The following is paper entitled "Site Remediation Alternatives for the Hays Army Ammunition Plant." This paper was written by Deborah Cohen, an undergraduate student in the Department of Civil and Environmental Engineering at Carnegie Mellon University, as part of an independent research project with Professor Mitchell Small. The paper addresses a series of hypothetical remediation alternatives for the site. (Note: The Hays site was cleaned of environmental contamination by the US Army before being donated to the URA of Pittsburgh.)
The Hays Army Ammunition plant of Pittsburgh, Pennsylvania ( Figure 1) is a brownfield site that is currently being redeveloped for future industrial use. Brownfields are defined as "abandoned, idled or under-used industrial and commercial facilities where expansion or redevelopment is complicated by real or perceived environmental contamination" (Office of Technology Assessment, p. 13). From 1942 to 1970 the U.S. Navy and Army used the Hays Army Ammunition plant to manufacture gun parts and projectiles. As a result of these industrial processes the main contaminants on this site are polychlorinated biphenols (PCBs) in transformers, oil, acid, kerosene, gasoline, and asbestos insulation (Weston and Baker Environmental Inc., p. 1). In order for a brownfield site to be reused as an industrial site it is necessary to determine whether the site complies with regulatory contamination limits. These environmental regulations have been established to protect the environment and the safety of the workers at the future facility.
An Environmental Baseline Study was prepared by Weston and Baker Environmental, Inc. in 1991 for the U.S. Army Corps of Engineers. This assessment found total petroleum hydrocarbons, volatile organic carbons (VOCs), metals, and PCBs in the soil (Weston and Baker Environmental Inc., p. 73-78). Based on applicable cleanup standards, the Pennsylvania Department of Environmental Protection (PADEP) has determined that only the petroleum hydrocarbons and PCBs will require remediation. The concentrations of metals and VOCs in the soil at the Hays site are at acceptable levels. Additionally, the groundwater at the site was found to be in compliance with all applicable standards. Because PCBs are found only in localized areas of shallow soil, excavation is the most practical remedial technology. The solution for the remediation of the petroleum hydrocarbon contamination is less certain. The main focus of this report is to investigate different remedial options for the petroleum hydrocarbons in soil at the Hays site.
In the fall of 1996 the Hays site will be used as a hot dipped galvanizing plant by GalvTech, L.P. Due to the future reuse of this site as an industrial facility, it is necessary to remediate current contamination from the site. Various methods of remediation will be described and compared by cost, duration, and probability of success to determine the most feasible technology. The soil remediation technologies investigated in this report are vacuum vapor extraction, soil washing, soil flushing, solvent/chemical extraction, and bioremediation.
The objective of this case study is to investigate alternative remedial technologies that may be used to reduce the levels of petroleum hydrocarbons in contaminated soils at the Hays Army Ammunition Plant. The PADEP regulations state that 500 ppm of total petroleum hydrocarbons is the maximum allowable concentration for an industrial site. These regulations are designed to protect environmental and human health. In this case the main concern is the safety of on-site personnel at the future GalvTech galvanizing plant.
The specific characteristics of the Hays site will influence which technology is most applicable for the remediation of petroleum hydrocarbons. The decision of what technology to use will be based on cost, duration, and probability of success. Data regarding characteristics of the contaminated soil is taken from the Environmental Baseline Study done by Weston and Baker Environmental, Inc. Petroleum hydrocarbons were detected in soils and sediments at concentrations up to 474,000 ppm. Additional levels were indicated in separate sections of the site, but these levels were below the regulatory limit of 500 ppm. Figure 2 shows concentrations of petroleum hydrocarbons at the Hays site.
Figure 3 shows the area of contaminated soil that requires remediation at the Hays site. The area of this section of soil is approximated by taking the area of a square of 300 ft by 75 ft, plus the area of a 45_ triangle with sides of 150 ft.
Area = Area of square + Area of triangle
Area = 300ft * 75ft + 1/2 * 150ft * 150ft
Area = 33,750 ft2
The average depth of contaminated soil is one foot. Using this depth it is possible to calculate the approximate volume of soil that requires remediation.
Volume = Area * Depth
Volume = 33,750 ft2 * 1 ft.
