groundwater and soil remediation (process design and cost estimating of proven technologies) || soil...

Chapter 10

Soil Washing

Soil flushing and soil washing remove contaminants from the outer surfaces andpores of soil particles. Soil flushing is an in situ process by which water in largequantities is infiltrated into the soil and recovered by extraction from trenches orwells for subsequent processing. Soil washing is an ex situ process, using water(usually with added agents) or a solvent to remove contaminants from excavated soil.Additives can be used to enhance removal of contaminants from the soil with eitherin situ soil flushing or soil washing. Water is used in two processes:

• Separation of coarse soil particles from fines• Removal of contaminants from soil particle surfaces

Water may also be used to slurry the soil so that it can be conveyed by pumping. Thetechnology can be applied for the aqueous removal of either metals or organiccontaminants. Alternatively, solvents are used to extract organics from contaminatedmedia. This chapter covers in situ soil flushing and ex situ soil washing and solventextraction.

The US EPA (1989a) reports removal efficiencies from four different soil washingsystems for coarse-grained soil fractions, as given in Table 10-1.

Table 10-1 Contaminant removal efficiencies with soil washing.

ContaminantMineral oilCyanides

ZincCadmium

NickelLead

Removal Efficiency(%)9894

67 to 8392

66 to 8975

ContaminantAromatics

PAHsCrude oil

HydrocarbonsChlorinated hydrocarbons

Phenol

Removal Efficiency(%)

81 to 99.8959796100100

Note that for the organic compounds, biodegradation may account for some of thereported removal efficiency. For soluble organics, such as phenol, removal tonondetect levels is expected. Part II of Table 3 in the US EPA (1990c) gives thepercentage of contaminant removal for a number of United States and European soilwashing schemes that have been marketed or pilot tested for sandy soils. Some of the

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reported removal efficiencies and residual contaminant concentrations are as given inTable 10-2.

Table 10-2 Contaminant removal efficiencies and residual concentrations.

ContaminantOil & GreasePCPArsenic trioxideVOCsSemivolatile organicsMost fuel productsAromaticsPAHsCrude oilPCBsCyanidesHeavy metal cationsOil

Efficiency(%)

50 to 8390 to 9550 to 80

98 to greater than 9998 to greater than 9998 to greater than 99

>819597

84 to 8895

Approximately 7095 to 99

Residual Concentration(rag/kg)

250 to 600<115

0.5 to 1.3<50<250

<2,2004515

2,3000.5 to 1.35 to 15<200

20

Exner (1995) indicates that with soil washing, coarse soil fractions generally havecleanup efficiencies of 90% to 99% for volatiles, 80% to 95% for semivolatiles, and50% to 90% for metals. With solvent extraction, organics removal efficienciesgenerally range from 90% to 99%.

10.1 Basic Principles of Soil Washing

Fine soil particles have large pore-surface areas relative to volume, and contaminantstend to stay adsorbed, so fine-grained soils are difficult to wash. This tendency isstronger when removing organic contaminants with water than with a nonaqueoussolvent.

According to Anderson (1993), soil particles smaller than 63 \im tend to be attachedloosely to the coarser particles. When water is used as the extractant, the physicalattachment forces (adhesion and compaction) are effectively broken by using attritionmills (see Section 10.4.2). The contaminants mainly stay with the fine fraction whenseparated from sand and gravel fractions. Separating out the readily washed coarsefractions reduces the volume of soil that must be further treated or disposed.

A wide variety of contaminated soils exist, containing 30% to 60% or more of coarse-fraction sands and gravels that adsorb relatively small amounts of contaminants andare readily washed. The volume of soil that remains contaminated after washing ismuch less than the original volume. With the particle size separation that aqueoussoil washing can accomplish, sandy soils may undergo a volume reduction of morethan 80% for final cleanup or disposal. However, successful metals removal from thecoarse fraction may require acid or chelant additions to the wash water. And

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thorough organics removal from sand fractions containing iron oxide may not bepossible (Dove, Bhanduri, and Novak, 1992,1993).

Soil washing is not often used for soils with different types of contaminants, such asmixtures of metals, volatile organics, and nonvolatile organics, unless sequentialwashing steps are used with different additives.

The fundamental process involved in the removal of contaminants from soil particlesurfaces, once particle separation has been accomplished, is the use of an extractingfluid with a higher affinity for the contaminants of interest than the soil organicmatter. This drives the contaminants off the soil surface and into the fluid. Therelative concentration of a contaminant at equilibrium between two phases in contactwith one another is described quantitatively by an equilibrium distributioncoefficient, K, by the following definition:

_ Concentration in Phase 1, mass/volume or massConcentration in Phase 2, mass/volume or mass (10-1)

The equilibrium concentration between a soil and water system is described by thedistribution coefficient, Kdi

_ Concentration in Soil, mass/volumed Concentration in Water, mass/volume (10-2)

The equilibrium concentration between water and a representative organic solvent,octanol, is described by the the octanol/water partition coefficient, Kow:

_ Concentration in Octanol, mass/volumeow Concentration in Water, mass/volume (10-3)

The extent of sorption of many contaminants to soil surfaces is highly correlated tothe amount of organic carbon in the soil, leading to an organic carbon normalizedsoil-water coefficient, Koc, described mathematically as:

KOC = Kd/decimal fraction of soil organic carbon (10-4)

Standard methods exist for the determination of these distribution coefficients from avariety of sources. These standard methods provide details of experimental methodsand procedures that should be used to determine these coefficients reliably, and theyinclude recommended experimental mixing vessel sizes and configurations, methodsfor determining equilibration times, recommended volumes of each phase, etc.Readers are referred to American Society of Testing and Materials (ASTM) methods(e.g., ASTM Standard E 1147-87 for Kow determinations) for reference proceduresfor measuring these coefficients.

The efficiency of solvent extraction should be directly related to the nature of theorganic contaminant in terms of its Kow and Koc values (i.e., relative affinity for an

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organic solvent and soil surface, respectively). For highly hydrophobic compoundswith large Kow values (i.e., greater than 10,000), the use of an organic washingsolution takes advantage of the contaminant's organic-phase affinity and, by massaction, can effectively remove contaminant from the soil organic matter. Forhydrophilic organic compounds (phenols, aromatics, and low molecular weighthalogenated hydrocarbons) with low Kow values (i.e., from 1 to 1,000), aqueous-based washing solutions should be selected to optimize recovery of these compoundsfrom contaminated soil because of these compounds' high aqueous-phase affinity.

The organic carbon fraction of soils tends to concentrate contaminants and makessoils difficult to wash. For contaminants with a large Kow in a soil high in organiccarbon, the contaminant equilibrium distribution shifts to the soil phase, and aqueous-based washing solutions become ineffective. To increase the efficiency of soilwashing in these soils, a more aggressive washing solution that is organic solvent-based must be used to drive the contaminant into this organic extraction fluid.Generally, soils with a high organic content are not good candidates for soil washing.

10.2 In Situ Soil Flushing

Flushing is accomplished by flooding the land surface within a berm, by usinginjection wells, by heavily spraying water on the land, or by applying water throughan infiltration gallery. The water infiltrates the soil and desorbs contaminants fromsoil particles. Horizontal flushing is possible in a shallow soil layer just above anaquitard (Anderson, 1993). In situ flushing is not generally applicable if more thanone type of contaminant is encountered, nor is it applicable in fine-grained or highlyheterogeneous soils or at sites where hydraulic control of injected water cannot beassured.

The wash water is recovered by pumping from wells screened below the water tableor from trenches and subsurface drains above the water table. The recovered water istreated, and usually most of it is recycled by reapplying it to the soil.

Anderson (1993) indicates that flushing is applicable for removing hydrocarbons,chlorinated hydrocarbons, metals, salts, pesticides, herbicides, and radioisotopes.The technology is less effective if pockets of soil with low hydraulic conductivityexist or if the contaminants are relatively insoluble or tightly bound to the soil.

Contaminants that have a low KOW (i.e., less than 10) are good candidates for removalby flushing (Hyman and Bagaasen, 1997). Additives that might be used includesulfuric, hydrochloric, nitric, phosphoric, or carbonic acid; alkaline agents; andsurfactants (detergents) (US EPA, 1990c). Table 10-3 summarizes the properties ofvarious surfactants that can be used for hydrophobic organic contaminants.

Nutrients can be added to recycled water for biodegradation of organic contaminantsthat can take place in the vadose zone soils and/or in an aquifer. However, permittingproblems may be formidable even for adding just water, as concerns often arise overthe potential of the flush water to transport contaminants into an aquifer or touncontaminated soil. Using acids, bases, surfactants, or nutrients raises even moreconcerns.

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Table 10-3 Surfactant characteristics.

Surfactant Type

Anionic

Cationic

Nonionic

Amphoterics

l)Carboxylic acid sails

2) Sulfuric acid ester sails3) Phosphoric andpolyphosphoric acid esters

4) Perfluorinated anionics5) Sulfonic acid salts

1) Long chain amines

2) Diamines and polyanilines

3) Quaternary ammoniumsalts4) Polyoxyethylenated long-chain amines1) Polyoxyethylenatedalkylphenols alkylphenolethoxylates2) Polyoxyethylenatedstraight chain alcohols andalcohol ethoxylates3) Polyoxyethylenatedpolyoxypropylene glycols4) Polyoxyethylenatedmercaptans5) Long-chain carboxylic acidesters6) Alkylamine "condensates,"alkanolamides

7) Tertiary acetylenic glycols1) pH-sensitive

2) pH-insensitive

Selected propertiesand uses

Good detergency

Good wetting agentsStrong surface tensionreducers

Good oil-in-wateremulsifiersEmulsifying agents

Corrosion inhibitor

Emulsifying agents

Detergents

Wetting agents

Dispersents

Foam control

Solubilizing agents

Wetting agents

Solubility

Generally water-soluble

Soluble in polarorganics

Low or varyingwater solubility

Water-soluble

Generally water-soluble

Water insolubleformulations

Varied (pH-dependent)

Reactivity

Electrolyte tolerant

Electrolyte-sensitiveResistant tobiodegradation

High chemical stabilityResistant to acid andalkaline hydrolysisAcid stable

Surface adsorption tosilicaeous materials

Good chemical stability

Resistant tobiodegradation

Relatively nontoxic

Subject to acid andalkaline hydrolysis

Nontoxic

Electrolyte tolerantAdsorption to negativelycharged surfaces

The US EPA (1990b) lists these information needs for considering potentialapplication of soil flushing:

• Characterization and concentration of contaminants• Depth and vertical and aerial distribution of contaminants• Partitioning of contaminants between solvents and soil• Effects of washing agent on physical, chemical, and biological properties of soil• Suitability of site for flooding and for installation of wells or subsurface drains• Site-specific groundwater flow rate and direction• Trafficability of soil and site

Anderson (1993) tabulates these factors for predicting success of soil flushing:

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• Soil hydraulic conductivity at least 10~5 cm/sec, preferably greater than 10~3

cm/sec• Soil carbon content less than 10%, preferably less than 1%• Contaminant water solubility at least 100 mg/L, preferably greater than 1,000

mg/L• Soil sorption constant, Kd, less thanl0,000 L/kg, preferably less than 100 L/kg• Contaminant vapor pressure less than 100 mm Hg, preferably less than 10 mm Hg• Contaminant liquid viscosity less than 20 cp, preferably less than 2 cp• Contaminant liquid specific gravity greater than 1, preferably greater than 2

10.3 Soil Washing and Solvent Extraction

Traditional ore refining techniques in the mining and metals industries includeseparating fines, which tend to be relatively enriched in metals. Equipment used inthose industries has been adapted to soil washing with water. The coarse materialsare separated by the washing process, producing clean coarse sands and gravel,thereby reducing the volume of soil that needs further treatment.

Besides the size of the soil particles, other soil characteristics that affect washabilityinclude humic content and cation exchange capacity. Contaminants partition stronglyto humic material, making washing of high humic content soils difficult. A potentialcandidate soil for washing is at least 60% greater than a 63-jam particle size and hasless than 20 wt% organic matter content. The US EPA (1991b) indicates that soilswith at least 50 wt% sand/gravel (particle size greater than 200 jam) wash the best.Soils with high clay and silt content are not good candidates for soil washing.

Equilibrium distribution studies with shaking of a soil sample in water, or with otherextractant fluid, determine contaminant partitioning. Another important laboratorytest is the determination of soil cation exchange capacity. The lower the cationexchange capacity, the better soil washing can succeed, especially for removal ofcationic metals.

Equilibrium distribution data help assess whether soil washing is feasible and whatextractant might work best. Table 10-4, adapted from the US EPA (1990c),summarizes physical and chemical characteristics that affect soil washing.

Table 10-5, from the US EPA (1989a), indicates that for sand, silt, and clay,respectively, for three types of contaminants, the following equipment is needed:some candidate wash media (extractants); applicable extraction equipment and solid-liquid separation equipment; and potential methods of treating wash media forrecycle (regeneration"). Note that the suggested equipment for extraction is not aproven technology in cases in which silts and clay soils are involved. Some exampleswill illustrate how Table 10-5 is used. The extractor-type inclined screw is applicablefor washing sand contaminated with hydrophobic and hydrophillic nonvolatileorganics but not with heavy metals/inorganics. Looking at the bottom three rows ofthe table, which apply to clay, an inclined screw is not applicable, whereas a stirredtank extractor is; water with pH control is an applicable extractant for removing

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Table 10-4 Physical and chemical characteristics that affect soil washing.

Key Physical ParameterParticle size distribution

>2mm0.25 - 2 mm

0.063 - 0.25 mmO.063 mm

Oversize pretreatment requirementsSimple soil washingComplex soil washingClay/silt fraction - unsuited for soil washing

Other Physical ParametersType, specific gravity, physical form,handling propertiesMoisture content

Affects pretreatment and transfer requirements

Affects pretreatment and transfer requirementsKey Chemical ParametersOrganics concentration, volatility,partition coefficient

Metals

Humic Acid

Determine contaminants and assess separation andwashing efficiency, hydrophobic interaction, washing fluidcompatibility, changes in washing fluid with changes incontaminants. May require preblending for consistentfeed. Use the jar test protocol to determine contaminantpartitioning.Concentration and species of constituents (specific jartest) will determine washing fluid compatibility, mobilityof metals, post-treatment.Organic content will affect adsorption characteristics ofcontaminants on soil. Important in marine/wetland sites.

hydrophilic nonvolatile organics but not for removing hydrophobic nonvolatileorganic s.

The best washing medium to use at a specific site depends on the type of contaminantbeing removed. A summary of washing media (extractants) is as follows:

• Cationic metals — acid or chelating agent• Amines, ethers — acid• Anionic metals — hydrogen peroxide• Insoluble organic compounds — water with surfactant or organic solvent• Polar organic compounds — water or liquid carbon dioxide

In cases in which acids are used for removing cationic metals, the pH of the washwater solution is usually just below 3. Chelants are ligands that are applied only toremove cationic metals by forming, at the molecular level, an organic ring structurearound and then bonding to positively charged metal ions.

Metals in anionic states, such as arsenite or arsenate, or in an anionic complex, suchas a metal cyanide ion, are difficult to remove by soil washing. Acid will not workunless the metal is first treated with a strong oxidizing agent, such as chlorine orperoxide, to change its oxidation state or to destroy the anion complex.

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Table 10.5- soil-contaminant technology martixe.

SOIL TYPECONTAMINANT

TYPE

EXTRACTANTEXTRACTOR TYPE

SOLID-LIQUIDSEPARATION

EXTRACTANT REGENERATION

X X 15.3 X X X X X X X X X X

X X 2 . 3 X X X X X X X X X

X X 12.5 X__X X X X X X

X X 0 . 6 X X X X X X X X_

X X 5.1 X X X X X X X

X X 2 . 3 X X X X X X

X X 0.6 X X X X X X

X X 1.7 X X X X

X X X

X X 3.4 X X X X X X X X X

X

X


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For water washing organic compounds such as hydrocarbon oils, surfactants andalkaline agents such as sodium hydroxide (caustic soda) are used. Just as oil can beobtained from tar sands by extracting it with sodium hydroxide (caustic soda)solution, hydrocarbons can also be extracted from sand or gravel with alkaline agents.Also, alkaline agents improve extraction efficiency when washing soils with highorganic content, for which aqueous surfactant washing is not applicable. Table 10-6from the US EPA (1989a) gives more details on what reagents are used in extractantfluids.

When washing soil with aqueous solutions, heating the water usually will aid inmobilizing contaminants and will improve removal efficiency.

After selecting potential extractants and performing equilibrium distribution tests,pilot tests or treatability studies (discussed in detail in Section 10.5) can be conductedusing the selected reagents to determine:

• Contaminant removal efficiency• Volume fraction of soil fines that are not cleaned• Ratio of wash fluid to soil and contact time needed during wash steps• Treatment requirements for used wash fluid• Percent of wash fluid that can be recycled

For extraction of contaminants with nonaqueous solvents, tests should be conductedto check for residual solvent in the washed soil.

Multi-stage wash steps and size classification steps can be applied. For example, soilwith oil and metals contaminants might be subjected to:

• Coarse screening to remove large rocks and debris• Magnetic separation of ferrous materials• Screening of 2-in. and larger cobbles, which are rinsed with water applied at the

screen• Fine screening or classifying to separate out particles larger than 5 mm (3/16 in.)• Attrition scrubbing of these larger particles with surfactant to remove oil• Classifying to separate out particles ranging from 0.25 to 5 mm (0.01 to 3/16 in.)• Washing these medium-coarse particles with acid or chelants to remove metals• Filtering and water rinsing the filter cake• Centrifuging (dewatering) and vacuum drying the fine soil fractions• Solvent extracting the remaining organic contaminants from the fine soil particles

Many of the surfactants used for soil washing can be treated by bacterial degradation.Acid wash water and rinse water may need treatment with neutralization,precipitation, or evaporation before disposal.

10.3.1 Aqueous Soil Washing for Particle Size Separation

Figure 10-1 shows a soil washing scheme with size separation equipment. The bulkof the soil fractions produced is clean gravel and sand. The fine-grained materials

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Table 10-6 Summary of process parameters and reagents for variouscontaminant groups.

ContaminantGroup

HydrophilicOrganicCompounds

SlightlyHydrophilicOrganicCompounds

HydrophobicOrganicCompounds

VolatileOrganics

Heavy Metals

PolychlorinatedBiphenyis

RadioactiveMaterials

OtherInorganics

Contaminant Examples

Alcohols (e.g., methyl,isopropyl, butyl)Phenols (e.g., picric acid,pentachlorophenol, creosote)DioxaneUrethaneRocket fuelAromatics (e.g., benzene,toluene, xylene)Halogenated hydrocarbons(e.g., trichloroethylene,ethylene dichloride vinylchloride, methylene chloride)

ChloroformTrichloroethaneOil and greaseChlorinated hydrocarbons (e.g.,endrin, lindane, DDT, dieldrin)Polynuclear aromatics (PAHs)

HexaneEthylbenzene

MercuryNickelZincLeadChromiumArsenicCadmiumCopperPCBs

Uranium mining andpurification wastesRadiumTritiumCyanidesAcids (e.g., sulfuric)Alkalis (e.g., lime, ammonia)SulfidesBeryllium

Process Parameters

Wash fluid pHHumic content in soilDegree of agitationTime, soil loading, andstagingWetting agent

Wash fluid pHHumic content in soilDegree of agitationTime, soil loading, andstaging

Wetting agent

Use of surfactantsCaustic agentExtraction stagesDegree of agitationTemperatureReactor configurationSoil:solution ratioExtraction stagesDecree of agitationTemperatureReactor configurationEffects of other metal cationsEffect of other anionsSoil classificationTemperatureChelant or acid concentrationChelation durationSoil loadingWash fluid pHUse of surfactantsExtraction stagesDegree of agitationTemperatureReactor configurationSoihsolution ratioWash fluid pHSoil:solution ratioTimeTemperatureDepends on contaminant(s)present

Potential Reagents

Alkaline pHCaustic or NaaCOs fordispersing humusWetting agent

Alkaline pHCaustic or Na2CO3 fordispersing humusWetting agent

Surfactants (e.g., Adse 799,HyonicNP-90, P & GInstitutional Formula Tide®)

Caustic or NazCOs fordispersing humusWater-soluble solvents (e.g.,acetone, ethanol)Water (hot)Coal

Chelants (e.g., EDTA, DTPA)Acids (e.g., hydroxyl-aminehydrochloride, citric, nitric, aquaregia, acetic, fluosilicic)

Surfactants (e.g., Adse 799,HyonicNP-90, P & GInstitutional Formula Tide®)

WaterInorganic saltsje^., NaCl, KCDAcids (e.g., HCI, HNO3)Chelants (e.g., EDTA, DTPA)Depends on contaminant(s)present

and sludge remaining would typically need a different type of treatment, such asfixation with cement.

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Figure 10-1 Aqueous soil washing process (From the US EPA 1990b).

Depending on how high the sand content is, soil washing can reduce the volumeneeding more expensive treatment (or permitted disposal) by a very large percentage.A demonstration unit operated at 5 to 10 ton/hr by Bechtel Hanford, Inc. (Richland,Washington) produced an 85% clean fraction. Alternative Remedial Technologies(Tampa, Florida) reports a metals removal case history with 83% of the volumecleaned with the following treatment scheme:

• Screening out oversized material• High-pressure spraying to break clods and form a slurry• Separating coarse- from fine-grained materials using a hydrocyclone• Using air flotation for the coarse fraction from the bottom of the hydrocyclone• Using Lamella (slant-plate) clarifiers dosed with polymers to settle fine-grained

materials• Filtering, which produced 55% dry solids cake

Hydrocyclones use a combination of centrifugal and gravitational forces to separatecoarse fractions. Other particle separation techniques are described in Table 10-7.

Some of the major soil dewatering techniques are described in Table 10-8.

10.3.2 Solvent Extraction for Removing Organic Contaminants

Solvent extraction is more effective than aqueous extraction for removing insolubleorganics. When solvent extraction is used instead of water washing, contaminant

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Table 10-7 Particle separation techniques (From Eagle, M.C., et al. "SoilWashing for Volume Reduction of Radioactively Contaminated Soils,"Remediation, Summer 1993. Copyright 1993 John Wiley & Sons, Inc.

Reprinted by permission of John Wiley & Sons, Inc.).

Technique

CommonNameBasicPrinciple

MajorAdvantage

MajorDisadvantage

GeneralEquipment

Lab TestEquipment

Sizing

screening

various diameteropenings andeffective particlesizeinexpensive

screens can plug,fine screens arefragile, dry screensproduce dustscreens, sieves

vacuum sieve/screen, trommelscreen

Settling Velocity

classification

faster vs. slowersettling, particledensity, size,shape of particlescontinuousprocessing, longhistory, reliable,inexpensivedifficulty withclayey, sandy,and humus soils

mechanical,non-mechanicalhydrodynamicclassifierselutriationcolumns

Specific Gravity

gravityseparationdifferences indensity, size, shape,and weight ofparticleseconomical, simpleto implement, longhistory

ineffectivefor fines

jigs, shakingtables, troughs,sluices

jig, shaking table

MagneticProperties

magnetic

magneticsusceptibility

simple toimplement

high operatingcosts

magneticseparators

lab magnets

Flotation

flotation

suspend fines byair agitation, addpromoter/collectoragents, skim oil frothvery effective forsome particlesizes

contaminant mustbe small fractionof total volume

flotationmachines

agitair laboratoryunit

Table 10-8 Dewatering techniques (From Eagle, M.C., et al. "Soil Washing forVolume Reduction of Radioactively Contaminated Soils," Remediation,Summer 1993. Copyright 1993 John Wiley & Sons, Inc. Reprinted by

permission of John Wiley & Sons, Inc.).

