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 ContentsRESIDUE MANAGEMENT
- WATER RESIDUES
- SOLID RESIDUES
- ORGANIC LIQUID AND OIL RESIDUES
- AIR AND GASEOUS RESIDUES
- DESCRIPTIONS OF TECHNOLOGIES
- SELECTION FACTORS
- COST ESTIMATING
- CONTAMINANT LOSSES
Residues are materials, products, or waste streams generated by components of a sediment remedial alternative. Residues may be water, wastewater, solids, oil fractions, or air and gas emissions. The management of these residues may involve treatment, containment, or discharge to the environment.
The types of residues anticipated from most sediment remedial alternatives and management options for them are provided below. Some sediment treatment technologies may generate unique residues, requiring special management considerations. At a minimum, the inert solid particles that were present in the original, untreated sediment, will still be present following the application of any treatment technology.
Water is likely to be the most important residue for consideration at most sediment remediation projects simply because of the volumes generated. The removal and transport technologies selected will have a profound effect on how much water residue is generated through the treatment process. For example, if the sediments are dredged hydraulically and transported by pipeline, a large area will probably be needed for gravity settling. In contrast, if the sediments were removed with a mechanical dredge and transported by truck, there would be much less "free water" to handle.
Some pretreatment and treatment processes may require the addition of even more water. For final disposal of sediments and solids residues, most of this water must be removed. Depending on how the sediments are handled, treated, and disposed, the volume of water that must ultimately be managed can be less than one-half of the volume of sediments (in place) dredged, or greater than five times this volume.
Water residues from a sediment remedial alternative are commonly referred to as effluent or leachate. The term "effluent" may be applied to a wide variety of water residues, including:
- Discharges from an active CDF
- Surface runoff from a landfill or CDF
- Sidestreams from a dewatering process (e.g., filtrate from a filter press or centrate from a centrifuge)
- Wastewater or condensate from a pretreatment or treatment process
The term "leachate" refers specifically to water that has flowed through the sediment, such as pore water, or precipitation that has infiltrated sediments in a CDF or landfill. The volume of leachate is generally much smaller than that of effluent, but the concentration of dissolved contaminants is typically higher.
The flow rate of effluents and leachates is highly dependent on their source. The effluent from a CDF during filling operations from a hydraulic dredge can be quite substantial--hundreds or even thousands of liters per minute. The duration of such discharges, however, is limited to the duration of dredging, which is typically on the order of weeks or months. Sidestreams from pretreatment or treatment operations are technology-dependent, but generally will produce smaller flows over a longer period of time (months to years). Once the remediation project is completed, the need for effluent treatment is limited to storm water (runoff), which could remain a long-term source if water comes into contact with contaminated sediments.
Leachate is generated over very long time periods, and therefore a permanent leachate collection and treatment system is a common requirement at municipal and industrial landfills.
Solid residues include the bulk of sediment solids following treatment as well as smaller fractions of solids separated from the sediments or produced by the treatment processes. For most remedial alternatives involving a properly designed and thorough treatment system, the treated solids will not require additional treatment and can be disposed using the technologies discussed in Chapter 8. Exceptions to this may include solid residues with special physical properties or concentrations of contaminants requiring special handling. Some treatment technologies produce small volumes of sludges. Other solid residues include debris and oversized materials separated during dredging or pretreatment, sludges from water or wastewater treatment systems, spent media from granular filters or carbon adsorption systems, and particulates collected from air pollution control systems.
Thermal desorption and solvent extraction technologies, as discussed in Chapter 7, can produce fractions of concentrated organic liquids and oil materials. These residues are generally small in volume but contain high concentrations of organic contaminants. An organic liquid fraction extracted from sediments with relatively low levels of PCBs may require treatment or disposal in accordance with TSCA requirements, because these processes concentrate the majority of the PCBs in a volume of oil and other organic liquids that is much smaller than the original sediment volume.
A number of treatment technologies produce emissions of air or gas that may require treatment before discharge to the atmosphere. Thermal destruction and thermal desorption treatment technologies commonly have substantial volumes of air and gas emissions, while (solvent) extraction and chemical treatment technologies are typically in closed reactors with incidental air venting.
"Active" biological treatment technologies, such as bioslurry processes, require an input of oxygen and are likely to have larger quantities of air emissions than passive bioremediation systems. Volatilization of organic contaminants may have to be controlled in some pretreatment and disposal technologies, as well as in treatment technologies. Processes that involve the agitation and mixing of sediments contaminated with volatile and semivolatile compounds should be considered as possible sources of contaminant emissions.
Technologies for treating wastewater from municipal and industrial sources are well established and well documented (Weber 1972; Metcalf & Eddy, Inc. 1979; Corbitt 1990). Averett et al. (in prep.) evaluated the applicability of these technologies to effluent and leachate from sediment remedial alternatives on the basis of cost, effectiveness, implementability, and availability.
