Soil washing is an ex-situ remediation technique that removes hazardous contaminants from soil by washing the soil with a liquid (often with a chemical additive), scrubbing the soil, and then separating the clean soils from contaminated soil and washwater (US EPA 1993, 1996). The concept of soil washing is based on the theory that contaminants are prone to bind to fine grained soils (silts and clays), which, in turn, are prone to bind to coarse grained soils. Therefore the main goal of soil washing is to separate these contaminated fines and washwater from the cleaned coarse grained soils (sands and gravels). The contaminated fines and washwater can then be treated or disposed of as necessary. The washed soils may be reused as backfill at the site if all contaminants are removed from the soil. The process of soil washing significantly reduces the volume of contaminated soil at a site, often making soil washing a pretreatment step for a different remediation technique (US EPA 1996, Sharma and Reddy 2004). Soil washing can be broken into six different steps (US EPA 1993):
The following summarizes the process of soil washing, which is depicted in Figures 1-4 below.
Figure 1: Schematic of Soil Washing Process (from US EPA 1996)
Figure 2: Aerial View of Soil Washing Plant (from ART Engineering)
Figure 3: Typical Soil Washing Equipment (from ART Engineering)
Figure 4: Separation Step (from ART Engineering)
Pretreatment is completed after the excavated soil has been placed in a staging area and before it can be washed through a mechanical process. In this step, large objects are removed from the soil so that a homogenous (diameters less than 2 inches) soil is prepared for the washing step. Removal is done through scalping, mechanical screening, and jigging and tabling. The oversized materials can consist of anything from construction debris to large pieces of rock or gravels. These materials are usually not contaminated; however, if treatment is necessary crushing and grinding may be needed to reduce the size of the materials (US EPA 1993, Griffiths 1995).
Once the soil has been pretreated, it is ready to be washed in the soil scrubbing unit. Separation of the coarse and fine grained materials occurs in this unit. Since the coarse grained soils are likely not contaminated, they are removed from the washing unit. The particle size cut point is usually between 63 (#230 sieve) and 74 microns (#200 sieve). Separation is done since the coarse and fine grained soils will require different final cleaning procedures. Separation of the coarse grained soils is commonly done by using mechanical screening such as trommels, while the fine grained soils are sorted out by hydrocyclones or other methods (US EPA 1993, Griffiths 1995). Figure 5 below shows an example of the EPA mobile soil-washing system. The separation of the coarse fraction as well as the use of hydrocyclones is depicted.
Figure 5: EPA mobile soil-washing system (from Griffiths 1995)
Once the coarse grained soils have been separated and removed from the scrubbing unit they may require additional treatment if contaminants have absorbed onto the soils. Common treatment methods include:
The water (and any fine materials that were found in the coarse fraction) that is still in the coarse grained soils is removed and then added back into the system so that it can be treated along with the fine grained soils (US EPA 1993).
Contaminants are predominantly found in the fine grained soils. Chemical additives are often added to solution during the scrubbing process in order to clean the soils (as shown above in Figure 5). The soils are mixed vigorously with the solution and then settled out (US EPA 1993).
The washwater used in the soil scrubbing unit will be contaminated and must be treated. Some of the contaminants that could be present in this washwater include (US EPA 1993):
Treatment must be done for the washwater in order to be reused in the soil washing process or to be disposed of in sewers (although disposing requirements are more stringent, making the recycling of the water into the system the preferred choice as long as it does not interfere with the washing process). The most common types of treatment of the washwater are (US EPA 1993):
The amount of residual material that is output during a soil washing process depends upon the grain size distribution of the original material. Contaminated fine grained soils and sludges may be disposed of in a landfill or, if they are still considered contaminated by regulations, may require further treatment before disposal. This further treatment could include (US EPA 1993):
Contaminated feed material could contain leaves, twigs, roots, or grass that must be removed since it is likely contaminated due to the vegetative materials porous and adsorptive behavior. The outcome of soil washing will also produce clean soils that can be used as backfill at the site. These soils may require additional washing or cleaning prior to placement (US EPA 1993).
Overall, there are many different soil washing systems that have been developed and the systems can vary from site to site due to location specific constraints for soils or contaminants. Soil washing is often also used in conjunction with another remediation technique since it can be used effectively to concentrate the contaminants down into a smaller volume of soil which then could be easily treated by another technique (US EPA 1996, US EPA 1993, Sharma and Reddy 2004).
