced by the process. A diagram of an incineration process is shown in Figure 7-1. Application of incineration to wet solids such as sediments is relatively uncommon; all traces of moisture must be driven off before the solids will burn. This requires the expenditure of large quantities of energy, which makes the process very expensive. Moreover, incineration tends to be a very controversial issue for communities where such facilities are to be sited.
As with most processes that destroy organic compounds, incineration does not remove heavy metal contamination. Most incineration processes increase the leachability of metals through the process of oxidation (exceptions include the slagging or vitrifying technologies, which produce a nonleachable, basalt-like residue). This increased leachability of metals would be advantageous only if the resulting ash were to be treated using a metals extraction process; otherwise, it is a distinct disadvantage. The leachability of metals is generally measured using the toxicity characteristic leaching procedure (TCLP) test. Incinerator ash that "fails" this test must be disposed of as a hazardous waste in accordance with RCRA.
Incineration technologies can be subdivided into two categories: conventional and innovative. Because gaseous emissions from incinerators present a potentially large contaminant loss pathway, the emission control system is a critical component for both categories. Conventional technologies include rotary kiln, fluidized bed, multiple hearth, and infrared incineration. These technologies, summarized in Table 7-1, typically heat the feed materials to between 650 and 980deg.C. An afterburner, or secondary combustion chamber, is generally required to achieve complete destruction of the volatilized organic compounds. All of these processes produce a dry ash residue.
In contrast, there are a number of innovative processes that are designed specifically for hazardous and toxic wastes. These proprietary technologies, listed in Table 7-2, operate at higher temperatures and generally achieve greater destruction and removal efficiencies compared with conventional incineration. Most of these technologies produce a dense slag or vitrified (glass-like) solid instead of a free-flowing ash. These technologies tend to be very expensive, but offer the advantage of producing a nonleachable end product.
In contrast to incineration, pyrolysis involves the heating of solids in the absence of oxygen. A pyrolysis system consists of a primary combustion chamber, a secondary combustion chamber, and pollution control devices. High temperatures, ranging from 540 to 760deg.C, cause large, complex molecules to decompose into simpler ones. The resulting gaseous products can then be collected (e.g., on a carbon bed) or destroyed in an afterburner at 1,200deg.C. A summary of proprietary technologies is provided in Table 7-3.
The Thermal Gas Phase Reduction Process is a specialized process in which a reducing agent (hydrogen gas) is introduced to remove chlorine atoms from PCBs or dioxins. In Hamilton, Ontario, a pilot-scale reactor was used to process PAH- and PCB-contaminated harbor sediments in July 1991. This process produced high destruction efficiencies for PAHs (99.92-99.99999 percent) and PCBs (99.999-99.99999 percent) in dilute sediment slurries (5-10 percent solids) (ELI Eco Logic International 1992). In late 1992, this technology was tested under the Superfund Innovative Technology Evaluation (SITE) Program with PCB-contaminated soil from a landfill in Bay City, Michigan (USEPA 1994b).
Pyrometallurgy, or smelting/calcination, is a nonproprietary form of pyrolysis. This commercial technology is commonly used to treat metal-bearing ores. High levels of metals or metal oxides can be recovered from waste materials of similar metal content because the effectiveness of recovery is directly proportional to the metal content of the waste. However, this process has the potential for forming toxic sludges and has high process costs (Averett et al., in prep.).
This category includes two related technologies: wet air oxidation and supercritical water oxidation. Both processes use the combination of high temperature and pressure to break down organic compounds. Typical operating conditions for both processes are shown in Table 7-4. As indicated in the table, wet air oxidation can operate at pressures of one-tenth those used during supercritical water oxidation.
Wet air oxidation is a commercially proven technology, although its use has generally been limited to conditioning of municipal wastewater sludges. This technology can degrade hydrocarbons (including PAHs), some pesticides, phenolic compounds, cyanides, and other organic compounds (USEPA 1987a). A bench-scale test using sediments from Indiana Harbor showed greater than 99 percent destruction of PAHs (USEPA, in prep.a). However, destruction of halogenated organic compounds (e.g., PCBs) with this process is poor. In bench-scale testing of the process conducted under the ARCS Program, using sediments from Indiana Harbor, it was found that only 35 percent of influent PCBs were destroyed (USEPA, in prep.a). It may be possible to improve oxidation through the use of catalysts (Averett et al., in prep.). One vendor of this technology is Zimpro Passavant (Rothschild, Wisconsin).
The supercritical water oxidation process is a relatively new technology that has received limited bench- and pilot-scale testing. Available data have shown essentially complete destruction of PCBs and other stable compounds. Vendors of this process include Modar, Inc. (Natick, Massachusetts) and VerTech Treatment Systems (Air Products and Chemicals, Allentown, Pennsylvania). Modar uses high-pressure pumps and an above-ground reactor. In contrast, VerTech uses a well between 2,500 and 3,000 m deep to achieve the necessary pressures.
Vitrification is an emerging technology that uses electricity to heat and destroy organic compounds and immobilize inert contaminants. A typical unit consists of a reaction chamber divided into two sections: the upper section introduces the feed material containing gases and pyrolysis products, while the lower section contains a two-layer molten zone for the metal and siliceous components of the waste. Wastes are vitrified by passing high electrical currents through the material. Electrodes are inserted into the waste solids, and graphite is applied to the surface to enhance its electrical conductivity. A large current is applied, resulting in rapid heating of the solids and causing the siliceous components of the material to melt. The end product is a solid, glass-like material that is very resistant to leaching. Temperatures of about 1,600deg.C are typically achieved.
Vitrification units demonstrated in pilot- scale and full-scale tests have solidified 300,000 kg/melt. Vitrifix N.A. (Alexandria, Virginia) is developing a full-scale unit for asbestos waste. Geotech Development Corp. and Penberthy Electromelt also offer vitrification systems.
In situ vitrification is a patented thermal destruction technology developed by the Battelle Memorial Institute's Pacific Northwest Laboratory. Although it was designed to treat contaminated soils in place, it could presumably be adapted to treat dredged sediments. This technology is available commercially from Geosafe Corp., (Kirkland, Washington).
Thermal desorption physically separates volatile and semivolatile compounds from sediments by heating the sediment to temperatures ranging from 90 to 540deg.C. Water, organic compounds, and some volatile metals are vaporized by the heating process and are subsequently condensed and collected as liquid, captured on activated carbon, and/or destroyed in an afterburner. An inert atmosphere is usually maintained in the heating step to minimize oxidation of organic compounds and to avoid the formation of compounds such as dioxins and furans. Figure 7-2 shows a typical process for thermal desorption. The temperature of the soil in the desorption unit and retention time are the primary variables affecting performance of the process. Heating may be accomplished by indirectly fired rotary kilns, heated screw conveyors, a series of externally heated distillation chambers, or fluidized beds (USEPA 1991c).
High-Temperature Thermal Processor
The high-temperature thermal processor (Remediation Technologies, Inc. [ReTec]) uses a Holoflite(TM) dryer, which is a heated screw conveyor, to heat the sediment and drive off water vapors, organic compounds, and other volatile compounds. The screws for the dryer are heated by a hot molten salt that circulates through the stems and blades of the augers, as well as through the trough that houses the augers. The molten salt is a mixture of salts, primarily potassium nitrate. Maximum soil temperatures of 450deg.C are attainable (USEPA 1992g). The motion of the screws mixes the sediment to improve heat transfer and conveys the sediment through the dryer. Off-gases are controlled by cyclones, condensers, and activated carbon. This technology was evaluated in ARCS Program bench- and pilot-scale demonstrations. Removal efficiencies from 42 to 96 percent were achieved for PAHs in Buffalo River sediments (USACE Buffalo District 1993). Greater than 89 percent of the PCBs in Ashtabula River sediments were removed by the ReTec pilot unit (USACE Buffalo District, in prep.).