Volume = 33,750 ft3
Because the contaminated soils are near the surface and are completely contained within the vadose zone, they are assumed to be completely unsaturated. This assumption makes it is possible to determine the weight of the soil. The top foot of soil is anticipated to be primarily a sandy silt or clay. According to the Soil Mechanics Design Manual, mixed soils, classified as sandy or silty clay, are estimated to have a dry unit weight (g) of 80-120 lb/ft3. Using this range of unit weights and the volume of soil that requires remediation, it is possible to estimate the weight of the soil.
Weight = Volume * Dry Unit Weight (g)
Weight = 33,750 ft3 * 80 lb/ft3 / 2205 lb/m ton= 1,224 m tons
Weight = 33,750 ft3 * 120 lb/ft3 / 2205 lb/m ton= 1,836 m tons
Weight Range = 1,224 - 1,836 metric tons
Knowing the classification, volume, and weight of the contaminated soil, it is now possible to estimate the costs and feasibility for five different remediation alternatives at the Hays site.
The following technologies will be evaluated for cleanup of the contaminated soil at the Hays site: vacuum vapor extraction, soil washing, soil flushing, solvent/chemical extraction, and bioremediation. The feasibility of each technology will be based on scientific feasibility, cost, project duration, and probability of success. First, the remediation technologies will be described and then their feasibility will be assessed.
The following descriptions of remediation technologies are derived from the third, fifth, and eighth volumes of the Innovative Site Remediation Technology series edited by William C. Anderson, Geotechnical Practice for Waste Disposal edited by David E. Daniel, and from Hazardous Waste Management by LaGrega, Buckingham and Evans.
Vacuum vapor extraction is an extraction process that removes contaminants from soil by creating an air flow field through the contaminated soil. It is potentially applicable to sites consisting of permeable, or sandy, soil where petroleum products have permeated into the subsurface. For this technology to work effectively the soil must have a permeability (k) greater than 10-8 cm2 or a hydraulic conductivity (i) of at least 10-3 cm/s. This technique can be performed both in situ and ex situ.
If ex situ methods are chosen, perforated pipes are placed in the soil to produce the necessary vapor flow throughout the soil. For in situ vacuum vapor extraction, vacuum blowers or pumps create a vapor flow which starts at the surface of the soil, drawing air from the atmosphere into the soil. This vapor flow then continues along the path of least resistance, and flows through the vapor extraction wells, whereupon the vapor is removed from the soil. This artificial vacuum increases the rate of volatilization of the petroleum hydrocarbons, and therefore increases the rate of removal of the contaminant from the soil. Once the contaminants within the flow field are volatilized, additional contaminants outside the flow field volatilize, diffuse into the vapor flow, and are removed from the soil. Figure 4 is a schematic diagram of a simplified in situ soil vapor extraction process.
When the air is removed from the soil, air treatment may be necessary. Vapor treatment systems can include catalytic and thermal destruction systems, adsorption units, and biological treatment systems. If air treatment is not necessary, it is possible to install air discharge stacks.
Vapor extraction is one of the most often used techniques for the removal of petroleum hydrocarbons; yet, it has a high degree of uncertainty. The design criteria, including the placement and number of vapor extraction wells, must be carefully specified to enable the optimal air flow field. The wells must be carefully situated in order to minimize the distance the volatilized petroleum hydrocarbons will travel to reach the extraction wells. Vapor extraction is a remediation that varies considerably depending on the specific site characteristics.
Soil Washing is an ex situ process that uses both physical and chemical techniques to remove contaminants from the soil. In an ex situ process, the contaminated soil is excavated from the site before remediation occurs. There are six basic steps to the soil washing process: (1) pretreatment, (2) separation, (3) coarse-grained treatment, (4) fine-grained treatment, (5) process water treatment, and (6) residuals management. Figure 5 shows a basic soil washing flow diagram.
After excavation the soil is put through a pretreatment process, which is the mechanical screening of the soil to remove large objects and oversized clumps of material. Depending upon contamination levels, this oversized material is either disposed of as solid waste or is returned to the site. Contamination levels of the oversized objects are usually low. Next, in the pretreatment process, crushing and grinding is performed in order to reduce clumps of materials and to free contaminated particle surfaces. Blending and mixing is performed in order to reduce variation in contamination throughout the soil.