Soil washing theory is based upon the physiochemical processes that occur between the solid particles of soil and the solution in which they are dispersed (Sharma and Reddy 2004). In general, soil washing is based on the principle that contaminants are likely to adhere to the fine grained soils, which, in turn, are likely to adhere to the coarse grained soils (through adhesion and compaction). Washing with water and possibly additives allows the coarse grained soils to separate from the contaminated fines. Several physiochemical processes such as desorption, complexation, dissolution/solubilization, and oxidation reduction are involved in soil washing (Sharma and Reddy 2004). Desorption occurs in soil washing when the washwater (and associated additives) is mixed with the soil. During this stage, the contaminants are desorbed from the soil particle surface. Dissolution or solubilization of the contaminants can occur due to pH changes that result from acid-base reactions of the washwater. The washwater may also cause the formation of complexes with the contaminants (which may be soluble). Oxidation-reduction reactions may also be initiated by the washwater, resulting in desorption or solubilization of the contaminants. The goal of soil washing is to effectively separate the coarse grained soils from the contaminated fine grained soils, and to remove the contaminants from the fines into the washing solution (Sharma and Reddy 2004). The contaminants may not be completely removed from the soil though. An equation that relates the contaminant concentration in soil washing is:
where Csi is the initial concentration of the contaminant in the soil (mg/kg), Ms is the total dry mass of the soil (kg), Csf is the final concentration of the contaminant in the soil after washing (mg/kg), Vl is the total volume of the washing solution (L), and Cl is the concentration of the contaminant in the solution (mg/L). After adequate washing time, the system will reach equilibrium and the distribution coefficient of the contaminant between the soil and the washing solution (Kd) is applicable (Sharma and Reddy 2004):
Equation 2 can be substituted into Equation 1 to find the removal efficiency, which is given by:
These equations are used to determine the effectiveness of soil washing at a site (Sharma and Reddy 2004).
Soil washing can effectively treat soils that are contaminated with different organic and inorganic contaminants. Soil washing has been proven to effectively remove the following contaminants from soil (US EPA 1993):
Studies have indicated that soil washing is good to excellent at removing volatile organic compounds (VOCs) and metals from sandy and gravelly soils (Sharma and Reddy 2004). A key factor in determining the applicability of soil washing at a particular site is the grain size distribution of the soils requiring treatment. The lower the silt, clay, and organic material levels, the more effective soil washing will be (soils with higher hydraulic conductivities work best). Soil washing may not be applicable if the contaminants adsorb strongly onto the soil particles, since the washing process is not always able to fully remove the contaminants from the soil surface. This situation would require an additional remediation technique to fully clean the soil. The use of soil washing as a preliminary technique is common, since it is able to reduce the volume of soils requiring treatment. Soil washing tends to be applicable at large sites, and is typically not cost effective unless there is at least 5000 tons of contaminated soil on-site (Sharma and Reddy 2004).
There are several advantages associated with using soil washing as a remediation technique. As a volume reduction technique, soil washing is very cost effective when it can reduce the amount of soil that needs further treatment or disposal. Soil washing, when performed under ideal conditions, can lead to a volume reduction of approximately 90% of the originally contaminated soil (Sharma and Reddy 2004). Since soil washing is performed on-site, the large volume of soil that is not contaminated after washing can be reused as backfill at the site.
Additionally, soil washing is performed on site in a closed system where the conditions, such as pH level and temperature, of the soil being treated can be controlled and closely monitored (Sharma and Reddy 2004). This on-site system saves money and time and generally the process can be run at a very high rate of around 100 cubic yards per day (US EPA 1996). The process also can remove a range of contaminants, both organic and inorganic, from the soil at the same time. Soil washing also only requires a few permits in order for it to be used, making it a relatively easy method to employ (Sharma and Reddy 2004).
There are also several disadvantages associated with using soil washing as a remediation technique. Soil washing requires a large area in order to set up the system (described in the following section). This could be an issue based upon the setting of the site. Soil washing is also predominantly effective with soils that are very coarse. The higher the percentage of coarse grains the more successful soil washing will be at remediating a contaminated site. This means that at very silty or clayey sites soil washing will not work very well as it will not reduce the volume of contaminated soils quickly as it would with a gravely soil. Further treatment of the soil through other more expensive treatment methods would then be necessary and the use of soil washing under these circumstances might not save any time or money. In general, soil washing is ineffective for soils containing more than 30 to 50% silt, clay or organic matter (Sharma and Reddy 2004, US EPA 1993).
Another disadvantage is that the used wastewater, which often contains chemical additives, may need specialized treatment which is generally difficult to do and expensive. At the end of the process there may also remain small volumes of contaminated sludge that require further treatment or disposal off site. Air emissions from cleaning equipment are another factor that can also increase the cost of the operation while reducing its appeal. Finally, exposure of the public to contaminants is also a concern as all the contaminated soil is being excavated and handled ex situ (Sharma and Reddy 2004). All of these factors will need to be accounted for before deciding upon soil washing as a part of the remediation process of choice.
The exact setup of a soil washing system can vary greatly depending on the needs of a particular remediation site. With this said, the exact field setup will greatly vary from project to project. Some of the fundamental equipment used in typical setups have been discussed earlier and numerous combinations of the aforementioned equipment are possible.
However, the largest factor which will need to be considered is how large of a footprint the soil washing system will take up. In general, the space requirement needed for a typical plant will range from approximately 100 x 200 ft to 125 x 250 ft depending on if it is a plant that can process 25 or 50 tons/hr, respectively (US EPA 1993). This area is the total space needed for all the necessary equipment for a washing plant as well as the space for soil and contaminant piles, as shown in Figure 6. It is possible to have plants that can clean soil at an even higher rate, though they will generally need an even larger footprint. There are also smaller systems available if space restrictions are an important factor. Although these smaller plants will clean the soil at a much slower rate which could greatly reduce the effectiveness of soil washing as a remediation technique.
Figure 6: Example of a Soil Washing Plant (from Aggregate Processing Solutions)
In summary the set up and system used at each site will likely vary greatly since many systems are available and may be custom tailored for site specific use. A schematic of a general soil washing system is shown in Figure 7 below. In addition to the mechanical equipment used for washing, construction equipment such as excavators and front-end loaders will be needed to move the soils. The processes shown in Figure 7 were described in the Main Concept and Description section above.
Figure 7: General Soil Washing Schematic (from US EPA 1993)
While Figure 7 shows the general schematic for a soil washing system, there are numerous variations and alterations possible depending on the differing remediation conditions.
The Harbauer soil washing system is shown in Figure 8 and is an example of a closed system that treats and reuses its washwater. This reduces the amount of washwater that needs to be disposed of or treated off site (US EPA 1993).
Figure 8: Harbauer soil washing system (from US EPA 1993)
The mobile soil washing system, shown in Figure 9, is an option that emphasizes the treatment of sands. It is referred to as mobile since it is installed on two trucks and it is used to determine if sands can be cleaned adequately with only the use of soil washing (US EPA 1993). It uses a countercurrent operation as well, seen in Cells 1-4, which clean the smaller particles much more aggressively than a typical soil washing system, resulting in more contaminant ending up in the wash fluid as it gets reused. Starting in Cell 4, the hydrocyclones separate the solids from the liquids, passing solids forward to Cell 3 and removing the fluids from the system. Clean wash fluid is being added to Cell 1 during this time, so as the solids work their way forward to Cell 1 they get progressively cleaner and are washed in progressively cleaner wash fluid. Therefore, ideally as they exit the hydrocyclone of Cell 1 they are clean and ready to be reused on site. This setup also results in heavily contaminated washwater leaving the system from Cell 4 and this liquid must be tested to see what additional steps need to be taken before reuse or disposal of the wastewater (US EPA 1993).
Figure 9: Mobile soil washing system (from USEPA 1993)
The use of flotation cells and flocculation is another means of separating contaminants out from a soil. An example schematic of this system is shown in Figure 10. The soil first needs to be sized, through any of the means seen in earlier discussed systems. The next step, which is crucial for the success of the washing process, is the choice of a reagent which will react with the contaminant and cause it to rise to the surface in a frothy state. With the success of that step, the contaminants will float on top of the washwater. The contaminants are then skimmed off easily and passed along to plate and frame filters, which are used to dewater the froth, resulting in a solid contaminant that can be disposed of easily. The rest of the washwater, with the now clean soil, has a flocculant added to it. This mixture is then sent to a thickening tank where the soil accumulates together and is dewatered by a plate and frame filter. This process yields a clean, water free soil that is ready for reuse (US EPA 1993).