Effluent/leachate treatment technologies may be categorized according to the type(s) of contaminants that are removed. This chapter discusses technologies that remove the following contaminant categories:
- Suspended solids
- Organic compounds
While there is some degree of overlap between the processes, these categories reflect the primary areas of treatment. There are a number of other contaminants that may also need to be addressed during a sediment remediation project, including:
- Sulfides (especially hydrogen sulfide)
- Oxygen demand (biological oxygen demand [BOD5]; chemical oxygen demand [COD])
Suspended Solids Removal Technologies
The removal of suspended matter is generally the most important process in the treatment of effluents and leachates from sediment remedial alternatives because most of the contaminants in water residues are associated with the solid particles. An effective solids removal system can significantly reduce contaminant concentrations, leaving behind only those contaminants that are dissolved or associated with colloidal material. Solids removal is a frequently required pretreatment for processes that remove dissolved contaminants (e.g., ion exchange, carbon adsorption). The primary technology types for suspended solids removal are sedimentation and filtration.
Sedimentation--Sedimentation is the basic form of primary treatment employed at most municipal and industrial wastewater treatment facilities. There are a number of process options available to enhance gravity settling of suspended particles, including chemical flocculants, CDFs, sedimentation basins, and clarifiers (Averett et al., in prep.). Of these, gravity settling in CDFs has been used most extensively with contaminated sediments.
CDFs have long served the dual role of a settling basin and storage or disposal facility for dredged sediments (see Chapter 8 for more information on CDFs). Gravity settling in CDFs, with proper design and operation, can take a hydraulically dredged slurry (typically having 10-15 percent solids by weight) and produce an effluent with 1-2 g/L suspended solids (USACE 1987b). Many CDFs on the Great Lakes produce effluents with suspended solids less than 1 g/L (e.g., 100 mg/L) by gravity settling alone.
At most CDFs, a hydraulically dredged slurry is discharged into the CDF at one end and effluent is released over a fixed or adjustable overflow weir at the opposite end, as shown in Figure 9-1. Settling times of several days are commonly achieved at larger CDFs. Improved settling efficiencies can be achieved by dividing the CDF into two or more cells or through operational controls to increase the detention time and prevent short-circuiting. As the CDF becomes filled, and detention times shorten, dredging production rates may have to be reduced or mechanical dredging used instead of hydraulic dredging to provide suitable settling efficiencies. Design guidance for sedimentation in CDFs is contained in Confined Disposal of Dredged Material (USACE 1987b).
Sedimentation basins or clarifiers are typically open, concrete or steel tanks with some type of solids collection system that operates on the bottom. Inclined plates may be incorporated into the tanks to improve solids capture for a given flow rate and reduce the size of the clarifier. Rectangular and circular clarifiers are commonly used in municipal and industrial wastewater treatment, but have only been used on a limited basis in applications with contaminated sediments. A cross flow, inclined plate clarifier was used at the ARCS Program's pilot-scale demonstration in Saginaw, Michigan (USACE Detroit District 1994).
Flocculating agents are routinely used in municipal and industrial wastewater treatment in conjunction with clarifiers. There are many proprietary surfactant-type polymers designed for this purpose, although inorganic chemicals such as ferric chloride may also be used. Schroeder (1983) found that low-viscosity, highly cationic liquid polymers were most effective for dredged material effluent treatment and required minimal equipment to implement.
A liquid cationic polymer flocculant was injected into the hydraulic discharge line at dosages of 10 ppm to enhance settling of sediments and fly ash dredged during construction of the Chicago Area CDF (USACE Chicago District 1984). Flocculants were also used during two demonstrations of soil washing technologies on the Great Lakes. Nonionic and anionic polymers were used during the ARCS Program's pilot-scale demonstration at Saginaw, Michigan (USACE Detroit District 1994). A coagulant and a polymer flocculant were used to promote the removal of silty-clay sediments during the pilot-scale dredging and sediment washing demonstration at Welland, Ontario (Acres International Ltd. 1993).
Filtration--Filtration is typically used as a polishing step for water that has been pretreated by flocculation and sedimentation in municipal and industrial wastewater applications. This technology is also widely used for treatment of drinking water. Granular media filtration has been used to treat effluents at most in-water and some upland CDFs in the Great Lakes using either filter dikes (Figure 9-2) or filter cells (Figure 9-3). Permeable dikes provide gravity filtration through horizontal flow, and are nonrenewable once clogged. Most in-water CDF dikes have a core of crushed stone. Some have discrete lenses of sand for filtration, as shown in Figure 9-2. Filter cells and sand-filled weirs are vertical-flow gravity filters that can be replaced or regenerated when exhausted. Filter cells may be incorporated into the CDF dike, as shown in Figure 9-3, or can be freestanding structures constructed of concrete, steel, or plastic.