Low-Temperature Thermal Treatment System
The low-temperature thermal treatment system (Roy F. Weston, Inc. [Weston]) also uses a Holoflite(TM) dryer, similar to the ReTec process. However, Weston's heating fluid is a thermal oil heated by a separate, gas-fired unit. Maximum temperature for the heating fluid is a limiting factor for this process. The typical oil medium has a maximum operating temperature of 350deg.C, which allows soils to be heated to approximately 290deg.C (Parker and Sisk 1991); however, higher temperatures would likely be required to effectively remove PCBs from sediments. Vapors from the contaminated material are passed through a particulate filter, scrubbers or condensers, and carbon adsorption columns, and may require additional post-treatment. In past demonstrations, Weston has attached an afterburner to the gas stream at temperatures as high as 1,200deg.C to destroy the organic compounds. Removal efficiencies >99 percent have been reported for volatile organic compounds; removal efficiencies of about 90 percent have been reported for PAHs (USEPA 1991c). Bench-, pilot-, and full-scale units are available. The capacity of the full-scale system is 6.8 tonnes/hour (Parker and Sisk 1991).
The X*TRAX thermal desorption system (Chemical Waste Management) uses an externally fired rotary kiln to heat soil to temperatures ranging from 90 to 480deg.C. Water and organic compounds volatilized by the process are transported by a nitrogen carrier gas to the gas treatment system. First, a high-energy scrubber removes dust particles and 10-30 percent of the organic compounds. The gases are then cooled to condense most of the remaining vapors. About 90-95 percent of the cleaned gas is reheated and recycled to the kiln. The remaining 5-10 percent is passed through a particulate filter and activated carbon and is then released to the atmosphere (USEPA 1992g). Pretreatment requirements include screening or grinding to reduce the particle size to less than 5 cm. Post-treatment includes treatment or disposal of the condensates and spent carbon. Removal efficiencies greater than 99 percent have been demonstrated for volatile organic compounds, pesticides, and PCBs. USEPA (1992g) reported that mercury, one of the more volatile metals, had been reduced from a soil concentration of 5,100 ppm to 1.3 ppm using this process. The X*TRAX system is available in bench- , pilot-, and full-scale units, although this particular thermal desorption process has not been demonstrated with contaminated sediments.
Desorption and Vaporization Extraction System
The Desorption and Vaporization Extraction System (DAVES[reg.]) process (Recycling Sciences International, Inc.) uses a fluidized bed maintained at a temperature of about 160deg.C and a concurrent flow of 540-760deg.C air from a gas-fired heater. As the contaminated material is fed to the dryer, water and contaminants are removed from the solids by contact with the hot air. Gases from the dryer are treated using cyclone separators and bag houses for removal of particulates and using a venturi scrubber, counter-current washer, and carbon adsorption system for removal of water and organic compounds. Onsite treatment of liquid residues is available as a part of the process. The mobile DAVES[reg.] unit has a capacity of 10-66 tonnes/hour. It is applicable to most volatile and semivolatile organic compounds and PCBs (USEPA 1992g). The process was tested with sediments from Waukegan Harbor, Illinois, with reported reductions in PCB concentrations from 250 ppm to <2 ppm (USEPA 1991c).
Low-Temperature Thermal Aeration System
The low-temperature thermal aeration system (Canonie Environmental Services Corp.) uses a direct-fired rotary dryer that can heat soil to temperatures of 430deg.C. The gas stream from the dryer is treated for particulate removal in cyclones and/or baghouses. Organic compounds may be destroyed in an afterburner or scrubbed and adsorbed onto activated carbon. The full-scale unit can process 11-15 m/hour. Effective separation of volatile organic compounds and PAHs from contaminated soils has been demonstrated (USEPA 1992g).
Anaerobic Thermal Processor Systems
The Anaerobic Thermal Processor[reg.] (ATP[reg.]) system (SoilTech ATP Systems, Inc.) also known as the AOSTRA-Taciuk process, consists of four processing zones. Contaminated material is fed into a preheat zone maintained at temperatures of 200-340deg.C where steam and light organic compounds are separated from the solids. The solids then move into a 480-620deg.C retort zone, which vaporizes the heavier organic compounds and thermally cracks hydrocarbons, forming coke and low molecular weight gases. Coked solids pass to a combustion zone (650-790deg.C) where they are combusted. The final zone is a cooling zone for the flue gases. The organic vapors are collected for particulate removal and for recovery or adsorption on activated carbon (USEPA 1992g). This system was used for the cleanup of PCB-contaminated sediments and soil from the Outboard Marine Corp. Superfund site in Waukegan Harbor, Illinois. A full-scale unit, rated at 23 tonnes/hour was used and produced PCB removals of 99.98 percent (Hutton and Shanks 1992). Pretreatment is necessary to reduce the feed materials to less than 5 cm. in diameter.
Summary of Thermal Desorption Technologies
Thermal desorption processes offer several advantages over thermal destructive processes, including reduced energy requirements, less potential for formation of toxic emissions, and smaller volumes of gaseous emissions. Disadvantages include the need for a follow-on destruction process for the volatilized organic compounds and reduced effectiveness for less volatile organic compounds. Table 7-6 [part i] [part ii] [part iii] provides a summary of various thermal desorption technologies, and Table 7-7 identifies factors that affect the efficiency of the thermal desorption process.
Immobilization alters the physical and/or chemical characteristics of the sediment to reduce the potential for contaminants to be released from the sediment when placed in a disposal site. The principal contaminant loss pathway reduced by immobilization is contaminant leaching from the disposal site to groundwater and/or surface water; however, contaminant losses at the sediment surface may also be reduced by immobilization processes. Solidification/stabilization is a commonly used term that covers the immobilization technologies discussed in this chapter. Table 7-8 lists some of the sediment characteristics that can affect the immobilization process.
Physical stabilization processes improve the engineering properties of the sediments, such as compressive strength, bearing capacity, resistance to wear and erosion, and permeability. Alteration of the physical character of the sediments to form a solid material (e.g., a cement matrix) reduces the accessibility of the contaminants to water and entraps or microencapsulates the contaminated solids within a stable matrix. Because most of the contaminants in dredged material are tightly bound to the particulate fraction, physical stabilization is an important immobilization mechanism (Myers and Zappi 1989). Solidification processes may also reduce contaminant losses by binding the free water in dredged material (a large contributor to the initial leachate volume from dredged material in a disposal site) into a hydrated solid.
Chemical stabilization is the alteration of the chemical form of the contaminants to make them resistant to aqueous leaching. Solidification/stabilization processes are formulated to minimize the solubility of metals by controlling pH and alkalinity. Anions, which are more difficult to bind in insoluble compounds, may be immobilized by entrapment or microencapsulation. Chemical stabilization of organic compounds may be possible, but the mechanisms involved are not well understood (Myers and Zappi 1989).
Binders used to immobilize contaminants in sediment or soils include cements, pozzolans, and thermoplastics (Cullinane et al. 1986b; Portland Cement Association 1991). In many commercially available processes, proprietary reagents are added during the basic solidification process to improve the effectiveness of the overall process or to target specific contaminants. The effectiveness of an immobilization process for a particular sediment is difficult to predict, and can only be evaluated using laboratory leaching tests. A diagram of an immobilization process is shown in Figure 7-3.
