9    DEVELOPMENT OF A SOIL WASHING SYSTEM
Steven B. Valine, Project Engineer
Dennis D. Chilcote, Vice President
Engineering BioTrol, Inc.
Chaska, Minnesota 55318
Adolfo R. Zambrano, Assistant Director
Hanna Research Center
Nashwauk, Minnesota 55769
INTRODUCTION
Developing an economical method to remediate soils requires a process development effort comparable to engineering developments in other fields. For soils contaminated with hazardous or toxic
materials, the development of a commercial cleanup process is a stepwise effort which includes:
• characterizing the soil
• developing a conceptual process flowsheet
• designing and conducting bench-scale verification tests
• completing a preliminary economic feasibility study
• designing and constructing a pilot facility
• conducting the pilot study
This process development approach has been used to engineer a soils washing system for contaminated soil. Following this pathway to developing a system results in several side benefits, including
intimate knowledge of the fundamentals of the operation and a valuable set of bench-scale test
protocols which can be used to model the performance of the field operation. Since site conditions are
expected to vary considerably, system flexibility is mandatory. A good basic understanding of both
the physico-chemical processes that are being utilized in the remediation effort and the process
equipment that is used to implement this technology is one key element in the design of a flexible
system. This paper describes the process of developing a soil washing system.
CHARACTERIZATION OF THE SOIL
Soils, including soils from hazardous waste sites, are characterized by a distribution of particle
sizes. If the soil is separated according to size, one often finds that the finest fractions (silts and clays)
contain the highest concentrations of contaminants. The finest fractions of soil have the highest
surface area per unit volume, and thus are most favored for adsorption-type phenomena. In addition,
the fine fraction of soil usually contains the natural organic component of soil, which could serve as a
sink for organic contaminants.
Somewhat coarser soil particles (in the range of -10 mesh to + 200 mesh) are often characterized by
surface irregularities enhanced by weathering, inorganic salt precipitation, and oxide formation. This
uneven and somewhat porous surface can provide a favorable environment for surface
contamination.
Very coarse particles (pebbles and stones) have a relatively low surface area to volume ratio per unit
mass. As long as this material is not porous, contamination is surficial and the effective concentration
per unit mass of material tends to be low.
Contaminated soils are often composed of coarse and fine mineral components and natural organic
components. In addition, the distribution of contaminants as a function of particle size can vary
significantly in soil from a contaminated site. Table I shows data for the distribution of particle sizes
and the concentration of pentachlorophenol (penta) relative to particle size for soil from a Superfund
site that currently supports an active wood treating operation. In this case, the higher concentration of
44th Purdue Industrial Waste Conference Proceedings, © 1990 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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