Gravity and pressure filters can be obtained as "package" units, or constructed onsite for larger applications. Package filtration units are available for purchase or lease. These units are typically mounted on a flatbed trailer for transportation to the site. The flow and filtration capacities of package units can often be designed to fit most small projects. Prefabricated filtration units were used as part of sediment remediation projects in Lorain, Ohio, and Waukegan, Illinois.
Gravity and pressure filters must be taken off line and backwashed periodically to remove accumulated solids. Continuous backwashing systems, which clean a portion of the filter at a time, are also available. The backwash water has high suspended solids content, and must be returned to the sediment disposal/holding area or handled in a sludge treatment system. The operation of one or more filters, including the backwash cycle, can be fully automated.
Filtration media used in Great Lakes CDFs are typically sand and/or graded stone. The filter cell at the Chicago Area CDF uses a combination of sand and anthracite. Alternative media can include limestone, crushed shells, activated carbon, or glauconitic green sands (zeolites). Beds constructed with ion exchange resins may effect ion exchange or precipitation reactions in addition to simple filtration (Averett et al. 1990 and in prep.).
Metals Removal Technologies
Metal contaminants are primarily associated with suspended particulates in most water residues from sediment remedial alternatives. Suspended solids removal technologies should therefore be sufficient to address metals removal needs for the majority of applications. Removal of dissolved metals from water residues can be conducted using ion exchange or precipitation. These technologies have been widely used for industrial wastewater treatment, but have not been applied to water residues from sediment remedial alternatives.
Ion Exchange--Ion exchange is a process in which ions held by electrostatic forces of charged functional groups on the surface of a solid are exchanged for ions of similar charge in a solution in which the solids are immersed (Weber 1972). The "solids" are specific resins (usually in the form of beads) that have an affinity for metallic ions. The most common configuration is the fixed bed system, in which the wastewater flows through resin contained in a column (Cullinane et al. 1986a). Ion exchange resins are either highly selective for specific metal contaminants or non-specific for a wide variety of metals.
Precipitation--Precipitation is a chemical process in which soluble chemicals are removed from solution by the addition of a reagent with which they react to form a (solid) precipitate. This precipitate can then be removed by standard flocculation, sedimentation, and/or filtration processes. Most heavy metals can be precipitated from water as hydroxides with the addition of a caustic (e.g., sodium hydroxide or lime). Alternatively, sodium sulfide or ferric sulfide may be added to precipitate metals as sulfides. The sulfide process is effective for certain metals, such as mercury, which do not precipitate as hydroxides. Precipitation processes produce a sludge that may have to be managed as hazardous waste due to the presence of concentrated heavy metals. Disposal costs for these sludges may therefore be significant.
Most organic contaminants, particularly the hydrophobic compounds, are strongly bound to sediment particulates and will be captured through the suspended solids removal technologies discussed above. Removal of dissolved organic contaminants may be necessary where unacceptable concentrations are present in water residues following sedimentation and/or filtration. Most of the organic contaminant removal technologies discussed here require that suspended solids be removed first.
Carbon Adsorption--Carbon adsorption is a technology that has been used widely in the drinking water treatment industry, and that is being used with increasing frequency in the wastewater and hazardous waste industry (Corbitt 1990). The process takes advantage of the highly adsorptive properties of specially prepared carbon known as activated carbon. The porous structure of the carbon provides a large internal surface area onto which organic molecules may become attached. Many organic substances, including chlorinated solvents, PCBs, PAHs, pesticides, and others, may be removed from solution using carbon adsorption.
Carbon adsorption is achieved by passing water residues through one or more columns containing granular activated carbon operated in parallel or in series. Carbon columns may be operated in either an upflow (expanded bed) or a downflow (fixed bed) mode. In theory, spent carbon may be regenerated. In practice, however, spent carbon must frequently be discarded, especially if high concentrations of PCBs are present.
Activated carbon was used to remove dissolved PCBs from the water drained from sediment storage lagoons and process water from the thermal desorption process at the Superfund remediation at Waukegan, Illinois (Sorensen 1994). Activated carbon was also used to remove phenols from water drained from a CDF used for the disposal of sediments dredged as part of a remediation project at Lorain, Ohio (Kovach 1994).
Oil Separation--Some sediments contain very high concentrations of oil and grease. In most cases, the oil and grease will remain attached to the sediment particulates and be captured by suspended solids removal technologies. In some cases, oil and grease is released from sediment particles, forming a slick, a suspension of discrete particles, or an emulsion in the water residue. In such cases, the oil and grease must be captured or removed prior to treatment processes such as ion exchange, carbon adsorption, and filtration, because oily compounds will foul the surfaces of exchange resins and filters.