After the pretreatment process, it is necessary to separate the soil into the coarse-grained (greater than ~70 microns) and fine-grained (less than ~70 microns) fractions of soil. This is done by mixing the soil with water or water plus surfactants. Hydroclones, mechanical screens, or hydrosizers are used to separate the soil from the spent fluid into two fractions: (1) the high volume, sand and gravel fraction and (2) the low volume, highly contaminated, silt and clay fraction. Some fine-grained solids will remain in the coarse-grained fraction of soil.
The coarse and fine-grained fractions of soil are treated separately. Although the larger portion of contamination is found in the fine-grained fraction, treatment may be required to remove any contaminants coating the coarse solids. Treatment methods for this contamination include surface attrition, acid or base treatment for solubilization, or the use of solvents for dissolving the contaminants. Next, dewatering of the coarse-grained fraction occurs. Because the coarse-grained fraction may contain up to 5% fine-grained solids, the dewatered material is sent to the fine-grained treatment process for recovery of these materials. The clean soil fraction of the coarse fraction is recovered so that it can be replaced to the originally excavated site.
The fine-grained treatment consists of the fine fraction removed at the separation step, and the fine materials that were carried along into the coarse-grained treatment. The fine-grained fraction of the soil contains the highest amount of contaminant due to the high surface area-to-volume ratio, and the strong cohesive properties of fine-grained soils. In this process the fine particles are settled out of the water by cycloning. The contaminated fraction of the fine soil then continues to the residual management process, while the water goes on to the process water treatment step. In the residuals management process the contaminated fine-grained particles and sludges are either landfilled or treated. In the water treatment process the washwater is either treated for recycling back to the process or for disposal. It is usually less expensive to reuse the water because it does not require such stringent standards.
Soil washing is often used as a pretreatment technology, to reduce the volume of contaminated soil. This reduction of volume, reduces the cost of the next step of remediation. According to the Superfund Innovative Technology Evaluation (SITE) Program reports, 90 to 98% removal of hydrocarbons may be achieved, when heat and surfactants are added to the washwater (Anderson, p. 2.2). Soil washing is recommended for petroleum and fuel residues that are contained in sandy or gravelly soils and is therefore applicable to the Hays site.
Soil flushing is an in situ process that uses water, dilute acids and bases, complexing and chelating agents, reducing agents, solvents, or surfactants to accelerate the movement of contaminants from a contaminated soil for recovery and treatment. Additionally, the flushing of the soil enhances the natural geochemical dissolution reactions that reduce the amount of contaminant in the soil, and the mobility of the contaminant. Such geochemical dissolution reactions include adsorption/desorption, acid/base reaction, solution/precipitation reactions, oxidation/reduction reactions, ion pairing or complexation, and biodegredation. The soil flushing also increases natural transport mechanisms such as advection, dispersion, molecular diffusion, and depletion through volatilization. A fluid is injected through either spraying, surface flooding, subsurface injection, subsurface leach fields, or other means. The injected fluid mobilizes the contaminants, and then recovery is necessary. Figure 6 shows an example of a soil flushing injection scheme.
Soil flushing is either considered conventional or unconventional. Conventional techniques use only water as the flushing agent. Conventional applications include natural restoration, well and capture methods in the vadose zone, and pump and treat systems in the saturated zone. For the Hays site we will only be dealing with the vadose zone. In this case study it is necessary to include surfactants to the process water, to increase the solubility of the petroleum hydrocarbons. This suggests that an unconventional method should be used, either primary, secondary, or tertiary. Primary methods use the natural energy within the system, such as neutral water drive or gravity drainage. Secondary recovery methods include waterflooding and pressure maintenance techniques. Tertiary recovery methods include the injection of materials that are not normally found in the soil, either gaseous, chemical or thermal materials. The chemical and thermal methods may include the addition of polymers, surfactants, or alkaline agents to aid in the dissolution of the petroleum hydrocarbons from the soil.
Once the solution has flushed the contaminants to a separate location, the contaminated fluids must be recovered. If surfactants and nutrients have been included in the flushing water, passive bioremediation might occur, and the contamination may be left in place. This technology performs best in homogeneous, permeable sands. It is applicable for petroleum hydrocarbons. Unfortunately, there are some concerns with this method, including the environmental concerns with the injected solvents and high costs for implementation which will be discussed in Section 4.0.