Immobilization technologies have been evaluated for treatment of contaminated sediments from both freshwater and saltwater environments. These investigations have shown that physical stabilization of sediments is easily achieved using a variety of binders, including proprietary processes. Results of leaching tests on the solidified products have been mixed; the mobility of some contaminants has been reduced while the mobility of other contaminants has been increased (Myers and Zappi 1992). The ARCS Program evaluated solidification/stabilization of Buffalo River sediments using three generic binders: Portland cement, lime-fly ash, and kiln dust. Leaching of lead, nickel, and zinc was reduced by the cement process, but leachate concentrations of copper were significantly higher for the solidified sediments compared to leachates from the untreated sediments (Fleming et al. 1991). Immobilization of organic compounds in sediments is generally thought to be less effective than for heavy metals; however, Myers and Zappi (1989) demonstrated reductions in PCB leachability in New Bedford Harbor sediments using a solidification process. The results of these studies demonstrate the importance of laboratory evaluations of appropriate protocols for specific sediments, binders, and contaminants prior to selecting an immobilization process for remediation.
Solvent extraction processes are used to separate contaminated sediments into three fractions: particulate solids, water, and concentrated organic compounds. Contaminants are dissolved or physically separated from the particulate solids using a solvent that is mixed thoroughly with the contaminated sediment. Most extraction processes do not destroy or detoxify contaminants, but they reduce the volume of contaminated material that must be subsequently treated or disposed. Volume reductions of 20-fold or more are possible, depending on the initial concentration of extractable contaminants in the feed material and the efficiency of separation of the concentrated organic (oil) stream and the water evaporated by the process. Another advantage of the volume reduction is that most of the contaminants are transferred from the solid phase to a liquid phase, which is more easily managed in subsequent treatment or disposal processes. The primary application of solvent extraction is to remove organic contaminants such as PCBs, volatile organic compounds, halogenated solvents, and petroleum hydrocarbons. Extraction processes may also be used to extract metals and inorganic compounds, but these applications, which usually involve acid extraction, are potentially more costly than those used for removing organic contaminants. Solvents used for extraction processes can represent a significant cost; therefore, a key component of an extraction process is to separate the solvents from the organic compounds and reuse them in subsequent extraction steps. Usually several extraction cycles are required to reduce contaminant concentrations in the sediments to target levels.
The principal pretreatment operation required for solvent extraction is screening or particle-size reduction to remove or reduce oversized debris (see Chapter 6). The maximum particle size depends on the scale and configuration of the extraction process, but the recommended maximum size is 0.5 cm (USEPA 1988b). A wide range of solids contents are acceptable for sediment treated by extraction processes. Some processes require that the feed material be pumped, which would require that water be added to the sediment to decrease the solids content.
Extraction processes can operate in a batch mode or continuous mode. Sediments and solvents are mixed together in an extractor (Figure 7-4). Extracted organic compounds are removed from the extractor using the solvent and are transferred to a separator where the solvent and organic compounds are separated from the water and the contaminants are separated from the solvent by changes in temperature or pressure, or differences in density. Concentrated organic contaminants are usually associated with an oil phase, which is removed from the separator for post-treatment. The solvent is recycled to the extractor to remove additional contaminants. This cycle is repeated several times before the treated solids are finally removed from the extractor.
When treated solids are removed from the extractor, traces of solvent will be present. The solvents selected for these processes generally vaporize or are biodegradable. Some processes include an additional separation step designed to further remove, by distillation or other means, most of the solvent from the product solids.
A number of process options for extraction are commercially available; however, most of them are proprietary. Most of the processes discussed in this chapter have been used in the USEPA SITE Program, and two of them have been demonstrated with contaminated sediments.
Basic Extractive Sludge Treatment Process
The B.E.S.T.[reg.] process (Resources Conservation Co.) uses a combination of tertiary amines, usually triethylamine (TEA), as the solvent. The first extraction is conducted at temperatures below 4deg.C where TEA is soluble with water and at a pH greater than 10. Hydrocarbons and water in the sediment simultaneously solubilize with the TEA, creating a homogenous mixture (USEPA 1992g). In the next step of the process, solids are separated from the liquid mixture by settling. The remaining solvent is removed from the solids fraction by indirect steam heating. Water is separated from the water-organic compound-TEA mixture by heating the solution to temperatures above the miscibility point (about 54deg.C). Organic compounds and TEA are separated by distillation, and the TEA is recycled to the extraction step. This process was demonstrated at the Grand Calumet River as a combination ARCS and SITE program demonstration in 1992 (USACE Chicago District 1994), and bench-scale tests were performed for Buffalo River, Saginaw River, and Grand Calumet River sediments (USEPA, in prep.a). A summary of the bench- and pilot-scale results for PCBs and PAHs is provided in Table 7-9.
CF Systems Solvent Extraction
The solvent extraction process offered by CF Systems uses compressed propane at supercritical conditions as the solvent. Sediment is screened to remove oversized material and debris and can then be pumped through the system as a slurry in a continuous mode. The solvent is mixed with the sediment under normal temperatures and high pressures. Organic compounds are extracted from the sediment and water into the solvent. The solvent-organic compound stream is removed from the extractor, and the propane is separated from the organic compounds by reducing the pressure and allowing the propane gas to vaporize. After recompression, the gas is recycled to the extraction step. Three or more extraction stages are usually required to achieve contaminant removal efficiencies of 90-98 percent (USEPA 1992g). This process was demonstrated using contaminated sediments from the New Bedford Harbor Superfund site during a SITE Program demonstration (USEPA 1990c,h).
The Carver-Greenfield process (Dehydro-Tech Corp.) is a physical process that can be used to separate oil-soluble organic compounds from contaminated sediments by dissolving the contaminants in a food-grade oil with a boiling point of approximately 204deg.C. Five to ten kilograms of carrier oil per kilogram of solids is combined in a mixing tank where the extraction takes place. Three or more extraction stages may be necessary. From the mixing tanks, the slurry is transferred to a high-efficiency evaporator where the water is removed. The oil is separated from the dewatered solids initially by centrifugation and then by a hydroextraction process that uses hot nitrogen gas to strip the remaining oil from the solids. After separating the contaminants from the oil by distillation, the oil is recycled to the extraction step and the concentrated contaminants are further treated or disposed. Low solids content is not a problem for this process, but particle size must be reduced to less than 0.5 cm in diameter. Demonstration projects have been conducted on drilling mud wastes, a relatively fine-grain material. The requirements of this process for fine particle sizes and wet feed material favor applications to contaminated sediments.
The term soil washing is generally used to describe extraction processes that use a water-based fluid as the solvent (USEPA 1990b). Many soil washing processes rely on particle-size separation to reduce the volume of contaminated material. These processes were discussed in Chapter 6, Pretreatment Technologies, and will not be addressed in this section. Other water-based techniques involve dissolving or suspending the contaminants in the water-based fluid. Because most sediment contaminants are tightly bound to particulate matter, water alone is not a suitable extraction fluid. Surfactants, acids, or chelating agents may be used with water to effect separation of some contaminants. The particle size and type of contaminant are important factors in the effectiveness of soil washing as an extraction process. Soil washing for clays and silts is only marginally applicable. The U.S. Bureau of Mines evaluated acid extraction for heavy metals in Great Lakes sediments from three AOCs under the ARCS Program and found minor reductions in sediment metal concentrations (Allen, in prep.). The use of surfactants may be successful for removing organic compounds from sandy sediments.
Other Extraction Processes
Other extraction processes are emerging that have the potential for removing organic, and perhaps inorganic, compounds from contaminated sediments. Table 7-10 [part i] [part ii] lists a number of extraction processes that are commercially available and are advertised as being applicable to contaminated sediments. This list was developed from those technologies in the SEDTEC database (Wastewater Technology Centre 1993). The table lists the name of the process, the classes of contaminants affected, and the extraction fluid or other medium used to separate the contaminants. Most of the vendors of these technologies do not specify a particular solvent, stating that it depends on the contaminant and material characteristics.
For the purposes of this document, the definition of chemical treatment is restricted to processes in which chemical reagents are added to a sediment matrix for the purpose of contaminant destruction. Certain immobilization, extraction, and thermal procedures also involve chemical inputs, but they are typically added to alter the phase of the contaminant, thus facilitating removal or binding the contaminant in the sediment. A clear distinction between categories cannot always be made, and some overlap may occur between this and other chapters of this document.
Chemical treatment technologies used during the removal component involve mixing chemical additives with sediments or with a sediment slurry. This mixing is typically done in batch operations in some type of process vessel. Chemical treatments may destroy contaminants completely, may alter the form of the contaminants so that they are amenable to other treatments, or may be used to optimize process conditions for other treatment processes. Treated sediments may then be permanently disposed of or put to some beneficial use, depending on the nature and extent of residuals, including reagents and contaminants.
For the ARCS Program, Averett et al. (1990 and in prep.) reviewed eight general categories of chemical treatment for suitability to dredged material. Chelation, dechlorination, and oxidation of organic compounds were considered most promising. The specific processes under these three categories that have been demonstrated to be useful or that are sufficiently developed for consideration are further described in this section. Other promising, emerging technologies are also discussed.
Chelation is the process of stable complex formation (a chelate) between a metal cation and a ligand (chelating agent). This process could also be considered an immobilization process, and some extraction processes also use chelating agents. Binding of the metal cation in a stable complex renders it unavailable for further reaction with other reagents in chemical or biological systems. The stability of a complex generally increases as the number of bonds increases between the ligand and the metal cation (Snoeyink and Jenkins 1980). A ligand forming a single bond is known as monodentate, a ligand forming two bonds is known as bidentate, while a ligand forming more than two bonds is known as polydentate. Ethylenediaminetetraacetic acid (EDTA) is a well-known example of a polydentate ligand (Brady and Humiston 1986). pH is one of the most important parameters that affects the treatment process. Efficiency varies with the chelating agent and dosage used (Averett et al., in prep.).
The ENSOL and LANDTREAT process uses a polysilicate as an adsorptive agent (LANDTREAT) to solidify metal hydroxide silicate complexes produced by the ENSOL, which contains sodium silicate and a proprietary chelating agent. The process is carried out in an enclosed, continuous-reaction chamber (Wastewater Technology Centre 1993). The process is available at the full-scale commercial level.
Dechlorination processes remove chlorine molecules from contaminants such as PCBs, dioxins, and pentachlorophenol through the addition of a chemical reagent under alkaline conditions at increased temperatures (USEPA 1990a,j). The resulting products are much less toxic than the original contaminants. Typically, chemical reagents are mixed with the contaminated sediments and heated to temperatures of 110-340deg.C for several hours, producing the chemical reaction and releasing steam and volatile organic vapors. The vapors are removed from the processor, condensed, and further treated using activated carbon. The treated residue is rinsed to remove reactor by-products and reagent and is then dewatered prior to disposal. Adjustment of the pH of the residue may also be required. The wastewater produced may require further treatment. Processing feed streams with lower solids contents, such as sediments, require greater amounts of reagent, increase energy requirements, and produce larger volumes of wastewater for disposal, all distinct disadvantages of this process for contaminated sediments. Four representative dechlorination processes are discussed in the following paragraphs, other vendors may offer similar processes.
APEG Chemical Dehalogenation Treatment--This process typically uses an APEG to treat aromatic halogenated compounds (USEPA 1990j). Potassium hydroxide (KOH) is most commonly used with polyethylene glycol (PEG), to form the polymeric alkoxide (potassium polyethyleneglycol [KPEG]), although sodium hydroxide is less expensive and has been used for this purpose. Another reagent is KOH or sodium hydroxide/tetraethylene glycol, which is more effective on halogenated aliphatic compounds. Dimethyl sulfoxide (DMSO) may be added to "enhance reaction rate kinetics" (USEPA 1990j). Products of the reaction are a glycol ether and/or a hydroxylated compound and an alkali metal salt-water-soluble by-products.
DeChlor/KGME Process--KGME is a proprietary reagent of Chemical Waste Management, Inc., and is the active species in a nucleophilic substitution (dechlorination) reaction. Principally used for liquid-phase halogenated compounds (particularly PCBs), KGME has been successfully used to treat contaminated soils in the laboratory. PCBs have been treated in both liquid and solid matrices (USEPA 1992g).
Base-Catalyzed Dechlorination Process--The base-catalyzed dechlorination process combines chemical addition with thermal inputs to dechlorinate organic compounds without the use of PEG (USEPA 1992g). The mechanism appears to be a hydrogenation reaction (Rogers 1993). The hydrogen source is a high-boiling-point oil plus a catalyst. The process has been used for both liquids and solids in in situ and ex situ applications. The SITE program demonstrated the process at a North Carolina site in 1993, and the Navy with support from the SITE program is also evaluating the process for PCB-contaminated soil.
Ultrasonically Assisted Detoxification of Hazardous Materials--This process affects the chemical destruction of PCBs in soil using an aprotic solvent, other reagents, and ultrasonic irradiation (USEPA 1992g). The dechlorination of PCBs in the process is believed to result from a nucleophilic substitution reaction, although this is presently unverified. The purpose of the ultrasonic irradiation is to add heat to the reaction. The technology is currently being tested using a moderate-temperature, heated reactor and reflux column (Kaszalka 1993). The process is suitable for ex situ application only; to be economically feasible the reagents must be recovered. This technology currently exists at the pilot-scale development level.
Chemical oxidation involves the use of chemical additives to transform, degrade, or immobilize organic wastes. Oxidizing agents most commonly used (singly or in combination with ultraviolet [UV] light) are ozone, hydrogen peroxide, peroxone (combination of ozone and hydrogen peroxide), potassium permanganate, calcium nitrate, and oxygen. The use of ozone, peroxide, and peroxone has come to be known as advanced oxidation processes. Strictly defined, oxidation is the addition of oxygen to a compound (creation of carbon to oxygen bonds) or the loss of electrons from a compound (increase in the positive valence). Oxidation is used to transform or break down compounds into less toxic, mobile, or biologically available forms. Theoretically, compounds can be decomposed completely to carbon dioxide and water. Adequate process control of pH, temperature, and contact time is important to prevent the formation of hazardous intermediate compounds, such as trihalomethanes, epoxides, and nitrosamines, from incomplete oxidation.
Oxidation is commonly used to treat amines, phenols, chlorophenols, cyanides, halogenated aliphatic compounds, mercaptans, and certain pesticides in liquid waste streams (USEPA 1991b). It can also be used on soil slurries and sludge. The effectiveness of oxidation depends on the organic compound as shown in Table 7-12.