Figure 10: Soil Washing Process Including Flotation Cells (from USEPA 1993)
For more heavily contaminated soils with larger solid masses, like rubble or other difficult to break down debris, the Deconterra soil washing process shown in Figure 11 is a useful option. The use of a crusher and multiple screens reduce the large clumps into their smaller constituents. This abrasion combined with flotation and washwater treatment cycles results in a system that is complex and costly yet has the potential to be able to efficiently treat soils with a wide range of particle sizes (US EPA 1993).
With this brief overview of different soil washing systems it is clear that each different system benefits from being customized to the specific remediation project at hand. Selection of the process most appropriate to the project will not only save money but will greatly increase the effectiveness of soil washing as a volume reduction and remediation technique.
Figure 11: Deconterra Process Flow Sheet (from USEPA 1993)
The average cost of a soil washing project is approximately $150 to $250 per ton (US EPA 1993). There are several costs associated with soil washing that depend on specific site requirements and remediation goals. The costs associated with soil washing can be broken into the following categories (CL:AIRE 2007):
Initial costs associated with soil washing could include bench scale tests and treatability studies prior to site implementation. Set-up and break down of the soil washing system could include permitting, design, infrastructure, transport, and commissioning costs. Costs associated with operation of the soil washing system could include: shift workers and labor costs, equipment purchasing or renting, fuel, electricity, chemicals, storage containers, and safety equipment. Chemical analysis and disposal may also add to the overall cost depending upon project specifications (CL:AIRE 2007). Figure 12 below was taken from an EPA publication that used the Remedial Action Cost Engineering and Requirements (RACER) software (FDTR 2006). The figure shows the breakdown of costs for soil washing projects of two different sizes. The cost of soil washing decreases significantly with increasing volume (for the table shown it decreases from $142 to $53 per cubic yard, which makes soil washing much more cost effective for large projects (FDTR 2006). This reduction in cost is likely due to the set-up, design, and purchasing of the equipment and remediation system.
Figure 12: Soil Washing Cost Comparison (from FTDR 2006)
Between 1984 and 2008, soil washing was chosen as the remediation technique in 1% (or 6 projects) of US EPA Superfund projects (US EPA 2010). Therefore, it is not as commonly chosen as other remedy techniques such as bioremediation or soil vapor extraction, but given certain site conditions that allow for soil washing it can be an effective technique (US EPA 2010). The use of soil washing will be reviewed for three case histories: the Twin Cities Army Ammunition Plant (Fristad 1995), the RMI Titanium Company (US DOE 1998), and the King of Prussia Technical Corporation (US EPA 1983, 1995). These case histories are important to examine since they demonstrate the full-scale applicability of soil washing for remediation.
The Twin Cities Army Ammunition Plant is located in New Brighton, Minnesota and is approximately 2,370 acres. The plant opened in 1941 and produced different types of ammunition for the United States during many wars. In 1982, the site was added to the National Priorities list for remediation. An ammunition test burn area of the site required remediation due to high levels of VOCs and heavy metals. A combination of soil leaching (where the metals could be removed and recycled) and soil washing was used to successfully remediate the site. The full scale system implemented at the site to treat more than 20,000 tons of soil is shown below in Figure 13. After examining the site it was determined that most of the contaminated soil was within the top 1 to 2 feet of soil; however, 16 trenches were excavated to a depth of 15 feet. X-ray fluorescence was used to guide the excavation. The soil was tested for treatment effectiveness after every 10 tons. The treatment of the soil occurred from Fall 1993 to Summer of 1995. The soil washing and leaching system was located approximately 100 yards from the excavation area, and was located on an existing concrete pad. The heavy metals were successfully leached out using different agents, and the soil was successfully washed in order to reuse it as backfill at the site. Overall, eight heavy metals were removed from the site and clean up criteria of less than 175 ppm were met. Initial levels of the heavy metals were as high as 86,000 ppm for lead and 100,000 ppm for copper (the average of all the heavy metals was 1,600 ppm). The innovative system allowed for all the soil to be reused on the site, no wastewater to be generated, and the heavy metals to be recycled (Fristad 1995).