Solvent/Chemical extraction (SCE) is an ex situ separation and concentration process in which a nonaqueous liquid reagent is used to remove contaminants from the soil. It is similar to soil washing, except that in this case concentrated chemical agents are used. Additionally, it is more effective than soil washing, because both physical and chemical remediation techniques are used.
At the start of the SCE process, the excavated contaminated soil enters a feed preparation system where the soil is screened, crushed, and further modified to enable the solvent to react appropriately. Next, a solvent is used to extract water and organics into the liquid phase. Typical solvents include alkanes, alcohols, ketones, or chlorinated solvents. The selection of extraction solvents, the solvent-to-solids ratio, the contact time, and the number of extraction stages will depend on which process is chosen and site specific characteristics. Next, the decontaminated solids are separated from the contaminated solvent by gravity separation, filtration, or centrifuging. After separation, the contaminated solvent is distilled or otherwise acted upon to recover the solvent. When SCE is complete, the soil has been separated into a treated fraction, a wastewater stream, and a concentrated contamination fraction. The concentrated contamination fraction may be subject to hazardous waste regulations. Figure 7 shows a simplified process flow for the solvent/chemical extraction process.
Seven SCE processes that have been developed include the basic extractive sludge treatment (B.E.S.T.), the CF systems organic extraction unit, the Dehydro-Tech's Carver Greenfield process, the Extraksol Process, the Low Energy Extraction Process (LEEP), the NuKEM development process (NKD), and the Soil Restoration Unit (S.R.U). The first three of these processes stated have been evaluated by the US EPA and were found to be effective on petroleum wastes when site specific characteristics are carefully considered.
The B.E.S.T. process uses triethylamine, a highly flammable substance, as the extraction solvent. This is an ex situ process, where the soil is crushed, screened, and pumped into a washer dryer unit. Upon completion of treatment, solids may be returned to land, while the water fraction may be discharged to publicly owned treatment works (POTW).
The CF Systems Process uses liquefied gases and supercritical fluids, usually liquified propane or propane and butane as the extraction solvent for contaminants in soils. This is also an ex situ process. The contaminated soil is put into a high-pressure contactor, where compressed propane is pushed upward through the soil. The contactor acts as a separator, and the clean soil is returned to the land, while the contaminants are otherwise disposed according to RCRA regulations. One drawback to this process is that propane is highly combustible.
The Carver-Greenfield Process first screens the soil to remove debris, and then grinds the soil so that the largest particle size is 6 mm. The soil is then turned into a slurry by adding the extraction solvent. Next, the water in the slurry is evaporated, condensed, and centrifuged. The contaminants are then sent to an oil/water separator. Some drawbacks to this method are the environmental impacts due to air emissions, dust releases, and hazards in transporting materials.
Bioremediation is the remediation technology recommended by Weston and Baker Environmental, Inc. in the Environmental Baseline Study for the Hays site. There are three forms of bioremediation: in situ, slurry phase treatment, and solid waste treatment, which includes prepared bed treatment and composting. The microbial processes are basically the same for each of these bioremedation technologies. The only differences are the mechanisms by which the nutrients, water, and oxygen are administered.
Biodegredation is a naturally occurring process in which microorganisms use organic compounds as an energy source. The microorganisms convert the organic carbons in petroleum products, into an inorganic carbon, such as carbon dioxide. Usually the microorganisms in the soil are sufficient to degrade the waste, although many companies sell microorganisms which they claim enhance the degrataion. To help microorganisms perform degradation, non-toxic conditions must exist, organisms must develop the enzyme systems capable of degrading the organic compounds, and proper nutrients must be supplied to the microorganisms to provide optimal metabolism and growth. Additional factors that affect biodegredation rates are: oxygen levels, temperature, pH, and amount of water in the soil.
In situ bioremediation occurs on the site of contamination. It is practiced in the aerobic mode, and therefore requires oxygen and nutrients to be supplied to the contaminated subsurface. Oxygen is the rate limiting factor in operating in situ bioremediation. Some aerobic bacteria produce an enzyme which releases the oxygen from its chemical bond to support aerobic biodegredation.