Technique

Basic Principle

Major Advantage

Major Disadvantage

General Equipment

Lab Test Equipment

Filtration

passage of particles

through porous

medium, particle

size

simple operation

more selective

separation

batch nature of

operation, washing

may be poor

drum, disk, horizontal

(belt) filters

vacuum filters,

filter press

Centrifugation

artificial gravity

settling: particle

size, shape, density,

and fluid density

fast, large capacity

expensive, more

complicated

equipment

solid bowl

sedimentation and

centrifugal perforated

basket

bench or floor

centrifuge

Sedimentation

gravity settling:

particle size, shape,

density, and fluid

density; flocculent

aided

simple, less

expensive equipment,

large capacity

slow

cylindrical continuous

clarifiers, rakes,

overflow, lamella,

deep cone thickeners

cylindrical tubes,

beaker, flocculents

Expression

compression with

liquid escape through

porous filter

handles slurries

difficult to pump,

drier product

high pressure

required, high

resistance to flow

in cases

batch and continuous

pressure

filter press, pressure

equipment

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removal is not limited to coarse fractions such as sand and gravel only. Solventextraction can be applied to a much wider range of soil particle sizes. However,separation of solvent from clay soils may be a problem, and centrifugation or thermaldesorption may be needed (US EPA, 1989a). A disadvantage of solvent extractionsystems is there is a risk of leaving significant amounts of solvent in the treated soil.Also, some solvents are mildly toxic and/or flammable.

In choosing a solvent, besides toxicity and safety/handling considerations, thefollowing factors are important: ability of the solvent to dissolve the contaminants,especially in the presence of soil moisture; volatility and the potential for airemissions if the soil wash system is not completely enclosed and vapors are notcontrolled; separability of the solvent/contaminant mix from the soil; and separabilityof the contaminants from the solvent so that the solvent can be recycled.

The common organic solvents that are used to extract hydrocarbon oils and othernonpolar organic contaminants are hydrocarbons with a boiling temperature rangeselected so that the range is lower than the boiling temperature range of thecontaminants. Then the solvent can be recovered for reuse by distilling it or bystripping it from the contaminants with steam or hot nitrogen. The US EPA (1992a)reports that solvent extraction is effective for removing petroleum wastes,halogenated solvents, and PCBs from sediments, sludges, and soils. The preferredhydrocarbon solvents are alkanes. Alcohols are effective solvents that do not sufferfrom moisture interference as much as hydrocarbons do, but they are expensive.

If the contaminated soil is wet, solvent extraction with hydrocarbons is improved byfirst evaporating the water, preferably under vacuum. Under vacuum conditions, thewater will evaporate rapidly without raising the temperature to 212° F. An exampleof such a solvent extraction system is the Carver-Greenfield system marketed byDehydro-Tech Corporation (East Hanover, New Jersey). Note that in the watervacuum evaporation step, some oil may be vaporized with the water. The watervapor and any evaporated oil are condensed, and the oil is decanted from the water inan oil/water separator.

Solvent extraction may have to be repeated for a given batch of soil in order toachieve effective contaminant removal. This can be accomplished by using anextraction system with multiple stages, by recycling treated soil, or by repeatedmixing of soil with solvent and successive solvent withdrawals.

An emerging technology is the use of liquified petroleum gas (LPG) or carbondioxide as the extracting solvent. These two compounds liquify by being placedunder pressure, by chilling, or by a combination of pressure and cooling. The LPG orcarbon dioxide solvent can readily be recovered in the vapor phase by depressuring,or warming the solvent/contaminant mixture drained from the soil. Such solventextraction systems are marketed by C. F. Systems (Woburn, Massachusetts).Liquified propanes and butanes extract nonpolar or hydrophobic organics. Residualsolvent, which contains contaminants, is displaced from the soil with warm water.Liquified carbon dioxide extracts polar organics. (Fluid extraction using supercriticalcarbon dioxide is not limited to polar organics.) The high volatility of propane or

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carbon dioxide solvent minimizes residual solvent concentrations in the treated soil.The process is effective for removing PCBs, PAHs, dioxins, and TPHs (Anon., 1993).

An innovative method of recovering solvent has been developed by ResourcesConservation Co. (Bellevue, Washington), with a system that uses triethylamine(TEA). This compound's water miscibility changes markedly over a relatively smalltemperature range. At temperatures less than 60° F, TEA is used to dissolve organiccompounds that are not soluble in water. The mix is chilled to a somewhat lowertemperature at which TEA is miscible with water. A mixture of TEA, organiccontaminants, and water is removed from the washing process and warmed to at least70° to 160° F. The process is described in the EPA Office of Solid Waste andEmergency Response publication "Tech Trends," No. 11, January 1993 (EPA/542/N-93/001) as follows:

"Contaminated material is screened to less than 1/2-inch diameter (1/8-inch for this demonstration) and added to a refrigerated premix tankwith a predetermined volume of 50% sodium hydroxide. After thetank is sealed and purged with nitrogen, chilled triethylamine solventis added. The chilled mixture is agitated and then allowed to settle,creating the non-homogenous mixture of moisture-free solids and thesolution of solvated oil, water and solvent. The solution is decantedfrom the solids and centrifuged. The solvent and water are removedfrom the solvent/water/oil mixture by evaporation and condensation ofthe solvent and water. Solids with high moisture content may requiremore than one cold extraction. For example, for this demonstration, asediment containing 41% moisture required two cold extractions.Once a sufficient volume of moisture-free solids is accumulated, it istransferred to a steam jacketed extractor/dryer where warmtriethylamine is added to the solids. The mixture is heated, agitated,settled and decanted to separate any of the organics not removedduring the initial cold extraction. The solids remaining in theextractor/dryer contain triethylamine following decanting. A smallamount of steam is injected to volatilize this remaining triethylamine.The hot extraction process can be repeated, when necessary, to furtherremove contaminants.

The products from the process are: (1) solids, (2) water and (3)concentrated oil containing the organic contaminants. The recoveredoil fraction can be dechlorinated or incinerated to destroy the organics.The triethylamine is recovered and reused in further extractions."

10.4 Main System Design Parameters for Soil Washing

10.4.1 Conceptual Designs

The first important steps in applying soil washing are developing a flow scheme andbalancing the mass flow rates of wash solution and soil. Because most soil washingschemes involve separating fractions of soil by particle size range and forming

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slurries, the mass balance should be done for each size fraction, and the percentage ofsolids content (or water content) should be determined for each step of the process.Then the equipment can be sized.

Equilibrium distribution studies and treatability test data are needed to estimate theamount of contaminant reduction and the particle size separations that might beattained, the water/soil ratio needed, additives requirements, and water treatmentconditions. The US EPA (1991b) describes applicability of jar tests and gives detailsfor planning pilot tests.

Some of the equipment is selected based on producing washed product size fractionsthat do not have excessive water content. A rough approximation of the particle sizedistribution that may result from a given washing and separation step can bepredicted from the type of equipment selected. For example, Besendorfer (1996)correlates separation percent for various size particulates when solids in water aretreated with hydrocyclones.

Minimizing the water/soil ratio is important, as this results in smaller equipment sizeand pumping horsepower requirements. Water treatment steps are chosen that allowmaximum recycle of wash water. It is also desirable to minimize the volume of waterdischarge that will need metals removal, organics destruction, or other finaltreatment.

10.4.2 Mass Balances

An example of a flow scheme and mass balance developed for a soil washing systemis shown in Figure 10-2. Refer to Table 10-9 for equipment identification.

The following figures show major components (as listed in Table 10-9) of thisscheme:

• Figure 10-3 shows the mass balance for the trommel screen Y-l and coarse flat-deck screens SC-2. A trommel screen is a water-washed cylindrical screenrotating within a drum. This step breaks down agglomerates, somewhat classifiesthe soil by particle size, and washes the coarse fraction.

• Figure 10-4 shows the mass balance for the spiral classifier Y-2 and a flat-deckscreen. A spiral classifier is an inclined device that is effective in separating outparticles larger than approximately 2 mm average diameter.