Figure 13: Schematic of Soil Leaching and Washing System (from Fristad 1995)
The RMI Titanium Company (RMI) of Ashtabula, Ohio operated from 1962 to 1988. During this time they performed uranium extrusion for the Department of Energy (DOE) which included working with both depleted and slightly enriched material. During the extrusion processes uranium particles ended up escaping the facility and settling on the surrounding soils in concentrations of up to 300 picocuries per gram (pCi/g) which is much greater than the total allowable limit of 30 pCi/g (USDOE 1998). As the DOE had contracted the work out to RMI, the DOE was held responsible for the site remediation and funded the RMI Decommissioning Project. In order to determine the feasibility of soil washing as a remediation strategy RMI Environmental Services (RMIES) and Alternative Remedial Technologies, Inc. (ART) were hired to conduct pilot tests on about 64 tons of soil. They set up a small scale soil washing system and proved that through a chemical leaching process using sodium carbonate extraction they were able to remove enough uranium to achieve the allowable limit of 30 pCi/g for almost all of the soil that was processed (USDOE 1998). Their treatment system consisted of a rotary batch reactor, screens, thickeners, and a filter press. The pilot plant operated from January 7, 1997 until February 14, 1997 while they processed the 64 tons of soil. The end result showed that the process was very successful at removing the uranium contaminant with removal efficiencies of 82% and a volume reduction of 95% (USDOE 1998). Not only did soil washing prove to be an incredibly efficient method but it also proved to be incredibly cost effective. The projected cost for soil washing all 20,000 tons of contaminated soil was approximately $325/ton while remediating the soil through excavation, transportation, burial and soil replacement was projected to cost $857/ton clearly making soil washing an incredibly cost effective remediation technique for the RMI site (USDOE 1998).
The King of Prussia Technical Corporation site is located in Winslow Township, New Jersey, and was used for waste processing from January 1971 to April 1974. It was estimated that 15 million gallons of liquid industrial waste were processed in six waste lagoons. On September 28, 1990 a Record of Decision (ROD) was issued for the site to clean up contaminated soils and sludge. Soil washing was chosen as the remediation technique to clean 19,200 tons of contaminated soil and sludge at the site. The full-scale soil washing unit was operated from June to October 1993. The primary contaminants on-site were nickel, chromium, and copper. Prior to treatment, the concentrations of the contaminants in the soil were 8,010 mg/kg for chromium, 9,070 mg/kg for copper, and 387 mg/kg for nickel. After treatment these concentration values were reduced to 660 mg/kg, 860 mg/kg, and 330 mg/kg, respectively (US EPA 1995). These values met the cleanup goals. The soil washing unit that was used was owned and operated by Alternative Remedial Technologies, Inc. (ART), and was designed to maintain a throughput of 25 ton/hour of soil. The system included a series of hydrocyclones, conditioners, and froth flotation cells, as depicted in Figure 14. After washing, the clean sand was reused as backfill at the site and the contaminated sludge was disposed of off-site. The in-situ soil on site was ideal for soil washing since it had an acceptable level of sand content, as shown in Figure 15, which depicts the particle size distribution of the soil. X-ray fluorescence was used to identify contaminated soils for selective excavation. On-site monitoring was also used to confirm contamination. Overall, the King of Prussia site was the first full-scale application of soil washing to remediate a Superfund site in the United States. Figures 16 and 17 show some photographs of the soil washing project site. Figure 16 depicts the site as well as the overall size of the soil washing system and the loading of the initial screening apparatus. Figure 17 depicts the soil screening, hydrocyclone usage, and sand treatment by flotation. The total cost of treatment, including soil washing and disposal off-site, was approximately 7.7 million dollars (US EPA 1983, 1995).
Figure 14: Soil Washing Unit used at King of Prussia Site (from US EPA 1995)
Figure 15: Particle Size Distribution of in-situ soils at King of Prussia Site (from US EPA1995)
Figure 16: King of Prussia Site Pictures I (from ART Engineering)
Figure 17: King of Prussia Site Pictures II (from ART Engineering)