In slurry-phase treatment, the contaminated soil is turned into a slurry by suspending the soil with water in a mixed reactor. The agitation of the slurry causes the breakdown of solid particles, desorption of contamination from solid particles, contact between organic waste and microorganisms, oxygenation of the slurry by aeration, and the volatilization of contaminants. The effectiveness of a slurry phase treatment system depends on the pretreatment, desorption, solids concentration, mixer design and retention time. The contaminated soil is usually pretreated to enhance desorption by adding surfactants and reducing soil particle size. Additionally, the concentration of waste to be treated can be increased by fractionating the soil. Next, the petroleum hydrocarbons must be desorbed from the soil particles. The concentration of solids, design of the mixer, and the length of time which the contaminated soil remains in the mixer, also affect the reaction rate.
Solid phase treatment is a category which encompasses land treatment, composting, and heaping. Land treatment involves the uniform application of contaminated soil to the land at a controlled rate. The contaminated soil is then mixed with the surface soil by spreading the waste over the surface or injecting just below the surface. This technique utilizes the natural physical, chemical and biological systems to degrade the contaminants in the soil. Usually, adjustments are made to stimulate bacterial growth, such as aeration, pH adjustment, and nutrient addition. Additionally, the soil is cultivated to mix the soil with the waste, and to provide the necessary aeration. In solid phase treatment there are three types of limiting factors: capacity limiting, rate limiting, and application limiting. Capacity limiting addresses the amount of immobilized materials that are introduced. Rate limiting refers to the amount of contaminated soil that is applied to the plot per given time period, which must not exceed the degradation rate. The application limit addresses the rate of volatilization and application of water and nutrients.
Bioremediation is a technology that is applicable to the remediation of the petroleum hydrocarbons located at the Hays remediation site. It may be applied to the site as an ex situ or in situ process.
The selection of the best technology depends on many factors. It has been determined that the possible remediation technologies for the Hays site include vacuum vapor extraction, soil washing, soil flushing, solvent/chemical extraction and bioremediation. Once the possible alternatives have been developed it is necessary to compare them by various metrics. These metrics include cost, duration of project, and probability of success. These metrics are dependent on specific site characteristics. A decision can then be made based on which factors are most important to the client (GalvTech), and which technologies satisfy PADEP regulations.
The cost of a remediation technology depends on many factors. If the process is an ex situ process, the cost of excavation must be considered. This could be a very expensive cost because approximately 33,750 ft3 of soil require excavation. For an in situ remediation technology, the cost of cleanup includes the capital costs of design, construction, and installation. Both in situ and ex situ technologies must consider the costs of operation and maintenance, monitoring, and utilities.
The duration of the remediation technology will be evaluated on whether it takes more than six months to reach the target contamination level. The probability of success of a technology will be defined as either high, medium or low based on technology specific considerations. This comparative analysis is summarized in Table 1.
Because vapor extraction is set up differently depending on the characteristics of a site, it is difficult to predict the cost of this technology. Due to the fact that the contaminated soil at the Hays site consists only of surface soil to one foot of depth, it is not worthwhile to install an in situ vapor extraction system. Therefore, for an ex situ vacuum vapor extraction system, the cost of excavation must be considered. The cost of ex situ vapor extraction is estimated to be about $100,000 per 230 L/sec capacity. Operation and maintenance costs are estimated at $1,250 per month (Anderson, p. 4.6) The time required for remediation is dependent on the specific characteristics of the soil, and the extent of sorption of the petroleum hydrocarbons in the soil particles. This technology is not recommended for the Hays site do to the complexity of in situ vapor extraction.
Soil washing is the second possible remediation technology considered for the Hays site. It has an estimated cost of $150 to $250 a ton, plus excavation costs (Anderson, p. 5.4). Excavation costs may range from $30 to $100 per cubic yard which corresponds to $37,500 to $125,000. For the Hays site, where it is necessary to remediate 1,224 to 1,836 tons of soil, the cost of remediation is $183,600 to $459,000. The total cost is estimated to be $221,100 to $584,000. This process would take more than six months. Although success is related to site specific correlations, there is a high probability of success for this technology at this site.