Oxidation is nonselective, and all chemically oxidizable material (including detritus and other naturally occurring organic material) will compete for the oxidizing agent. It is not applicable to highly halogenated organic compounds (Averett et al., in prep.). Certain contaminants, such as PCBs and dioxins, that will not react with ozone alone require the use of UV light with the oxidizing agent. This limits the effectiveness of the process with slurries because the UV light cannot penetrate the mixture.
The LANDTREAT and PETROXY process uses a synthetic polysilicate (LANDTREAT) for adsorption of organic compounds to facilitate the oxidation by the PETROXY reagent, which includes a combination of hydrogen peroxide and other additives. A secondary reaction is the conversion of heavy metal cations to metal silicates on active sites of the LANDTREAT (Wastewater Technology Centre 1993).
Chemical and Biological Treatment Process--This process combines chemical oxidation and biological treatment for the purpose of enhancing biodegradation processes (USEPA 1992g). The mechanism provides oxygen for biological use, oxidation of organopollutants, and alteration of the soil matrix. The process produces chemical intermediates that are both more biodegradable and, due to the apparent alteration of the soil matrix, more bioavailable. This can be beneficial with high waste concentrations that would typically be toxic to microorganisms.
D-Plus (Sinre/DRAT)--This process (Wastewater Technology Centre 1993) involves the use of chemical inputs to stimulate enzymes and to provide a favorable chemical environment (alkaline, reducing, anaerobic) for hydrogenation, dehalogenation, and hydrolysis chemical reactions. A biochemical process, the technology uses heat to break carbon-halogen bonds and to volatilize light organic compounds. Although not yet available on a commercial scale, it may be feasible at the current stage of development to treat up to 900 tonnes of contaminated sediments. There is potential for future development of in situ application as well.
Summary of Chemical Treatment Technologies
Table 7-13 [part i] [part ii] [part iii] lists the processes discussed above and presents specific applications, limitations, specifications, and efficiencies of these processes.
Bioremediation, sometimes called biorestoration, is a managed or spontaneous process in which microbiological processes are used to degrade or transform contaminants to less toxic or nontoxic forms, thereby remedying or eliminating environmental contamination. Microorganisms depend on nutrients and carbon to provide the energy needed for their growth and survival. Degradation of natural substances in soils and sediments provides the necessary food for the development of microbial populations in these media. Bioremediation technologies harness these natural processes by promoting the enzymatic production and microbial growth necessary to convert the target contaminants to nontoxic end products.
Biological treatment has been used for decades to treat domestic and industrial wastewaters, and in recent years has been demonstrated as a technology for destroying some organic compounds in soils, sediment, and sludges. Bench-scale testing of bioremediation was conducted for the ARCS Program with sediments from Great Lakes sites (Jones et al., in prep.a). The chemical and physical structure of organic compounds affects the ability of microorganisms to use them as a food source. The degradation potential for different classes of organic compounds is illustrated in Figure 7-5. Bioremediation of organic compounds in sediment is a complex process, and its application to specific compounds is based on an understanding of the microbiology, biochemistry, genetics, metabolic processes, structure, and function of natural microbial communities. Microbiology must be combined with engineering to develop effective bioremediation processes. The ARCS Program conducted a workshop on bioremediation of contaminated sediments to document laboratory research and field applications of this technology. The proceedings of this workshop (Jafvert and Rogers 1991) provide an excellent discussion of the state of the art with an emphasis on the microbial and chemical processes involved.
Many of the more persistent contaminants in the environment, such as PCBs and PAHs, are resistant to microbial degradation because of 1) the compound's toxicity to the organisms, 2) preferential feeding of microorganisms on other substrates, 3) the microorganism's lack of genetic capability to use the compound as a source of carbon and energy, or 4) unfavorable environmental conditions in the sediment for propagating the appropriate strain of microorganisms. Alteration of the environmental conditions can often stimulate development of appropriate microbial populations that can degrade the organic compounds. Such changes may include adjusting the concentration of the compound, pH, oxygen concentration, or temperature, or adding nutrients or microbes that have been acclimated to the compound. A summary of sediment characteristics and environmental conditions that limit bioremediation processes, and actions to minimize the effects of these limitations, is presented in Table 7-14.
Biodegradation of refractory organic compounds is not uncommon in nature, but can take many years. The key to improving the usefulness of bioremediation for cleaning up contaminated sediment sites is to determine how to accelerate the rate of biodegradation to detoxify the target compounds in a finite time period (i.e., weeks or months rather than years).
Ideally, biodegradation of organic compounds in sediments would be accelerated in situ. However, because of the complexity of the sediment-water ecosystem; the difficulties in controlling physical and chemical, as well as biological, processes in the sediment, and the need to adjust environmental conditions for various stages of the biodegradation process; limited effectiveness has been demonstrated for in situ bioremediation. Much research is underway in the area of in situ treatment, and future efforts will likely overcome some of these difficulties for certain sites and specific contaminants. However, the best current prospects for successful bioremediation of xenobiotic compounds are engineered treatment systems in which environmental conditions can be carefully controlled and adjusted as the biotransformation processes progress with time.
Biodegradation is accomplished either aerobically or anaerobically. Aerobic respiration is energy-yielding microbial metabolism in which the terminal electron acceptor for substrate oxidation is molecular oxygen, and carbon dioxide and water are the end products. Free oxygen must be present for aerobic reactions to occur. Anaerobic respiration is energy-yielding metabolism in which the terminal electron acceptor is a compound other than molecular oxygen, such as sulfate, nitrate, or carbon dioxide, and methane, sulfides, and organic acids are the typical end products. Aerobic processes generally proceed more quickly and provide a more complete degradation of the organic compounds than anaerobic processes. However, some compounds can only be changed by anaerobic organisms. For example, dechlorination of the more highly chlorinated PCBs by anaerobic processes has been demonstrated in laboratory and field studies. On the other hand, the less chlorinated PCBs are susceptible to degradation by aerobic organisms. Sequential anaerobic treatment followed by aerobic processes appears to offer an effective destruction technology for PCBs (Quensen et al. 1991).
This section addresses surface bioremediation techniques in which sediments are removed from the waterway and treated in bioslurry reactors, contained land treatment systems, compost piles, or CTFs. Pretreatment requirements for these processes include removal of oversized particles for bioslurry reactors and possible adjustment of solids content for all of the processes. One of the advantages of bioremediation technologies is that the physical and basic chemical characteristics of the treated sediments are very similar to the feed material, allowing a wide range of choices for beneficial use of the treated sediment.
Bioslurry reactors are a relatively new technology that has been applied to contaminated solids mostly in the last 5-10 years. There have been a number of pilot-scale applications, but few full-scale installations. Bioslurry reactors are best suited to treating fine-grained materials that are easily maintained in suspension. In a bioslurry system, a sediment-water slurry is continuously mixed with appropriate nutrients under controlled conditions in an open or closed impoundment or tank. Aerobic treatment, which involves adding air or another oxygen source, is the most common mode of operation. However, conditions suitable for anaerobic microorganisms can also be maintained in the reactor where this oxic state is an essential step in the biodegradation process. Sequential anaerobic/aerobic treatments are also possible in these systems. Contaminants with potential for volatilization during the mixing and/or aeration process can be controlled using emission control equipment. A schematic diagram of an aerobic bioslurry process is shown in Figure 7-6. Systems for treating soils or sediments are often operated in batch mode, because typical retention times are on the order of 2-12 weeks. Once the treatment period is completed, the solids may be separated from the water and disposed of separately. The slurry solids concentrations range from 15-40 percent; therefore, adjustments in solids contents for slurry treatment of sediments may be minor.