Oil booms and skimmers are routinely used in CDFs to capture oil and floating debris. Coalescing plate separators employ a medium that provides a surface for the aggregation of small, emulsified oil droplets, which can then be removed by gravity separation. Emulsified oils are much more difficult to separate from water. Chemical de-emulsifying agents, heat, and/or acids are generally effective for breaking emulsions. Once the emulsion is broken, the oil is amenable to the treatment processes described above.
Oxidation--Oxidation is used to partially or completely degrade organic compounds. Complete oxidation of organic compounds can theoretically reduce complex molecules to carbon dioxide and water. Halogenated organic compounds will produce minor amounts of mineral acids (e.g., hydrochloric acid). However, oxidation is often not complete, resulting in the formation of simpler "daughter" compounds that are usually much less toxic or persistent than the original contaminants (Weber 1972).
Two forms of oxidation that might be applicable to water residues from sediment remedial alternatives are chemical oxidation and UV-assisted oxidation. Chemical oxidants suitable for treating wastewater include oxygen, ozone (O3), hydrogen peroxide (H2O2), potassium permanganate, chlorine (or hypochlorites), and chlorine dioxide (Weber 1972). The oxidizing power of hydrogen peroxide and ozone can be significantly enhanced through the use of UV light. This technology is effective for treating a wide variety of organic compounds, including PCBs and PAHs.
Most of the sediment solids generated by pretreatment or treatment technologies will be disposed using the technologies discussed in Chapter 8. Treated solids may be suitable for beneficial uses, while residues that are still contaminated will likely require confined disposal or subsequent treatment. Sand reclaimed from a CDF in Duluth through a crude soil washing process has been used for road construction fill (Bedore and Bowman 1990). Sediments from Waukegan Harbor treated with a thermal desorption process were confined onsite because of the residual concentrations of PCBs and heavy metal contaminants.
Many of the thermal treatment processes produce solid residues with very little moisture. For example, the solid residues from the thermal desorption process demonstrated at Buffalo, New York, were almost all greater than 99 percent solids by weight (USACE Buffalo District 1993). Fine-grained sediments that have been almost completely dewatered may be difficult to handle and transport without substantial losses as wind-blown dust. Water residues or excess process water may be used to wet the sediments to a manageable consistency.
The easiest place to wet the treated solids is immediately as they exit the treatment process, perhaps by applying a water spray to the residues on a belt or screw conveyer. Other options are to mix the dry residues with wet sediments that are not to be treated or to solidify the residues through the addition of cement, binding agents, and water. These options would require a large mixing tank and agitator.
Other solid residues likely to require special handling include debris and oversized materials removed during dredging or pretreatment, treatment process residues with special properties, spent filter media or carbon from water treatment systems, and particulates collected by air pollution control systems.
Large debris that might be separated during dredging or rehandling may be suitable for salvage or scrap if the contaminated sediments can be washed off. If this is not practical, it may still be necessary to cut or compact the debris into smaller pieces for transport to a landfill. Smaller debris and oversized materials separated during pretreatment will likely require confined disposal.
Filter media and carbon used to treat water residues and particulates collected from air pollution control systems may contain high concentrations of contaminants. These materials may be suitable for co-treatment or co-disposal with the sediments. Granular filter media from the filter cells at the Chicago Area CDF have been routinely disposed in the CDF.
Fractions of concentrated organic materials from thermal desorption and solvent extraction technologies are likely to be relatively small in volume, provided that the treatment process made a good separation of organic and water fractions and there was a good recovery of solvent (if used). For example, 15 kg of oil was collected from 415 kg of sediment during the demonstration of a solvent extraction process at the Grand Calumet River in Indiana (USACE Chicago District 1994). In contrast, a poor separation of oil and water fractions during the pilot demonstrations of a thermal desorption process at the Buffalo and Ashtabula Rivers resulted in a mixed (oil-water) residue with a mass equal to more than one-half that of the feed material (USACE Buffalo District 1993; USACE Buffalo District, in prep.).
Because of their relatively small volume and high concentrations of contaminants (with good separation), subsequent treatment of organic residues is quite feasible and, in many cases, required by regulation. Thermal destructive, chemical treatment, and bioremediation technologies discussed in Chapter 7 may be used to treat organic residues. Some of these technologies were originally developed to treat oil/organic wastes and therefore are more fully developed for organic residues than for sediments. These technologies are also likely to be more efficient with the highly concentrated organic residue than with the sediments.
Oil residues collected from the thermal desorption process used at the Waukegan, Illinois, Superfund cleanup and from the solvent extraction process demonstrated at the Grand Calumet River, Indiana, were incinerated at a licensed TSCA facility. The oil residue from the thermal desorption process demonstrated at Buffalo, New York, and Ashtabula, Ohio, was sent to a commercial oil treatment facility.
Storage onsite, or at a licensed landfill, may be a short-term option for organic residues if a treatment facility is not readily available. The applicability of confined disposal as a permanent option for organic residues will depend largely on regulatory requirements.