Although soil washing may seem like an ideal, cost effective method of remediation, there are a few limitations. Contaminants should be uniform throughout the soil. The soil should have low percentages of fine grains, and low humic contents. Additionally there must be a source of process water and electric power. The applicability of soil washing is based on three characteristics, the soil's mix of contaminants, particle size distribution, and specific gravity. This alternative is a possibility for the remediation of the Hays site; however, it takes an extended period of time.
Soil flushing is very similar to soil washing except it is performed in situ. This prevents the need for excavation and hence eliminates that expense. The estimated cost of this technology is $79 to $165 per cubic yard (Anderson, p. 5.7). The volume of contaminated soil at the Hays site is approximately 33,750 ft3, which correlates to 1,875 yd3. For this volume of soil, the estimated cost of soil flushing is $148,125 to $309,375.
Although soil flushing seems like a suitable method of remediation, it does have its limitations. Removal efficiency can be affected by low permeability, low hydraulic conductivity, and a lack of process water. Also, the surfactants may become ineffective due to hardness in process water, high clay content, or biodegradation of the surfactant. An additional complication is the disposal of the contaminated elutriate (the used washwater). Additionally, there are potential environmental consequences to be considered because soil flushing increases contaminant mobility.
The cleanup time of this technology depends on site-specific soil and contaminant characteristics. Other concerns include the fact that the contamination for this specific site is in the vadose zone. Before soil flushing can be effective, the soil must be saturated, and transport predictions through the vadose zone is not reliable. Therefore, this process is not recommended for the Hays site.
Solvent/Chemical extraction is always an ex situ process, therefore it is again necessary to consider excavation costs. Costs have been estimated at $95 to $700 a ton (Anderson, p. 5.2). Costs for the Hays site would be approximately $116,280 to $1,285,200, plus excavation costs of $37,500 to $125,000. This corresponds to a possible total cost of $153,780 to $1,410,200. These costs are highly variable due to the fact that there are many different extraction technologies. The chemicals used in this process are highly toxic, therefore environmental disposal of these materials must be considered. This process is not recommended for the Hays site due to the unnecessarily high cost of cleanup.
Bioremediation is the recommended technology for the Hays site. It has variable costs depending on what type of bioremediation is used: in situ, slurry phase treatment, or solid waste treatment. In situ treatment would avoid the cost of excavation. An average estimate for remediation, not include excavation, is $150 to $200 per ton, which correlates to $183,600 to $367,200 for the Hays site. It is possible to complete the remediation in less than 6 months, which is an advantage for the client.
Costs and duration greatly vary for the different technologies depending on site specific characteristics. Because of these variations, it is difficult to make a recommendation for the Hays site based primarily on cost and duration of the project. Regardless, it should be considered that ex situ remediation will include an additional cost for excavation. Due to this variation in cost and duration, the recommendation for the remediation of the Hays site includes consideration of the simplicity of the alternative technologies. Due to the avoided excavation costs, the possible short duration of remediation, and the simplicity of the technology, in situ bioremediation is recommended for the Hays site.
Anderson, William C., ed., Innovative Site Remediation Technology: Soil Washing/ Soil Flushing, United States: Library of Congress, 1995.
Anderson, William C., ed., Innovative Site Remediation Technology: Solvent/ Chemical Extraction, United States: Library of Congress, 1995.
Anderson, William C., ed., Innovative Site Remediation Technology: Vacuum Vapor Extraction, United States: Library of Congress, 1995.
Anderson, William C., ed., Innovative Site Remediation Technology: Bioremediation. United States, Library of Congress, 1995.
Daniel, David E., Geotechnical Practice for Waste Disposal, London: Chapman & Hall, 1993.
LaGrega, Michael D.; Buckingham, Philip L.; Evans, Jeffrey C., Hazardous Waste Management, New York: McGraw-Hill, Inc., 1994.
Naval Facilities Engineering Command, Soil Mechanics, Design Manual 7.01, Alexandria Virginia: U.S. Navy, 1986.
Technology Assessment Board of the 103d Congress, State of the States on Brownfields Programs for Cleanup and Reuse of Contaminated Sites, Office of Technology Assessment, 1995.
Weston and Baker Environmental, Inc., Environmental Baseline Study (EBS), 1991.
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Last Updated: August 24, 1998