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ENGI 7718 – Environmental GeotechniquesENGI 9621 – Soil Remediation Engineering

Spring 2011 Faculty of Engineering & Applied Science

Lecture 5: Soil Vapor Extraction

2Source: Suthersan, 1997

Schematic of SVE implementation in the field

5.1 Introduction

Vapor extraction wells + blowers or vacuum pumps

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An accepted, recognized, and cost-effective technology for remediating soils contaminated with volatile and semi-volatile organic compounds

known in the industry by various other names, such as soil venting and vacuum extraction

Inducing airflow in the subsurface with an applied vacuum enhancing the in situ volatilization of contaminants extraction of air laden with contaminant vapors can be achieved with vertical extraction wells or horizontal extraction pipes

(1) Soil vapor extraction

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Usage of a similar approach to soil vapor extraction, but with a different objective

The intent of bioventing to induce airflow to provide oxygen to maximize the aerobic biodegradation of the compounds, in contrast to volatilization

Usage of the same blowers used in SVE systems provide specific distribution and flux of air through the contaminated vadose zone stimulate the indigenous microorganisms to degrade the contaminants

e.g. degradation of benzene

(2) Bioventing

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An in-situ technology Can be implemented with a minimum disturbance to

site operations Very effective in removing the volatile contaminant

mass present in the vadose zoneHas the potential for treating large volumes of soil at

acceptable costsThe system can be mobilized and installed very

quicklyCan easily integrate with other technologies for site

cleanup

(3) Advantages

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5.2 Governing Phenomena

5.2.1 Airflow characteristics

Soil air permeability

describes how easily vapors flow through the soil pore space

the airflow and the air permeability are linearly dependent a higher air permeability will result in a higher flow rate at the same vacuum

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Source: Suthersan, 1997

Typical Airflow patterns and vacuum distribution during SVE

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5.2.2 Contaminant properties

Source: Suthersan, 1997

Contaminant distribution into various phases

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(1) Solubility, vapor pressure, and Henry’s law constant

Source: Suthersan, 1997

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(2) Soil adsorption coefficient

Difference in adsorption potential under wet and drier soil conditionsSource: Suthersan, 1997

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(3) Biodegradability of contaminant

The ease of biodegradation affect on efficiencies of bioventing operations

Alkynes > Alkenes > Carboxylic Acids >Alcohols > Straight Chains > Aromatics > Chlorinated Aromatics > Branched Chains

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Weathering Process of changing in organic contaminant mixture characteristics

During SVE initially remove primarily the more volatile, lighter-end fractions then the extracted vapor will likely be composed mostly of heavier compounds

The degree of weathering impact the overall SVE effectiveness

(4) Weathering

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Sand ==> Silt ==> Claydecreased sizedecreased porosity increased available surface decreased SVE efficiency

5.2.3 Soil properties(1) Soil porosity

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(2) Moisture content

increased moisture content decreased of Kddecreased adsorptive capacity increased SVE efficiency

Adsorption of contaminants as a function of moisture content

Source: Suthersan, 1997

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(3) Soil heterogeneity

Removal of contaminants from heterogeneous soilSource: Suthersan, 1997

Dead zone molecular diffusionHomogenerous subsurface better for SVE

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(4) Surface seals

Effect of surface seal on vapor flow pathsSource: Suthersan, 1997

usage of air inlet wells to control the subsurface air flow

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(5) Depth of water table

Water table rise during SVE operation

Source: Suthersan, 1997

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5.3 Applicability

SVE applicability monograph Source: Suthersan, 1997

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SVE more applicable where the contaminated unsaturated zone is relatively permeable and homogenerous

A candidate site for SVE

Source: Suthersan, 1997

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5.4 Pilot testing

SVE pilot testing schematic

Source: Suthersan, 1997

It’s important to conduct pilot testing before field system design and application

21Typical SVE well

Source: Suthersan, 1997

22Monitoring well

Source: Suthersan, 1997

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5.5 System design

Most widely used approach in industry today

The extraction flow rate vacuum, number of wells and their locations selected based on the information obtained from a field pilot test

The pilot test provide steady state vacuum vs. distance relationships for a given site

Radius of influence approach

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(1) Determination of the radius of influence of the extraction well

Vacuum vs. distance plot on a semi-log paperSource: Suthersan, 1997

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(2) Determination of the required number of wells from the radius of influence

Source: Suthersan, 1997

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(3) Estimation of the total extraction airflow rate

Airflow generation plot

Source: Suthersan, 1997

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Operation of two smaller-capacity blowers in series or parallel instead of a single large-capacity blower cost-effective way

Use of blowers in series or parallelSource: Suthersan, 1997

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5.6 Vapor treatment technologies

(1) Thermal oxidation sufficiently heating a VOC in the presence of oxygen to convert VOC to harmless end products

Schematic diagram of a recuperative thermal oxidizerSource: Suthersan, 1997

Key parameters: combustion T + residence time + degree of mixing

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(2) Catalytic oxidation combines a heat exchanger with a catalyst. the catalyst inside the combustion unit lowers the activation energy for combustion

Catalyst Pt, Pd, CoO, CuO, MnO2 …Catalyst deactivators Pb, Ar, S, Si, P, Hg, Zn… Source: Suthersan, 1997

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(3) Adsorption gaseous VOC molecules contact a solid adsorbent and bond via weak intermolecular forces replacement/regeneration desorption

Schematic diagram of a fixed regenerative bed carbon adsorberSource: Suthersan, 1997

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(4) Condensation remove VOCs from a non-condensable gas stream by lowering gas stream temperature at constant pressure or increasing gas stream pressure at constant temperature

Schematic diagram of a refrigerated condenser

Source: Suthersan, 1997

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(5) Biofiltration

In a biofilter => microorganisms grow on materials such as soil or compost + supplemented with synthetic materials including activatedcarbon, which provides bulking and structural stability

Source: Suthersan, 1997

Components of a biofiltration system

33VOC removal in air streams by membrane separationSource: Suthersan, 1997

(6) Membrane filtration use of a semi-

permeable membrane to separate VOCs from an air stream VOC-laden air contacts one side of a membrane that is permeable to organic vapors but is relatively impermeable to air

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5.7 Cost considerations

Costs equipment cost + operation cost$10 to $50 per yd3 of contaminated subsurface material

Source: Suthersan, 1997