The emission of contaminants to the air is a potential contaminant loss pathway for most sediment remediation components. These air emissions may be a point source, such as the stack or vent from unit operations for a treatment process, or a diffuse source, such as volatilized organic compounds from the surface of a CDF. Although organic compounds are usually the contaminants of concern, inorganic contaminants (heavy metals) may be associated with dust generated by remediation processes that remove water from the sediment. Thermal processes that separate volatile heavy metals such as mercury from the sediment are also a potential source of air contamination.
Point sources are generally easier to control because they are already contained and can be piped through an air pollution control system. Point vapor sources from sediment treatment processes can be treated by adsorption (activated carbon or other media), condensation, spray towers, scrubbers, packed columns, thermal oxidation systems, or catalytic oxidation systems. Particulate control may be accomplished by cyclones, scrubbers, bag filters, and similar systems.
Fugitive emission controls for process equipment such as those used for pretreatment and treatment technologies generally require enclosing the entire process in a structure, either a building or an inflatable bubble. Gases vented from these systems would be pumped through a treatment unit, probably activated carbon.
Volatile emissions from large surface areas, such as CDFs or storage tanks, are more difficult to control. Volatilization from these sites may be reduced by limiting the contact between the contaminated sediment or supernatant and air. Options for covering the CDF include buildings or bubbles, floating covers, foams, and sorbent materials. Mixing and splashing during filling from a pipeline can be reduced by submerging the discharge below the surface. The rate of volatilization can also be reduced by shielding the wind from the pond surface through the construction of fences around the perimeter of the facility.
The need for treatment of water residues from a sediment remedial alternative is controlled primarily by the regulatory requirements on the discharge. Water residues may be discharged directly into a waterway or into a municipal wastewater treatment plant. The former is termed "direct discharge," while the latter is an "indirect discharge." Both discharges are regulated under the Clean Water Act (PL-92-500), but the treatment requirements may be quite different.
Water that is returned from any dredged material disposal operation back to a river, lake, harbor, wetland, or other "waters of the United States" is considered "dredged material" and regulated under §404 and §401 of the Clean Water Act. This would include the effluent from a CDF and water separated from dredged sediments during pretreatment. Water from treatment processes and leachate from disposal facilities may be regulated under section 402 of the Clean Water Act (NPDES). Regardless of which of these permitting authorities applies, the direct discharge must meet State water quality standards for the receiving waterway. In some cases, NPDES effluent limitations are based on technology standards (e.g., Best Available Technology).
For direct discharges, the flow rate will usually not be limited. Mixing zones may or may not be allowed for the initial dilution and dispersion of the discharge. Discharge to a small stream or lake with little dilution may not be feasible for some water residues.
Discharges to a wastewater treatment facility are permitted through the local sewer authority or municipality. A "pretreatment" or "industrial discharger" permit must be obtained in accordance with section307 of the Clean Water Act. Sewer use charges are likely to be levied, although these are usually considerably less than the cost of building a separate treatment system. Effluent limitations for conventional pollutants (e.g., BOD, nitrogen, phosphorus) and heavy metals are generally less stringent than direct discharges, because the water undergoes further treatment at the municipal wastewater treatment plant. However, limitations for toxic organic compounds, such as PCBs, PAHs, and phenolic compounds, may be nearly as strict as those for direct discharge. Representative pretreatment standards for three municipalities are shown in Table 9-1.
Discharges to municipal wastewater treatment facilities are typically through existing sewer systems. The rate of discharge may be limited by the capacity of the wastewater treatment facility or the sewers. Small volumes of water residues can also be trucked from unsewered areas to the wastewater treatment facility.
A sediment remedial alternative may have water residues from several sources. Initially, each water stream should be evaluated separately. Some water residues may be suitable for combining for treatment, while others may have to be treated separately.
Once it has been determined that a water residue from a sediment remedial alternative must be treated, the selection of treatment technologies is determined primarily by the following factors:
- Characteristics of the water residue to be treated
- Required effluent quality
- Flow rate (both magnitude and variability)
The quantity and quality of a water residue reflect the characteristics of the sediments being processed and the remediation component at which the residue is generated. The rate of flow will depend on the processing rate of the component generating the water residue and the water storage capacity available.
Other factors that may influence technology selection include:
- Land availability
- Power requirements
- Operator availability and experience
Suspended Solids Removal
The treatment of water residues requires a sequence of steps to achieve the desired effluent quality. In most sediment remedial alternatives, the first and most important step will be the removal of suspended solids. Gravity settling is capable of removing between 90 and 99 percent of suspended solids. Selection factors for suspended solids removal technologies are summarized in Table 9-2.
