The International Information Center for Geotechnical Engineers

Soil Washing

Field Setup

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.

 

quarry2

 

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.

 Fig 3

Figure 7: General Soil Washing Schematic (from US EPA 1993)

 


Different Soil Washing System Designs

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).

Fig 4

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).

 

 Fig 5
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).

Fig 6Figure 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.

  Fig 7

Figure 11: Deconterra Process Flow Sheet (from USEPA 1993)

 

Add comment

NOTE: The symbol < is not allowed in comments. If you use it, the comment will not be published correctly.

Security code
Refresh
*Please insert the above-shown characters in the field below.

The Geoengineer.org Corporate Sponsors: