Ex situ separation can be performed by many processes. Gravity separation and sieving/physical separation are two well-developed processes that have long been primary methods for treating municipal wastewaters. Magnetic separation, on the other hand, is a much newer separation process that is still being tested.
Gravity separation is a solid/liquid separation process, which relies on a density difference between the phases. Equipment size and effectiveness of gravity separation depends on the solids settling velocity, which is a function of the particles size, density difference, fluid viscosity, and particle concentration (hindered settling). Gravity separation is also used for removing immiscible oil phases, and for classification where particles of different sizes are separated. It is often preceded by coagulation and flocculation to increase particle size, thereby allowing removal of fine particles.
Magnetic separation is used to extract slightly magnetic radioactive particles from host materials such as water, soil, or air. All uranium and plutonium compounds are slightly magnetic while most host materials are nonmagnetic. The process operates by passing contaminated fluid or slurry through a magnetized volume. The magnetized volume contains a magnetic matrix material such as steel wool that extracts the slightly magnetic contamination particles from the slurry.
Sieving and physical separation processes use different size sieves and screens to effectively concentrate contaminants into smaller volumes. Physical separation is based on the fact that most organic and inorganic contaminants tend to bind, either chemically or physically, to the fine (i.e., clay and silt) fraction of a soil. The clay and silt soil particles are, in turn, physically bound to the coarser sand and gravel particles by compaction and adhesion. Thus, separating the fine clay and silt particles from the coarser sand and gravel soil particles would effectively concentrate the contaminants into a smaller volume of soil that could then be further treated or disposed.
The target contaminant groups for ex situ separation processes are SVOCs, fuels, and inorganics (including radionuclides). The technologies can be used on selected VOCs and pesticides. Magnetic separation is specifically used on heavy metals, radionuclides, and magnetic radioactive particles, such as uranium and plutonium compounds.
Physical separation often precedes chemical extraction treatment based on the assumption that most of the contamination is tied to the finer soil fraction, which alone may need to be treated. Separation is also useful when heavy metal contaminants occur as particulates (e.g., in small-arms ranges). One advantage of physical separation processes is that high throughputs can be achieved with relatively small equipment.
Factors that may limit the applicability and effectiveness of these processes include:
- High clay and moisture content will increase treatment cost.
- Gravity separation processes rely on a difference in the solids and liquid phase densities. Specific gravity of particles will effect settling rate and process efficiency. Additionally, settling velocity is dependent on the viscosity of the suspending fluid, which must be known to estimate process efficiency and to size equipment.
- Special measures may be required to mitigate odor problems, resulting from organic sludge that undergoes septic conditions.
In addition, particle size distribution; soil type, physical form, handling properties, and moisture content; contaminant type and concentration; texture; and organic content need to be investigated.
Gravity separation and sieving/physical separation are full-scale, well-established technologies used mostly for treatment of wastewater and contaminated soil, sediment, and sludge. Magnetic separation is a promising new technique used to remove radioactive contaminants from soils. It has recently been tested at the bench-scale level at DOE sites.
Additional cost information can be found in the Hazardous, Toxic, and Radioactive Wastes (HTRW) Historical Cost Analysis System (HCAS) developed by Environmental Historical Cost Committee of Interagency Cost Estimation Group.