If the sediments are to be dredged or transported hydraulically, laboratory settling tests should be conducted to predict settling properties and aid in the design of the settling/containment area (USACE 1987b). Additional information on these tests is provided in Table 8-3. The USACE manual Confined Disposal of Dredged Material (1987b) provides guidance on the design and operation of CDFs for removal of suspended solids. The SETTLE routine of the ADDAMS model (as discussed in Chapter 8) can be used to predict gravity settling in a CDF (Schroeder and Palermo 1990).
Flocculants can be used to enhance suspended solids removal, but are generally only recommended for application after primary settling. Schroeder (1983) discusses approaches for applying flocculants to a CDF effluent and compares the effectiveness of several flocculants. With secondary settling, removal efficiencies of 90 percent and greater were readily achieved. Jar tests with a sediment slurry, after allowing for primary settling, are a simple and inexpensive means for selecting flocculating agents and dosage rates.
Filtration systems can provide suspended solids removal efficiencies of up to 90 percent (one pass), but are generally only recommended for water residues with relatively low suspended solids concentrations (less than 300 mg/L). Loadings with higher solids concentrations will cause rapid filter clogging. Guidance on the design of filtration systems for CDFs is provided in Krizek et al. (1976). Laboratory filtration tests are generally not necessary to predict suspended solids removal efficiencies.
Filtration systems typically have a fixed design removal efficiency and flow rate, which may be problematic if the influent water residue has highly variable flow rates or suspended solids concentrations. Flocculant dosages can be adjusted to meet changing flows and suspended solids concentrations, offering greater flexibility in operation. "Package" filtration units can be leased for projects with limited flow rates, and require little space. Filtration may be cost prohibitive for projects with large flow rates. Flocculation and secondary settling can accommodate large flows, but require a secondary settling tank or basin.
Metal and Organic Contaminant Removal
The need for water residue treatment beyond suspended solids removal is determined by laboratory tests to predict the concentrations of dissolved contaminants. The modified elutriate test was developed to predict the quality of an effluent from a CDF during hydraulic dredging/discharge following primary settling (Palermo and Thaxton 1988). The character of surface runoff and leachate from a CDF may be predicted using the methods in Lee and Skogerboe (1983) and Myers and Brannon (1991), respectively. Additional information on these tests is provided in Table 8-3.
Tests for predicting dissolved contaminant concentrations in water residues from treatment technologies will have to be developed on a case-by-case basis. Water residues produced in bench- or pilot-scale demonstrations can be evaluated, but may not adequately reflect the water residues from a full-scale application because of differences in materials handling equipment and the effects of smaller-scale operation.
If water residues require both organic compound and metal treatment technologies, site-specific conditions will dictate which process is to come first. It may be preferable to remove the organic compounds first, because they can interfere with metals removal processes. This is particularly true when metals are chemically or physically bound to organic compounds (e.g., methyl mercury, tetraethyl lead). Conversely, it may be preferable to remove metals in conjunction with suspended solids removal. This would, for example, produce a relatively clean waste stream to be polished with activated carbon.
Reported treatment efficiencies can be used as an initial screening tool in process option selection. However, it is generally necessary to conduct treatability studies with the actual water residue to determine the ultimate feasibility of a specific technology. Treatability studies are particularly important for determining the feasibility of advanced treatment methods (e.g., carbon adsorption, ion exchange) or technologies that are under development (e.g., microfiltration). Selection factors for treatment technologies are presented in Table 9-3 for metals removal and Table 9-4 [part i] [part ii] for organic compound removal.
The disposition of solid residues from a sediment remedial alternative will generally be determined by the following factors:
- Material physical and chemical characteristics
- Volume of material
- Regulatory requirements
Treated sediments that have little residual contamination may be suitable for the beneficial use disposal technologies discussed in Chapter 8. Laboratory tests for predicting contaminant mobility and impacts (see Table 8-3) can be used to screen these disposal options. The selection factors for beneficial use discussed in Chapter 8 should apply to solid residues as well as untreated dredged material.
Treated sediments and other solid residues with elevated levels of residual contamination will require subsequent treatment or confined disposal in most cases. Although the physical and chemical properties of treated solids may be quite different from those of the untreated sediments, the selection factors for treatment technologies (Chapter 7) and for confined disposal technologies (Chapter 8) should still apply.
Treated sediments, filter media, and carbon used to treat water residues and particulates collected from air pollution control systems should be tested to determine if their disposal is regulated by TSCA or RCRA.
The disposition of organic residues is most likely to be controlled by regulation. Thermal desorption or solvent extraction of sediments containing relatively low concentrations (1-5 ppm) of PCBs will probably produce an organic residue with concentrations over 50 ppm PCBs, which must be disposed in accordance with TSCA regulations. In most cases, treatment at an existing, licensed facility will be more cost effective than setting up a second treatment process onsite. As of June 1994, there are four commercial incinerators in the United States licensed to treat TSCA-regulated materials. Other treatment processes (i.e., dechlorination, oxidation, pyrolysis, bioremediation, etc.) may be feasible if an operating, licensed facility is unavailable.
TSCA has specific requirements for the storage, labeling, and transport of PCBs. These, or equally conservative, requirements are likely to be necessary for the storage and handling of organic residues from a sediment remedial alternative. In addition to contaminant control safeguards, the organic residue should also be evaluated for its fire/explosion hazard potential.
Contaminant losses to the air during sediment handling, storage, or treatment are affected by the following factors (USEPA 1992i):
- Contaminant Volatility--The tendency of a contaminant to volatilize from sediments can generally be related to Henry's Constant, which is directly proportional to vapor pressure and the molecular weight of the contaminant and inversely proportional to the solubility of the contaminant in water. Compounds such as PCBs having relatively low vapor pressures, but low aqueous solubilities, may have high Henry's constants and be relatively volatile--hence the need to evaluate potential losses to the atmosphere during sediment remediation (see Myers et al. in prep).
- Residence Time--The longer the sediment or contaminated water is exposed to the atmosphere, the larger the fraction of contaminant lost by this pathway. Long storage periods should be avoided where air emissions are an issue.
- Surface Area--Air emissions are generally directly proportional to surface area. The exposed surface area should be minimized to reduce the mass of contaminant volatilized.
- Turbulence--Agitation or aeration increases the contact time between the contaminated liquid or slurry and increases volatilization.
- Wind Speed--Wind blowing across a CDF or pond or across exposed sediment increases the rate of volatilization. Site location or fences to divert the movement of air can reduce the effects of wind (Thibodeaux et al. 1985).
- Temperature--Volatilization increases with increased temperatures. Operations in cooler weather would reduce contaminant losses.
- Extent of Competing Mechanisms--Contaminant reduction by adsorption, settling, biodegradation, or other treatment techniques could occur at a faster rate than the processes necessary for volatilization, reducing the concentration difference between water and air and consequently the volatilization rate.
The selection of technologies for control of volatile emissions depends on the type of source (point or diffuse), whether vapors or particulates are the concern, and the practicality of capturing or controlling the emission. Selection factors for emission controls for the various components and key technologies of sediment remediation are provided in Table 9-5.
Most vendors of treatment technologies with point souces of air/gaseous emissions should have some operating experience with one or more control systems. The compatability of a specific process unit with a treatment technology will depend on the character and rate of the emission. Control of diffuse emission sources requires changing one of the factors discussed above to reduce the rate and/or mass of volatilization or particulate loss, or requires capturing the emission for treatment by one of the processes used for point sources. The cost for construction and maintenance of structures to capture fugitive emissions is one obvious disadvantage; another disadvantage is the additional health and safety requirements for the personnel who have to operate the equipment and the associated increase in cost and decrease in efficiency. Operation of these structures will require a leak detection and repair program to maintain their effectiveness.
Volatile losses at facilities with large surface areas, such as CDFs, may not be practical to contain and treat. Operational practices may be the only option for minimizing volatile loss. Disposal sites for sediment have their highest emission rates when there is no free water and the sediment is moist, and before a crust forms on the surface. Volatilization losses may be reduced by maintaining ponded water over the sediments or by capping the CDF surface with clean sediment prior to removing the free water.
Cost estimates provided by vendors of sediment treatment technologies do not typically include the costs for managing all residues. When evaluating cost data, it is important to identify residue management that is included and that which is not. Costs for the storage, handling, and transportation of residues need to be estimated along with other residue management costs.
The regulatory requirements for residue management may cause increased costs. If the feed material is not RCRA-or TSCA-regulated, but one or more residues are regulated by these statutes, the regulatory requirements can be relatively simple, provided the residues are not stored or treated onsite. If a RCRA-regulated residue is produced, the treatment process must be registered as a hazardous waste generator. If a RCRA-or TSCA-regulated residue is stored or treated onsite, there are substantial cost increases because of the regulatory requirements.
Considerable cost data are available on technologies to treat wastewater from municipal and industrial applications. Relatively little cost data are available from applications with contaminated sediments, except for CDFs (see Chapter 8). CDFs perform both effluent treatment and disposal functions, and the costs of these are not readily separated. Consequently, if a CDF or similar facility is used for sediment storage, dewatering, rehandling, and/or disposal in a remedial alternative, the costs for effluent treatment (gravity settling) are included in the facility costs.
Features of a CDF that are primarily for effluent treatment include cross dike(s) to enhance settling or provide for secondary settling after flocculant addition, overflow weir(s), oil booms, and special filter dikes. These features may not be included in the basic CDF cost estimate, and should be added as water residue treatment cost items.
Water residue treatment costs are summarized in Table 9-6 [part i] [part ii]. The capital cost of water pollution control structures and equipment is largely dependent on flow rate and contaminant loading. Table 9-6 [part i] [part ii] illustrates example costs based primarily on flow capacities. For metal and organic compound removal technologies, this provides a reasonable basis for comparison. For suspended solids removal technologies, solids loadings are a more critical factor for estimating costs.