• Figure 10-5 shows the mass balance for a series of attrition mills Y-3 andhydrocyclones Y-4 and Y-5. Attrition mills wash the medium-sized particles withwater using two opposed-pitch, relatively high-speed mixers in each mill.Cyclones separate larger-sized particles by centrifugal and gravitational action,with no moving parts —just a high-velocity, horizontal, tangential inlet for thewater/soil slurry.

• Figure 10-6 shows the mass balance for flotation cells Y-7 and a belt filter Y-8.Surfactant is first mixed in a conditioner, and froth is formed in the cells. Thefiner particles are trapped by the froth bubbles and are skimmed off the tops ofthe cells.

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Figure 10-2 Example of a soil washing scheme.

Idl WOtCATESEOmPMEKTW,HQ.$t£ eaUPMEHT U$T f C«oescflBTWHANocwwrfrrneouoeo


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Table 10-9 Generic soil washing major equipment list.

SI.NO.1

2

3

4

5

6

7

8

9

TitleDump Hopper

Feeder Conveyor

Rock Screen

Rotary Scrubber

Coarse Gravel Screen

Spiral Classifier

Fine Gravel Screen

Attrition Scrubber

Pri. Sand CycloneFeed Sump

I.D. No.D-l

T-l

SC-1

Y-l

SC-2

Y-2

SC-3

Y-3(A through D)

D-2

DescriptionVolume: two times the dumping bucketMaterial: CS with liners for sloping sidesAccessories- Bar grid with 6-in. opening and slidegate at outlet

Capacity: 25 tphWidth and Length: to suitType: Horizontal VibratingNo. of Decks:- TwoFeed Capacity: 25 tphDock Openings: 4 inch and 2 inchFeed Capacity: 20 tphSize (dia x length): 4 ft x 12 ft (inside linersLiners: Mn steelInternals: LiftersFeed and Discharge Sleeves: Lined and with spiralflights

Discharge trommel: RequiredRetention time: 3 minutesType: Horizontal VibratingNo. of Decks: TwoFeed Capacity: 25 tphDeck Openings: 1 inch and 3/16 inchFlow rate (Slurry): 165 gpmSolids: 22 tphSolid Sp. Gr.: 2.6Water: 135 gpmSands: 10 tph (dry basis)Classification: 250 micronsAccessories: Water Sprays at the beachManual hydraulic lift for screwType: Horizontal VibratingNo. of Decks: OneFeed Capacity: 10 tph (dry basis)Deck Openings: 2 mm- wedge wireFlow rate (Slurry): 100 gpmSolids percentage: 25 to 30 %Number Required: 4Arrangement: 4 in SeriesRetention Time: 20 mins (total)Approximate Size: (Dia x hi): 4 h x 5 hImpellers: TurbineLiners: Tanks and Shafts lined for abrasion resistanceInclude 'Sand Relief

Maximum Particle Size: 2 to 3 mm silicaVolume: 600 galsType: Conical with cylindrical topMaterial: CSAccessory: Mixer for keeping contents in suspension

Qty1

1

1

1

1

1

1

4

1

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Table 10-9 Generic soil washing major equipment list (continued).

SI.NO.10

11

12

13

14

15

161719

20

21

22

23

TitlePd. Sand Cyclone Feed Pump

Primary Sand Cyclone

Sec. Sand Cyclone Feed Sump

Sec. Sand Cyclone Feed Pump

Sec. Sand Cyclone

Recycle Water O'Head Tank

Not UsedNot UsedSpiral Overflow Sump

Conditioner Feed Pump

Conditioner

Flotation Cell

Sludge Filter

I.D. No.G-l

Y-4

D-3

G-2

Y-5

D-4

D-5

G-3

Y-6

Y-7(A through D)

Y-8

DescriptionType: Centrifugal, Horizontal, SlurryFlow rate (Slurry): 100 gpmSp. Gr. of Slurry: 1.2TDH: to suit ArrangementSpeed Control: Mechanical Variable -RemoteFlow rate (Slurry): 100 gpmSolids: 7.2 tphSolid Sp. Gr.: 2.6Sp. Gr. of Slurry: 1.2Classification: 250 micronsVolume: 850 galsType: Conical with cylindrical topMaterial: CSAccessory: Mixer for keeping contents insuspensionType: Centrifugal, Horizontal, SlurryFlow rate (Slurry): 160 gpmSp. Gr. of Slurry: 1.1TDH: to suit ArrangementSpeed Control: Mechanical Variable -RemoteFlow rate (Slurry): 160 gpmSolids: 6 tphSolid Sp. Gr.: 2.6Sp. Gr. of Slurry: 1.1Classification: 250 micronsVolume: 550 galsType: Conical with cylindrical topMaterial: CS

Volume: 850 galsType: Conical with cylindrical topMaterial: CSAccessory: Mixer for keeping contents insuspensionType: Centrifugal, Horizontal, SlurryFlow rate (Slurry): 180 gpmSp. Gr. of Slurry: 1.2TDH: to suit ArrangementSpeed Control: Mechanical Variable -RemoteType: Double Impeller high speedRetention time- 1 minVolume: 180 galsPower Draw: 40 HPFlow rate (Slurry): 180 gpmRetention Time: 6 minsApproximate Size- 40 eft each- hog troughType: Double Bell Press filterCapacity: 1 tph (Based on Dry Solids )

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1

1

1

1

001

1

1

4

1

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Table 10-9 Generic soil washing major equipment list (continued).

SI.NO.

24

25

26

27

28

29

30

31

32

33

34

35

Title

Fine Sand Cyclone Feed Sump

Fine Sand Cyclone Feed Pump

Fine Sand Cyclone

Thickener

Sill & Clay Filter Feed Pump

Silt and Clay Filter

Thickener Overflow Sump

Thickener Overflow Pump

Spillage pump

Treated Water Tank

Booster Pump

Additive Metering Pumps

I.D. No.

D-6

G-4

Y-9

Y-10

G-5

Y-ll

D-7

G-6

G-7

D-8

G-8

Y-12

(A through F)

Description

Volume: 850 gals

Type: Conical with cylindrical top

Material: CS

Accessory: Mixer for keeping contents in

suspension

Type: Centrifugal, Horizontal, Slurry

Flow rate (Slurry): 180 gpm

Sp. Gr. Of Slurry: 1.2

TDH: to suit Arrangement

Speed Control: Mechanical Variable -Remote


Solids: 12 tph

Solid Sp. Gr.: 2.6

Sp. Gr. of Slurry: 1.2

Classification: 75 microns

Type: Delta Stack or Equal


Type: Centrifugal, Horizontal, Slurry


Sp. Gr. Of Slurry: 1.3


Speed Control: Mechanical Variable -Remote

Type: Double Belt Press filter

Capacity: 8 tph (Based on Dry Solids)

Volume: 1000 gals

Material: CS

Type: Centrifugal, Horizontal, Water

Flow rate: 150 gpm


Type: Vertical Sump Pump



Volume: 1500 gals

Material: CS

Type: Centrifugal, Horizontal, Water

Flow rate: 150 gpm

Discharge Pressure: 45 psig

Flow Rate: 0-10 gpm

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In, Ton/hr Out, Ton/hr

1 . Raw soil2. Spray-gun water3. Recycled water4. Fresh water7. Recycled water8. Fresh water

Solids20.000.340.08

0.05

20.47

Water3.53

15.003.601.802.401.20

27.53

5. Rock, 6" x 2"9. Gravel; 2" x 3/16"10. To Classifier

Solids1.800.60

18.0720.47

Water0.060.03

27.4427.53

Figure 10-3 Mass balance for screens.

In, Ton/hr

10. From Screen1 1 . Recycled water14. Recycled water15. Treated water

Solids18.070.170.220.52

18.98

Water27.44

7.5014.482.08

51.50

13. To Conditioner16. Fine gravel;

3/1 6" x 5/1 6"17. To Attrition Mill

Solids10.26

2.686.04

18.98

Water33.56

0.7817.1651.50

Figure 10-4 Mass balance for a spiral classifier and a flat-deck screen.