The degradation of PCBs using the bioslurry reactor technology was investigated by General Electric Co. (Abramowicz et al. 1992). Researchers concluded that between 35 and 55 percent of the initial PCBs were degraded over a 10-week test period in reactors amended with biphenyl. Remediation of contaminated sediments from Toronto Harbor, Ontario, was tested in pilot-scale reactors in 1992 (Toronto Harbour Commission 1993). Although complicated by analytical interferences, the results showed that oil and grease was completely degraded in several week's time, with a partial degradation of PAHs.
Contained Land Treatment Systems
Contained land treatment systems, which have been demonstrated in Europe, require mixing of appropriate amendments with the sediments, followed by placement of the material in an enclosure such as a building or tank and on a pad or prepared surface (USEPA 1991d). The enclosure protects the material from precipitation, moderates temperature changes, allows moisture control, and provides the capability to control volatile organic compound emissions. A schematic diagram of a contained land treatment system is shown in Figure 7-7.
Leachate from the sediment is collected by underdrains for further treatment as necessary. The layer of sediment treated for each lift is generally no deeper than 6-8 in. (15-20 cm). Regular cultivation of the sediments and the addition of nutrients, and in some cases bacterial inocula, are typically required to optimize environmental conditions for rapid bioremediation. The excess water associated with the sediment as it is placed in the treatment bed may create operational problems for startup and will likely require that the system be designed for lateral confinement of the material.
Composting is a biological treatment process used primarily for contaminated solid materials. Bulking agents (e.g., wood chips, bark, sawdust, straw) are added to the solid material to absorb moisture, increase porosity, and provide a source of degradable carbon. Water, oxygen, and nutrients are needed to facilitate bacterial growth. Sediment solids contents will likely be sufficient for composting operations, and in some cases dewatering of the sediment may be necessary as a pretreatment step. Available composting techniques include aerated static pile, windrowing, and closed reactor designs (USEPA 1991d). Volatilization of contaminants may be a concern during composting and may require controls such as enclosures or pulling air through the compost pile rather than pushing air into and out of the pile. Use of composting to treat sediments should increase permeability of the sediment, allowing for more effective transfer of oxygen or nutrients to the microorganisms. A pilot-scale demonstration of composting is being conducted for Environment Canada's Cleanup Fund at a site in Burlington, Ontario. Approximately 150 tonnes of PAH-contaminated sediments from Hamilton Harbor were placed in a temporary shelter and tilled periodically with additions of a proprietary organic amendment (Seech et al. 1993). The treatment was executed over an 11-month period. Sediments that were tilled with the amendment showed reductions of PAHs of over 90 percent, while controls with tillage and no amendment showed reductions of 51 percent. Controls with no tillage or amendment showed reductions of 73 percent (Grace Dearborn Inc., in prep.).
Contained Treatment Facility
CDFs routinely used for dredged material may be used as contained treatment facilities for bioremediation of sediments. These facilities often provide long-term to permanent storage. The size of the CDF and the depth (1.5-5 m) of sediments may limit the capability to control conditions compared to other bioremediation systems. These limitations are similar to those for in situ bioremediation processes for contaminated soil sites, except that engineering the biotreatment system for upland CDFs is not as difficult compared to in situ systems. A pilot evaluation of a contained treatment facility for PCB-contaminated sediments is underway at the Sheboygan River AOC. Rather than a diked disposal facility, the contained treatment facility is constructed with sheet pile walls and includes an underdrain system that could be used for leachate control and to add nutrients, oxygen, and other additives. The ARCS Program has contributed to the scientific assessment of the operation; a report documenting these investigations will be published at a later date; however, these experiments were inconclusive as of early 1994. Bioremediation in a CDF would offer an economical process for reducing sediment organic contamination, but more research is needed to develop techniques for implementation.
Selection factors for treatment technologies will be discussed in terms of three general categories: target contaminants, sediment characteristics, and implementation factors. The discussion is based on selection of a type of technology (e.g., thermal destruction, extraction, immobilization) for a particular project. Selection of a process option within a technology type will require further evaluation using treatability studies and consideration of the factors affecting the technologies discussed earlier in this chapter. In addition, the evaluation of the overall remedial alternative must consider the effects of each step of the process on preceding and succeeding steps.
Selection of a treatment technology for a particular contaminated sediment site should first consider the contaminants of concern and the effectiveness of each technology in destroying, removing, or immobilizing those contaminants. Table 7-16 rates the effectiveness of each of the major technology types on organic and inorganic compounds typically found in contaminated sediments. For many contaminant/technology combinations, effectiveness of removal or destruction has been demonstrated; however, as the table notes, in some cases the effects are not known or the process is only partially effective in treating the contaminant. A note is also made where a technology may increase contaminant loss for a nontarget contaminant present in the sediment. When both organic and inorganic contaminants are targeted, more than one technology may be required to accomplish project objectives.
Table 7-17 shows how three major sediment characteristics can affect the performance of various treatment technologies. These characteristics are predominant particle size, solids content, and high contaminant concentration. Particle size may be the most important limiting characteristic for application of treatment technologies to sediments. Most treatment technologies are very effective on sandy soils and sediments. The presence of fine-grained material adversely affects treatment system emission controls because it increases particulate generation during thermal drying, it is more difficult to dewater, and it has greater attraction to the contaminants (particularly clays). Clayey sediments that are cohesive also present materials handling problems in most processing systems.
Another sediment characteristic that affects process performance is solids content. Two classes of solids contents are shown in Table 7-17: high, representing material at near the in situ solids content (30-60 percent solids by weight); and low, representing a hydraulically dredged sediment (10-30 percent solids by weight). Technologies that require the sediments to be in a slurry for treatment are favored for the lower solids contents; however, high solids contents are easily changed to lower solids contents by water addition at the time of processing. Changing from a lower to a higher solids content requires more processing. Thermal processes are adversely affected by lower solids contents primarily because of increased energy consumption. Dechlorination processes are adversely affected because of increased chemical costs and increased wastewater treatment requirements.
The last set of characteristics shown in Table 7-17 is the presence of organic compounds or heavy metals in high concentrations. Incineration and oxidation processes are generally favored for higher organic carbon concentrations (not necessarily the target contaminant). Higher metal concentrations may make a technology less favorable because of the increased mobility of certain metal species following application of the technology.
A number of other factors may affect selection of a treatment technology other than its effectiveness for treatment. Seven of these factors are listed in Table 7-18. Each of these factors must be weighed for each technology. The table indicates with a check mark the technology-factor combination for which the factor may be critical to evaluation of the technology. For example, vitrification and supercritical water oxidation have only been used for relatively small projects and would be very difficult to implement for full-scale sediment projects. Regulatory compliance and community acceptance become prominent issues for any type of incineration system. Land requirements are more of a concern for solidification and solid-phase bioremediation projects. Residuals disposal must be addressed for those processes (i.e., thermal desorption, extraction, soil washing) that generate a contaminated, potentially hazardous, waste stream. Wastewater treatment and air emission control are more of a concern when the technology generates these releases.
It is evident from the previous discussion that there may be several different types of technologies that have potential for successfully remediating a specific contaminated sediment site. A screening process, considering such factors as contaminant type and sediment physical characteristics, will typically narrow the range of applicable technology candidates, but will not reduce them to a single process option.
To proceed from a site screening analysis or remedial investigation to the selection of an optimum technology for full-scale application in the remediation of a contaminated sediment site, there are several types of tests that can be used to further reduce the range of options. The following sections discuss the various testing options, the implications surrounding them, and some general cost ranges for such tests.