Because of the importance of flow rates to the cost of water residue treatment, the ability to store water and treat it over extended periods can be cost effective. This is particularly relevant if hydraulic dredging or transport is used and large volumes of water residues are created in a relatively short period of time. A comparison of the approximate volumes of water residue produced from dredged sediments (volume of water per unit volume of sediment) is as follows:
Hydraulic dredge, 10 percent slurry--1,200 gal/yd (6,000 L/m)
Hydraulic dredge, 20 percent slurry--440 gal/yd (2,200 L/m)
Mechanical dredge, 20 percent expansion--40 gal/yd (200 L/m)
For the above example, it is assumed that the sediment has an in situ solids concentration of 50 percent, and that the final solids concentration after settling and consolidation is also 50 percent.
If sufficient land is not available for gravity settling and for storing water for treatment, mechanical dredging should be used to minimize the water residue produced. If the available land allows for water storage, hydraulic dredging may be feasible if the dredging rate is compatible with the storage and treatment system.
For water residues with limited flow rates, leased treatment equipment or contracted treatment services are likely to be most cost effective; however, some specialized treatment equipment is only available for purchase. The second-hand market may also offer opportunities for savings.
The operation and maintenance costs of water treatment systems are highly dependent on flow rate. However, other variables, such as suspended solids loading, contaminant concentrations, and water chemistry also have a significant impact on operating costs. Some technologies require experienced operators. Water treatment systems can also produce solid residues, such as spent filter media, activated carbon, and sludges, that require disposal.
Costs for the treatment or disposal of solid residues will generally be the same as those discussed in Chapters 7 and 8. The physical and chemical properties of treated sediment solids are likely to be more homogeneous than those of the untreated sediments. Consequently, solid residues may require little or no pretreatment and may be treated more efficiently and at lower unit costs.
Solid residues will require storage onsite until the material can be treated further, disposed onsite, or transported for offsite treatment or disposal. Duplicate storage areas may be necessary for storing one batch of residue while another is awaiting test results to show that the materials were treated to acceptable levels for subsequent treatment or disposal. Solid residues with high concentrations of contaminants (i.e., spent filter media and carbon, treatment sludges, particulates from air pollution control systems) may require special containers for storage, and may require disposal in RCRA-or TSCA-licensed facilities.
Incineration is likely to be the preferred treatment alternative for organic residues from extraction processes. The unit cost for incineration at a TSCA-licensed facility is between $0.55-$1.00/kg (Payne 1993). The availability and unit costs of other treatment processes are difficult to predict because there are so few operating, licensed facilities.
Most vendors of thermal treatment processes do include the costs for air pollution control equipment in their unit costs. Costs for controls of nonpoint emissions from other treatment technologies and from pretreatment and disposal technologies must be estimated separately. These costs may include shelters or bubbles to contain air emissions, air treatment systems, and operational controls. Secondary costs include increased operating costs and decreased production by treatment or pretreatment units that must operate inside air containment structures.
Residuals are releases or discharges from a sediment remedial alternative to the environment that are managed or controlled. The contaminant concentrations in the residual and the type and level of control exercised determine the contaminant loss.
Water residues must be treated to a level that meets regulatory requirements. The total contaminant loss can be readily calculated from the estimated effluent contaminant concentration and the volume of water to be discharged. For a more conservative analysis, the effluent contaminant concentration may be assumed to be equal to the discharge standard. Methods for predicting effluent and leachate contaminant losses are discussed in Myers et al. (in prep.). Additional losses can occur in the event of failure of the treatment system, resulting in the discharge of untreated water. Such accidental losses cannot be predicted, but should be preventable with suitable process control.
Another contaminant pathway from water residue treatment is volatile losses from the surface of sedimentation basins or in the off-gasses from process equipment. Volatilization from sedimentation basins can be estimated using the same procedures derived for CDFs (Myers et al., in prep.). Air emissions from water treatment equipment are likely to be minimal due to the relatively small surface areas and residence times involved.
Contaminant losses from the treatment or disposal of solid residues can be estimated using the procedures discussed in Chapters 7 and 8.
Contaminant losses from the treatment of organic residues can be estimated using the procedures discussed in Chapter 7.
Air and gaseous emissions from point sources, or fugitive sources that have been contained, will be treated in pollution-control equipment to a level that meets regulatory requirements. The total contaminant loss can be readily calculated from the estimated emission contaminant concentration and the volume of air/gas to be discharged. For a more conservative analysis, the emission contaminant concentration may be assumed to be equal to the discharge standard.
Volatile losses from fugitive and nonpoint sources that cannot be contained may be estimated using the methods discussed in Myers et al. (in prep.) for CDFs.