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17. From Screen22. Freshwater

Solids Water Solids Water6.04 17.16 23. Medium sand 5.11 3.41

27.27 26. Water to Sprays 0.87 38.506.04 44.43 27. Water to Flotation 0.06 2.52

6.04 44.43

Figure 10-5 Mass balance for attrition mills and hydrocyclones.


13. From Classifier27. From Hydroclones

Solids Water10.26 33.56 30. Sludge0.06 2.52 30A. To Thickener

10.32 36.08 31. ToHydroclone

Solids Water0.52 0.77

1.30

9.80 34.0110.32 36.08

Figure 10-6 Mass balance for flotation cells and belt filter.

Figure 10-7 shows the mass balance for a hydrocyclone Y-9 and a thickener filtersystem Y-10 and Y-11 for the relatively coarse material that does not float in thecells.

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In, Ton/hr

31. From Flotation30A. From Filter

Out, Ton/hr

Solids Water9.80 34.01 32. Fine sand

1.30 36. Water to Treatment

Solids Water3.22 2.26

29.506.58 3.559.80 35.31

9.80 35.31 35. Silt & clay

Figure 10-7 Mass balance for hydrocyclone and thickener filter system.

The mass balance for each major stream is summarized in Table 10-10, along withthe particle size distribution estimated for each numbered stream. In Table 10-10,TPH is short tons per hour, and GPM is US gallons per minute.

When chelants are used for removal of metal cations, the US EPA (1989a) indicatesthat the following process parameters should be considered:

" Naturally occurring, noncontaminant metals compete for the chelant. Proper pHcontrol and chelant selection can minimize excessive chelant consumption thatoccurs with such competition. For example, raising the pH to 7 to 9 favorschelation of divalent lead over trivalent iron.

• Separation of soil particle size fractions before applying chelant washing may beneeded for removing some metals.

• Extended contact time in the chelant washing step is needed to chelate themaximum amount of metal.

10.4.3 Treatment of Wash Water

• Anderson (1993) lists the following materials and contaminants that may be in thewash water: coarse sand, silt and clay (with adsorbed contaminants), dissolved

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Table 10-10 Generic soil washing material balance.

ABCDEFG

SIZE FRACTION

6 in. to 3 in.3 in. to 2 in.

2 in. to 3/16 in.3/16 in. to 2 mm2 mm to 250 urn250 urn to 75 urn

<75 umTOTAL

SIZE DISTRIBUTION OF SOIL (%)COARSE

ROCK4

4

MED ROCK

5

5

COARSEGRAVEL

3

3

FINEGRAVEL

10

10

MED SAND

26

26

FINE SAND

16

16

SILT ANDCLAY

3636

TOTAL

45310261636100

XTOTAL

4912224864100

STREAM NO.

DESCRIPTION

GRAIN SIZE NOMINALABCDEFG


2 in. to 3/16 in.3/16 in. to 2 mm2 mm to 250 um250 um to 75 um

<75 umTOTAL DRY SOLIDS

WATERWATER

DRY SOLIDSTOTAL STREAMTOTAL SLURRY

TPHTPHTPHTPHTPHTPHTPHTPHTPHGPM

%TPHGPM

1

RAW SOIL

6 in. X 00.8

10.62

5.23.27.220

3.5314.12

8523.53

45

2WATER

GUNRECYCLED

0.170.170.3415.060.02.2

15.3460.1

3SPRAYWATER

RECYCLE

0.040.040.083.614.42.2

3.6815

4SPRAYWATER

TREATED

01.87.2

1.87

5COARSE &MED ROCK6 in. to 3 in.

0.81

1.80.060.2297

1.863

6ROTARY

SCRUBBERFEED

0.62

5.23.417.4118.6223.8795.4943.8

42.49124

7SPRAYWATER

RECYCLE

0.0250.0250.052.49.62.2

2.4510

8SPRAYWATER

TREATED

01.24.8

1.25

9COARSEGRAVEL

2 in. to 3/16 in.

0.6

0.60.030.1395

0.631


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Table 10-10 Generic soil washing material balance (continued).

STREAM NO.

DESCRIPTION



2 in. to 3/16 in.3/16 in. to 2 mm2 mm to 250 urn250 um to 75 urn


WATERWATER



%TPHGPM

10SCREEN-2

UNDERFLOW3/16 in. to 0

25.23.447.4418.0727.44109.7739.7

45.51138

11SPRAYWATER

RECYCLE

0.0850.0850.177.5302.27.6730

12SPIRALSANDS

3/16 in. to 250 um

2.05.1

0.340.377.811.385.5185

9.1918

13SPIRAL

OVERFLOW250 um to 0

0.103.097.0710.2633.56134.2523.443.82

150

14SPRAYWATER

RECYCLE

0.110.110.22

10402.2

10.2240

15SPRAYWATER

TREATED

6.024

6.024

16FINE

GRAVEL3/16 in. to 2 mm

2.0

2.000.22

190

2.224

STREAM NO.

DESCRIPTION



2 in. to 3/16 in.3/16 in. to 2 mm2 mm to 250 um250 um to 75 um


WATERWATER

DRY SOLIDSTOTAL STREAMrr^x-^r^, i , r-,, > T T > 1 - » X r


%TPH/"!!-»» *

17

SCREEN-3UNDERFLOW

2 mm to 250 um

18

PRIM CYCLFEED

2 mm X 0

5.10.460.480.0017.1668.63

2623.191 0

5.10.460.486.0417.1668.63

2623.19

1 0

19

PRIM CYCLUNDERFLOW

2 mm X 0

5.10.050.075.213.48

1460

8.6900

20

PRIM CYCL

0.410.410.8213.6854.725.714.5c/:

21

SEC CYCLFEED

2 mm X 0

5.10.050.075.2130.7512314.5

35.961-21

22

WATERADDITION

21-19

0.0027.27109

27.27inn

23

SEC CYCLUNDERFLOW

2 mm to 250 um

5.10

0.015.113.411460

8.5201


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STREAM NO.

DESCRIPTION



2 in. to 3/16 in.3/16 in. to 2 mm2 mm to 250 |im250 urn to 75 urn

<75 jimTOTAL DRY SOLIDS

WATERWATER



%TPHGPM

24SEC CYCL

OVERFLOW21-23

250 urn X 0

0.050.060.1027.34

1090.4

27.44110

25COMBINEDOVERFLOW

20+24250 urn X 0

0.460.470.93

41.02164.082.20741.95

166

26TOTAL

RECYCLE2+3+7+11+14

250 urn X 0

0.4350.4350.8738.51542.3

39.37155

27

FLOTN ADDITNRECYCLE, 25-26

250 urn X 0

0.020.040.062.5210.082.32.58

10

28

FLOTATIONFEED, 13+27

250 urn X 0

0.13.117.1110.3236.08144.3322.246.4160

29

FLOATS

0.160.360.522.088.31202.69

30

FILTERED

250 urn X 0

0.160.360.520.783.12401.34

STREAM NO.

DESCRIPTION




<75 urnTOTAL DRY SOLIDS

WATERWATER



%TPHGPM

30A

SLUDGEFILTRATE

0.001.3

5.19

1.35

31FINE SANDCYCL FEED

28-30250 urn X 0

0.13.046.839.97

34.01136.0222.4

43.81151

32

FINE SANDCYCL UFLOW

250 urn to 75 urn

0.12.740.553.392.269.0360

5.4814

33FINE SAND

CYCL OFLOW31-32

75 urn x 0

0.36.286.58

31.75126.99

17.238.33

137

34TAIL THICK

FEED33+30A+35A

75 urn x 0

0.36.286.5841.73166.91

13.648.31

177

34A

TAIL THICKUNDERFLOW

0.36.286.5812.2348.91

3518.8159

35

SILT ANDCLAY

75 urn x 0

0.36.286.583.5514.18

6510.13

24


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STREAM NO.

DESCRIPTION




<75 urnTOTAL DRY SOLIDS

WATERWATER



%TPHGPM

35ASILT FILTER

FILTRATE34A-35

00

0.008.6834.73

8.6835

36WATER TO

TREATMENT34-34A

00

0.0029.5118

29.5118

37TREATED WATER

ADDITIONS4+8+15+22

36.27145.08

36.27145

TOTAL IN

1+37

0.81

0.62

5.23.27.2

20.0039.8159.2

59.8190

TOTAL OUT

5+9+16+23+32+35+30+36

0.81

0.62

5.23.27.2

20.0039.8159.2

59.8190


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salts, naturally occurring organic matter, undesirable pH conditions, solubilizedheavy metals, and other contaminants such as hydrocarbons.

A frequently used scheme for washing petroleum-contaminated soil usesbiodegradable surfactants. The spent wash water is treated in a bioreactor. Finalcleanup of the soil can be done biologically in a slurry bioreactor or by spreading thesoil in a land treatment system and adding nutrients.

Some soil washing systems use activated carbon for removal of dissolved organicsfrom the wash water.

Fine materials that stay suspended in wash water can be removed using twoalternative methods: by coagulation, flocculation, and sedimentation; or byultrafiltration. A batch ultrafiltration scheme is shown in Figure 3-4.

When acid is used to leach metals from soil, the wash water can be neutralized withsodium bicarbonate or caustic soda, and the pH can be raised to precipitate metalhydroxide sludge. With polymer additions, the sludge can be separated from thewater by using a clarifler or an ultrafiltration unit. Acid addition is then used for finalwash water pH correction prior to discharge.

Ion exchange can be used for wash water cleanup. The conventional method usesfixed-bed ion exchange resins or zeolites following water filtration. Some innovativesoil washing techniques use ion exchange resin beads suspended in the wash water.

10.5 Treatability Studies for Soil Washing

The US EPA (1991b) recommends treatability studies to determine the following:

• The recoverable clean soil fraction• The volume and characteristics of fine soil and sludge fractions requiring

treatment or disposal• The efficacy of additives• The degree to which additives can be recovered and recycled• The ratio of additives to soil• The ratio of soil to wash water

The first three factors provide information regarding the necessity and costs ofdownstream treatment options. The last three factors help estimate costs of suppliesand utilities and the sizes of major equipment components. Treatability studies canalso help determine contaminant removal efficiency, the processing conditionsneeded for recycling or disposal of wash water or solvent, and the percentage of eachstream that can be recycled. For solvent extraction, treatability studies are used todetermine how much solvent remains with the treated soil.

The US EPA (1991b) recommends that the tests in Table 10-11 be done on soilsamples before considering washing as a candidate process.

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Table 10-11 Laboratory tests for soil washing.

Parameter

Particle sizedistribution

Cation exchangecapacity

Test

Sieve with #10 and #60screens or equivalent

Ammonium acetate

Sodium acetate

Analysis Method

ASTM D422

EPA 9080

EPA 9081

For the particle size distribution test, the US standard sieve series No. 10 and No. 60provide grain-size separation into three fractions approximately as follows: more than2 mm, 0.25 to 2 mm, and less than 0.25 mm.

The recommended equilibrium distribution tests include the following: additions ofsurfactants, acid, base, or chelants; use of heated water and cold water; wash step;and rinse step. Treated soil should be grain-size separated using a No. 10 sieve. Ingeneral, at least 50% contaminant reduction should be experienced in the coarsefraction if washing is to be considered further. If further consideration and testing areindicated, in addition to the basic test requirements listed above, remedy selectiontests should be done that include separating grain sizes into finer fractions, varyingwash times, varying rinse water-to-wash water ratios, and analyzing the wash water.

A good starting ratio for soil to water is 1/3, using 10 to 30 min of shaking. The USEPA (1989a) suggests that a 1/1 soil/water ratio is preferred, but ratios up to 1/3 aremore practical.

Laboratory equilibrium distribution tests can be carried out to determine for eachcontaminant the partitioning coefficient — the ratio of these two parameters: (1) themass concentration that remains in the soil sample, usually expressed as mg/kg; (2)the liquid concentration, e.g., mg/L. If the concentration in the soil is below thecleanup target level, a washing process has the potential for success without anothertreatment process being needed in series with washing. The concentrations ofcontaminants in the liquid are determined so that liquid treatment requirements can bedesigned.

The equilibrium distribution tests are carried out at various soil/water ratios. Theremay be a certain ratio above which the cleanup target level can be met. The ratiofinally selected for full-scale washing operations is critical. The lower the soil/waterratio, the larger the treatment equipment must be for a given soil batch size.

If removal of cationic heavy metals by water washing is to be attempted, equilibriumdistribution tests at lowered pH levels and/or with chelating agents should be carriedout. A weak acid, such as acetic acid, should be tried first. Dilute sulfuric acid orhydrochloric acid would be a second choice. A starting point for testing chelants is touse 20% EDTA (ethyl diamine tetraacetic acid) at a pH of 2 to 3. For metal-forminganions (e.g., anion complexes of arsenic or selenium), hydrogen peroxide solution ora chlorine-based oxidizing agent may prove effective.

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The water from the equilibrium distribution tests can be used for other experimentson treatability of the water.

The octanol/water partition coefficient should be determined from laboratory tests forany contaminants for which the coefficient is not known from literature sources. Ahigh coefficient favors solvent extraction, whereas a low coefficient favors washingor flushing with water solutions.

For testing in situ soil flushing schemes, a column of soil can be flooded in alaboratory setup as shown by Anderson (1993). Or a column of soil can be used in anapparatus similar to a dynamic ion exchange bench-scale system. The reduction insoil contaminant concentrations can be measured with water as the extracting fluidand with various agents (e.g., acids, alkaline agents, surfactants) added to the water.

Pilot testing of size separation processing can be carried out by utilizing small-scaleequipment developed in the mining and metallurgical industries for the classifyingand benefaction of ores. Steps such as screening, spiral classification, flotation,hydrocyclonic separation, and hydraulic classification or elutriation may be carriedout in series in a soil washing scheme. Pilot testing can be used to determine theparticle size distribution that can be attained with each step and the level ofcontamination in each size fraction. The required water/soil ratio can be confirmed,and water recycling can be evaluated to reduce water consumption and the volume ofwater needing final treatment. For chelation processes, the contact time for washingthat is needed to attain maximum removal of contaminant metal cations can bedetermined.

Anderson (1993) gives the following examples of equipment components that can beused in conducting treatability studies:

• Grizzly bar screen for removal of debris larger than 50 mm (2 in.)• Tramp iron and steel separator• Density media separator for removal of leaves, twigs, roots, plants, shells, etc.,

based on specific gravities of these materials• Rotary trommel screen for initial soil breakup and classification, with screened

coarse product greater than 9.52 mm (0.375 in.)• Attrition scrubber that contacts soil particulate smaller than 9.52 mm with

chemical wash solution to mobilize fines smaller than 74 jam (200 mesh)• Hydrocyclone for separating less than 74-um particulate from sand and gravel in

the underflow, which is 70% to 75% solids• Reverse-slope dewatering module, which is a screening system with high-

frequency shaking for final rinsing, dewatering, and desliming• Wash water clarifier system for flocculation, sedimentation, and densification of

soil particulate less than 74 um• Oil and grease separator for wastewater• Dissolved air flotation unit for removal of undissolved hydrocarbons• Continuous belt filter for dewatering materials less than 63 urn (230 mesh),

producing filter cake with 40% to 60% solids.

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Some soil washing systems incorporate adsorption and/or ion exchange mediasuspended in the wash water. Bench-scale or pilot-scale tests can be used to evaluatevarious media for their effectiveness and capacity for the contaminants of concern.

10.6 Cost Estimating for Soil Washing

The US EPA (1991b) data indicate that after simple equilibrium distribution tests areconducted, bench-scale treatability studies for remedy selection cost $20,000 to$100,000, and field pilot tests, $100,000 to $500,000.