The need for technology testing, either in the laboratory (bench-scale) or on a larger scale in a field setting (pilot- or full-scale), is a function of both the particular sediment contamination problem and the state of development of the technology. As Averett et al. (in prep.) have noted, the application of hazardous waste or mineral processing technologies to full-scale sediment remediation projects is in its infancy at this time. The recent completion of the cleanup of the Outboard Marine Corp./Waukegan Harbor Superfund site, which employed a thermal desorption unit to treat more than 11,000 tonnes of contaminated sediments, is the only full-scale, sediment treatment project completed in North America to date.
Until the implementation of the ARCS Program in the United States and the Contaminated Sediment Treatment Technology Program (COSTTEP) in Canada, very few treatment technologies had been evaluated for contaminated sediments in the laboratory or in the field. Through these programs, however, as of summer 1993, about 30 technologies have been tested on sediments in the laboratory. Pilot-scale demonstrations in the field have now been conducted with 12 processes. The experience gained through these programs, in addition to other studies conducted by the Corps and through the SITE Program, has helped advance the state of knowledge on the general effectiveness of treatment technologies for contaminated sediments and will serve as a useful guide for others attempting to select a technology for their site.
Because of the unique characteristics of each contaminated sediment site, some amount of laboratory testing will be necessary to determine if the technology being considered is capable of obtaining the desired treatment efficiencies. Spatial variabilities within a given site may require testing of several sediment samples with different physical and/or chemical characteristics. Only in very rare cases will there be no testing required prior to full-scale remediation efforts. At a minimum, the technology vendor will need to set operating parameters for its full-size treatment unit, requiring at least the performance of glassware simulations of the main components of the treatment technology using samples representative of the specific sediments to be remediated.
The need for pilot-scale tests, using process equipment that closely mimics the unit operations of a full-scale technology, will have to be determined on a case-by-case basis. The decision to conduct pilot-scale tests is a joint one between the parties responsible for the cleanup, the Federal and State agencies regulating the cleanup, and the technology vendor. It is sometimes beneficial, for contracting purposes, to allow the technology vendor flexibility in reaching established treatment goals, as opposed to conducting extensive testing prior to the full-scale operations. Minor changes in field operations can adversely affect processes for which very narrow operating parameters were specified.
The purpose of conducting bench-scale, or laboratory, tests on small quantities of sediments (typically less than 1 kg) can range from simply determining gross process efficiencies to setting specific operating parameters for a full-scale technology application. Each sediment sample is unique, combining different contaminant types and concentrations with certain physical characteristics, and all of these variables can affect the ability of a technology to "treat" the sediments.
In an ideal situation, specific cleanup goals will have been set for a site, either expressed as a maximum residual concentration of a specific contaminant (e.g., 2 mg/kg PCBs) or as a minimum percent of the contaminant that must be removed from the raw material. In addition, the contaminant concentrations that are expected in the final, treated products would ideally be measurable using current analytical techniques. By working with the technology vendor, an experimental design can be established to determine the optimum configuration of a process (e.g., operating temperature, residence time, extraction cycles) to meet the cleanup goals. A factorial design, varying two or more parameters in a systematic pattern, is useful to examine the sensitivity of a process when treating the sediment of concern. The USEPA document, Guide for Conducting Treatability Studies Under CERCLA (USEPA 1989b), is an excellent reference on this subject.
Under the ARCS Program, bench-scale tests were conducted with no specific treatability goals established. Instead, vendors were directed to optimize the application of their process to one or more sediment samples, keeping in mind that economics would be a prime consideration in the full-scale application of the technology by the users of the information generated by the ARCS Program. A two-phased approach was used. During Phase I, the vendors were allowed to adjust operating parameters to determine optimum conditions. During Phase II, the process was run under these optimum conditions, with extensive analyses conducted on all the feed and residual materials produced by the technology to determine process efficiency. A matrix of the parameters analyzed in these tests is provided in Table 7-19.
The selection criteria listed in Table 7-16 should serve as a starting point for other technology evaluations. Contaminants should be added or deleted from the list as appropriate for the specific sediment sample and technology being evaluated. Chemicals used in the process that may be problematic if encountered in treatment residuals should also be monitored. In addition, if concerns exist over the status of the untreated sediments being regulated as a hazardous waste (e.g., the sediments fail the TCLP test for one or more parameters), or if the technology may alter the sediments such that the solid residue produced by the process may fail the TCLP, then appropriate analyses of the raw and treated materials should be conducted.
Quality assurance and quality control issues should receive utmost priority in conducting any evaluation of treatment technologies. Quality assurance project plans (QAPjP) were prepared and followed for all of the bench-scale tests performed under the ARCS Program, in accordance with the Quality Assurance Management Plan (QAMP) for the overall ARCS Program (USEPA 1992c). The ARCS QAMP serves as a useful guide for conducting sediment sampling and analysis activities, and is recommended for further information on this subject.
In addition to analyzing for contaminant concentrations in raw and treated materials, an attempt should be made to perform a mass balance analysis for each bench-scale test. However, the degree of certainty that can be obtained with a mass balance analysis is highly dependent on the representativeness of that sample for the sediments as a whole. Any error in this analysis is magnified when the total mass of the contaminant is calculated by multiplying the contaminant concentration by the total weight of the sample. Weights for all materials entering or exiting a process should be accurately and precisely determined. The masses measured directly for materials such as solids, water, and oil may produce more reliable mass balance results.
The need for pilot-scale demonstrations and testing of a technology will be influenced by the state of development of the technology (whether pilot- or full-scale treatment units exist), the success of previous testing on similar sediment types, and the vendor's confidence in scaling up from bench-scale test results. An additional factor may be the need to demonstrate to the local community that a technology is safe, effective, and aesthetically acceptable. This can be best accomplished through an onsite, pilot-scale demonstration.
Certain critical elements of a sediment remediation process can also be analyzed more realistically during a pilot-scale demonstration than in a bench-scale test. Because a pilot-scale unit uses pieces of equipment and process flow patterns that more closely simulate the full-scale technology, the ability for the unit to deal with the physical characteristics of the contaminated sediments is better evaluated. In addition, the effects of particle size, solids content, and high contaminant concentrations can be evaluated more easily than in the laboratory. The pilot-scale demonstrations conducted under the ARCS Program were most successful in expanding the body of knowledge for engineering issues concerning the application of treatment technologies to contaminated sediments.
The experimental design for a pilot-scale testing program should follow the same logic as that described for the bench-scale test. If bench-scale tests precede the pilot-scale test, the optimum settings for the operating parameters should already be established. The pilot-scale test can then be used to evaluate the effects of other variables (e.g., solids content in the feed material, processor throughput rates, operating temperatures) on the effectiveness of the process.
The larger-scale, high-volume processes in the pilot-scale demonstration may require the sampling and analysis of additional process streams including: air emissions (including carbon canisters used as emission control devices), wastewater discharges, chemical reagent or solvent stocks, and multiple solid product streams (e.g., cyclone residuals). Monitoring of some of these process streams may be necessary to ensure compliance with permits obtained for the demonstration.
The success of a treatability test is usually judged by comparing the concentrations of the contaminants of concern in the untreated sediments with those in the treated solids produced by the process. The evaluation can be made as to whether the residual contaminant concentrations are below the established cleanup goals or the percentage removal from the untreated sediments meets or exceeds an established guideline. These cleanup goals or removal guidelines may be established by regulation or on a project-specific basis.