In situ soil flushing costs range from $80/yd3 of soil using surface flooding to$165/yd3 using subsurface injection (Anderson, 1993). These costs include treatmentof used wash water but exclude excavation, debris removal, and treatment/disposal ofresiduals.

Studies by Bechtel Environmental Inc. in 1994 indicated that ex situ soil washingcosts are typically $100 ± $30/ton of soil for physical separation. Additional leachingor further processing adds $25/ton to $150/ton. These figures exclude costs fortransport, setup, and demobilization of equipment, analyses, excavation, hauling, andbackfill. Higher unit costs are encountered if the quantity of contaminated soil is lessthan a few thousand tons or if removal of relatively high concentrations of heavymetals is involved. Exner (1995) indicates that soil washing costs are in the range of$80/ton to $200/ton, whereas solvent extraction is in the range of $150/ton to$400/ton. Anderson (1993) indicates that soil washing costs are in the range of$150/tonto$250/ton.

Remediation cost estimating/process design software is available for various types ofsoil washing from a number of companies (e.g., COMPOSER GOLD from BuildingSystems Design, Atlanta, Georgia, and RACER/ENVEST™ from Talisman PartnersLtd., Englewood, Colorado). COMPOSER GOLD includes estimating software forin situ soil flushing and ex situ soil washing and solvent extraction.

For in situ soil flushing, Talisman Partners Ltd. use a computerized ENVEST modelthat estimates the cost of these items:

• Installing and removing an infiltration gallery with 2-in. PVC piping arms in 12-in. deep by 12-in. gravel-filled trenches lined with filter fabric, at 5-ft spacing, fedby a PVC header in a 24-in. deep by 24-in. trench — The program output definesthe cost of dismantling; the user can delete this cost if it is not applicable at aparticular site.

• An earth berm around the site 12 in. high and 24 in. wide at the top" Optional use of additives to the flushing water, such as alkylbenzene sulfonate

detergent at 0.05 wt%, sulfuric acid from 220-lb drums at 4.1 lb/1,000 gal, orsodium hydroxide (caustic soda) solution from 100-lb drums at 1.5 lb/1,000 gal— The program does not include the cost of makeup water, wells, or extractionwell pumps, and it assumes recirculation of water pumped from a downgradientwell. The program does not include the cost of treating recirculated water. Costs

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for a mixing tank and a pump for feeding the infiltration gallery are estimated bythe program.

• The quantity of water — The user can choose the number of soil pore volumes,ranging from one to 20. If this parameter value is unknown, the program defaultsto 10 pore volumes. The program assumes that the soil porosity is 30%. Theprogram user can adjust the number of pore volumes in proportion to porosity ifthe actual soil porosity is known to be different than 30%.

• Operation costs, including electric power, acid or caustic addition, and surfactant— Another model is available for estimating sampling and analysis costs.Because water treatment costs are excluded, no operating labor, other than what isaccounted for in the sampling/analysis model, is estimated.

• Area of contaminated soil — The value of this parameter must be given by theuser.

• Depth to groundwater — The model assumes that the flushing solution willinfiltrate through and past the soil contaminant plume and down to an aquifer (orto a hydrological confining formation), from which it can be extracted fortreatment and recycling.

• The level of protective clothing/equipment needed by site operators• Soil vertical permeability — The water application rate is proportional to

permeability. If the user does not input the vertical permeability, one can choosefrom three categories of soil, from which the model infers vertical permeabilityand water application rate:

• Silty to fine sand (0.001 cm/sec permeability; apply water at 3in./day)

• Fine to coarse sand (0.01 cm/sec permeability; apply water at 6in./day)

• Coarse sand to gravel (0.1 cm/sec permeability; apply water at 12in./day)

The model calculates the theoretical duration of applying flushing based on thenumber of pore volume flushes, porosity, area of contaminated soil, depth togroundwater, and permeability. For the first flush, an additional 33% volume ofsolution is assumed, to account for soil retentivity. The model multiplies thetheoretical duration by a safety factor of two and then derives the operating costs.

The EN VEST computer model for estimating soil washing costs is based on theBioTrol (Chaska, Minnesota) system. The 1993 costs using this model, with a leased,mobile 20-ton/hr unit, were $72/ton for treating 20,000 tons of soil or $66/ton for60,000 tons. The scheme includes screening, flotation, attrition scrubbing,classifying, and dewatering of separated sand, and it includes the use of a slurrybioreactor for the silt/clay fraction, followed by dewatering. The model assumes thata mobile, 20-ton/hour system will be used. The program user must state the tons ofsoil to be processed and the level of protective clothing/equipment needed by thesystem operators. Most soil washing operations require only coveralls, safetyfootware, gloves, and eye protection. Breathing protection is required for handlingdusty soils feeding the process (Anderson, 1993). The costs estimated with thisENVEST model exclude treatment or disposal of residuals.

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The ENVEST model for solvent extraction for removal of organic contaminants is forsystems handling up to 18,000 ydVmo. The required input parameters are:

• Designation of whether the medium being remediated is soil, sludge, or liquid• Throughput in cubic yards per month• Quantity of soil to be treated (not recommended for less than 5,000 yd3)

For better estimating accuracy, values may be included in the input for thesesecondary parameters:

• Whether crushing and screening are required pretreatment steps• Downtime allowed (10% is the default value)• Treatment difficulty, in which 0% is not difficult and 100% requires extensive

treatment

The model assumes that wastewater and solvent are stored on-site in tanks or inmobile tankers and that treated soil is stockpiled. No treatment of liquid wastes andno backfilling of treated soil are included in the estimated costs.

Other ENVEST models that can be applied to arrive at total costs include:

• Water distribution• Overhead electrical distribution• Fencing and signage• Excavation, hauling, backfill, etc.

10.7 Summary of Important Points for Soil Washing

• Contaminants tend to stay adsorbed on fine-grained soil fractions, which aredifficult to extract with water.

• Coarse fractions tend to be relatively clean or are readily washed. Separating outcoarse fractions with water washing reduces the volume of soil that must bedisposed of or treated.

• Solvents can remove organic contaminants from fine-grained soil fractions, aswell as from coarse-grained soil fractions.

• With in situ flushing, infiltrating water dissolves contaminants. The water withthe contaminants is recovered, treated, and recycled through the soil.

• Various additives (such as acids, chelants, alkaline agents, or surfactants) may beused to improve the extraction efficiency of the soil washing system.

• Permitting for soil flushing may be difficult, because additives and contaminantsmay be transported to an aquifer or to uncontaminated soil.

• Ore refining techniques and equipment can be adapted for separating fines andwater washing in an environmental soil washing application.

• Laboratory equilibrium distribution tests help quantify how much contaminantpartitions into the washing fluid versus the amount retained with the soil, as wellas what extractant might work best.

• The lower the soil cation exchange capacity, the more efficient soil washing willbe, especially for removing cationic metals.

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Alkaline agents are used as additives in aqueous soil washing systems to improvethe removal of hydrocarbon oils from soil.Extraction solvents can be recovered by distillation or stripping.Solvent extraction may have to repeated for a batch of soil, because a singleextraction may be insufficient for achieving the remediation goal.Liquified propane or carbon dioxide may be the preferred solvent for low-boilingorganic contaminants, as well as semivolatile organics.A flow scheme and a mass balance for the washing solution and for the soilshould be developed for each soil washing project.If the treatment scheme includes separation by soil particle size fractions, a massbalance should be conducted for each size fraction.The percent solids content should be determined for each process step in a massbalance.Aqueous streams from petroleum contamination soil washing systems can betreated using biodegradation or activated carbon adsorption.Biotreatment can be used for final cleanup of petroleum-contaminated soil.Water treatment may include clarification and/or filtration.Treatability tests determine contaminant removal efficiency; volume fraction offines not cleaned; ratio of soil to wash fluid; additive requirements, efficacy, andrecycling ability; washing contact time needed and treatment requirements forused wash fluid; and percent of wash fluid that can be recycled.Computerized cost-estimating models are available for in situ flushing, ex situwater washing, and ex situ solvent extraction.

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groundwater and soil remediation (process design and cost estimating of proven technologies) || soil...

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