Consideration must also be given to the potential transformation and fate of contaminants. This is a concern with any process that uses heat to treat chlorinated hydrocarbons, particularly PCBs, because dioxins and furans can be formed at temperatures less than those required for complete destruction by incineration. Any process that causes a chemical transformation to occur should also be evaluated to determine the possibility of the formation of intermediary products that may be of concern. If any such products are expected, they should also be analyzed for in the appropriate process stream. In addition, those technologies that extract or separate contaminants from the sediment matrix require that all residuals be analyzed for the extracted contaminants, to ensure that unexpected and uncontrolled losses are not occurring. It may be necessary to develop specialized analytical protocols for unusual matrices (e.g., activated carbon or condensed oils).
General cost estimating guidance was provided in Chapter 2. This section provides guidance for estimating the costs associated with the treatment step of the overall remedial action process. Treatment costs will in most cases be the step requiring the largest expenditure of funds. Unfortunately, costs for the treatment step are the most difficult to estimate accurately. Treatment technologies have not been widely applied to full-scale remediation projects for soils or sediments. Historical project construction data and data for relatively standard construction practices are available for other components, such as removal and disposal, but such data are not available for treatment technologies. Most treatment cost estimates are based on information provided by the vendor. Though vendors may act in good faith in providing cost information, comparability of the data from various vendors is often poor because of variability in the items included in the estimates, the effects of variable sediment characteristics on process operations, and other uncertainties in the process.
The costs directly attributable to the treatment component are discussed below in terms of the cost elements generally used by the SITE Program for evaluating treatment costs based on field (usually pilot-scale) tests for the treatment technologies. The relative importance of each element in selecting various treatment technologies depends on the unit operations involved in the process, the importance of chemical additives for the process, the energy requirements and costs, and project-specific factors.
Site Preparation Costs--These costs are for the site used to construct and operate the treatment facility. This element includes site design and layout, surveys and site logistics, legal searches, access rights and roads, preparation of support facilities, decontamination facilities, utility connections, and auxiliary buildings. Where the site is used for more than just the treatment technology (e.g., pretreatment or disposal of residues), site preparation costs may be partially included in the costs for other components.
Permitting and Regulatory Requirements--This element includes permits, system monitoring requirements, and development of monitoring and analytical protocols and procedures.
Capital Equipment--Major equipment items, process equipment, and residual materials handling equipment are included in this element. The annualized equipment cost is based on the life of the equipment, the salvage value, and the annual interest rate.
Startup and Fixed Costs--This element includes mobilization, shakedown, testing, insurance, taxes, and initiation of environmental monitoring programs. Mobilization costs represent a larger share of the total treatment costs for smaller-scale projects.
Labor Costs--Labor charges for operational, supervisory, administrative, professional, technical, maintenance, and clerical personnel supporting the treatment processes must be estimated for this element.
Supplies, Consumables, and Utilities Costs--Fuel, electricity, raw materials, and supplies required to process the material are included in this element.
Residue Treatment and Disposal Costs--Treatment systems may generate one or more residues (e.g., water, oil, solids, sludges, air/gas) that require further treatment before discharge or disposal. Technologies for treatment and disposal of these residues are discussed in Chapter 9.
Monitoring and Analytical Costs--Field and laboratory costs for monitoring the conditions of the treatment process and the quality of residues are included in this element.
Facility Modification, Repair, and Replacement Costs--This element includes design adjustments, facility modifications, scheduled maintenance, and equipment replacement.
Demobilization--Once the sediment cleanup project is completed, all equipment will have to be dismantled and removed from the treatment site and the land will likely have to be restored to its original condition.
Real Estate and Contingencies
Other major cost items that should be included in the overall estimate are land purchase or lease and overall contingency costs.
Table 7-20 lists a number of factors that affect the cost of treatment technologies included in the VISITT database (USEPA 1993b). In USEPA's query of vendors for the database, the vendor was asked to identify the factors that most affected the cost of each process. The top three factors listed in Table 7-20 were the cost factors identified most frequently by the vendors. These factors are waste quantity, initial contaminant concentration, and target contaminant concentration. A wide range of sediment remediation technologies may be available for a given project, and the costs will vary depending on the volume of sediment to be treated and the contaminant concentrations in the feed and treated material. Table 7-21 [part i] [part ii] lists selected vendors from the VISITT database, the cost range reported by each vendor for a technology type, and the three major cost factors affecting that vendor's costs. Although this table shows cost information for individual process options (vendors), the comparability of these costs (within a given technology type) is limited. In other words, a vendor should not be selected based on the costs shown here. This table should only be used to compare the range of costs and cost factors for the various technology types.
Few remediation projects, including those at Superfund sites, have employed the treatment technologies discussed in this section. However, through demonstrations conducted by the SITE Program, the ARCS Program, the Canadian Cleanup Fund, and others, example costs for a number of technologies applied to specific sites have been documented. Information selected from published SITE and ARCS Program reports is presented in Table 7-22. These data were generated based on operational data from field demonstrations of a few cubic meters. The field data were extrapolated to projects of a specific size based on the particular site. For the four ARCS Program demonstration projects, a range of project sizes and associated costs was reported.
Estimating costs for treatment technologies requires defining the project requirements, acquiring treatability data for the sediments, determining cleanup levels, reviewing available cost reports for treatment technologies, and communicating with vendors of the technologies. A consistent set of rules, site conditions, sediment characteristics, target cleanup levels, and cost elements should be provided to each vendor to obtain information for a comparative analysis of treatment costs.
Estimating Contaminant Losses
Methods for estimating or modeling contaminant losses from various combinations of treatment technologies are complicated by the wide range of chemical and physical characteristics of contaminated sediments, the strong affinity of most contaminants for fine-grained sediment particles, and the limited application of treatment technologies to contaminated sediments. Basic mathematical models may be available for simple process operations, such as extraction or thermal vaporization applied to single contaminants in relatively pure systems. However, such models have not been validated for the sediment treatment technologies discussed in this chapter because of the limited database on treatment technologies for contaminated sediments or soils.
Standard engineering practice for evaluating the effectiveness of treatment technologies for any type of contaminated media (solids, liquids, or gases) is to perform a treatability study for a sample that is representative of the contaminated material. In a management review of the Superfund Program, USEPA (1989b) concluded that "To evaluate the application of treatment technologies to particular sites, it is essential to conduct laboratory or pilot-scale tests on actual wastes from the site, including, if needed and feasible, tests of actual operating units prior to remedy selection." The performance data generated by the treatability studies will usually provide a reliable estimate of the contaminant concentrations for the residual sediment following treatment. Contaminant concentrations and weights for waste streams generated by a technology can also be determined from treatability studies, but the need for this information must be clearly identified as one of the objectives of the treatability study so that appropriate data will be collected. Treatability studies may be performed at the bench-scale and/or pilot-scale level.
Most treatment technologies include post-treatment or controls for waste streams produced by the processing. The contaminant losses can be defined as the residual contaminant concentrations in the liquid or gaseous streams released to the environment. For technologies that extract or separate the contaminants from the bulk of the sediment, a concentrated waste stream may be produced that requires treatment offsite at a hazardous waste treatment facility, where permit requirements may require destruction and removal efficiencies greater than 99.9999 percent. The other source of contaminant loss for treatment technologies is the residual contamination in the sediment after treatment. Wherever the treated material is disposed, it is subject to leaching, volatilization, and losses by other pathways. The significance of these pathways depends on the type and level of contamination that is not removed or treated by the treatment process. Various waste streams for each type of technology that should be considered in treatability evaluations are listed in Table 7-23.