lecture 12: in situ air sparging and vacuum extractionbaiyu/engi 9621_files/spring 2012... · for...
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ENGI 9621 – Soil Remediation Engineering
Spring 2012 Faculty of Engineering & Applied Science
Lecture 12: In Situ Air Sparging and
Vacuum Extraction
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A remediation technique has broad appeal since about 1985 due to its projected low costs relative to conventional approaches
For the remediation of volatile organic compounds (VOCs) dissolved in the groundwater, sorbed to the saturated zone soils, and trapped in soil pores of the saturated zone
Often in conjunction with vacuum extraction systems to remove the stripped contaminants
In situ air sparging12.1 Introduction
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Source: Hardisty, 2005
Schematic of Air Sparging with Vacuum Extraction3
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12.2 Applicability
In order for air sparging to be effective the VOCsmust transfer from the groundwater into the injected air, and oxygen present in the injected air must transfer into the groundwater to stimulate biodegradation
The criterion for defining contaminant strippabilityHenry’s law constant being greater than 1×10–5 atm-m3/mol
Compounds with a vapor pressure greater than 0.5 to 1.0 mmHg can be volatilized easily
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Examples of Contaminant Applicability for In Situ Air Sparging
Removal of fuel oil aerobic biodegradation
Source: Suthersan, 1997
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Qualitative presentation of potential air spargingmass removal for petroleum compounds
Source: Suthersan, 1997
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12.3 Description of the process
(1) Air injection into water-saturated soils
Air injected into the saturated zone groundwater necessarily be displaced the displacement of groundwater will have both a vertical and lateral component
Water table mounding a local rise in the water table caused by the vertical component
Mounding an indicator of the “radius-of-influence” of the sparge well during the early stages of air sparging
The magnitude of mounding depends on site conditions and the location of the observation wells relative to the sparge well vary from a negligible amount to several feet in magnitude
(2) Mounding of water table
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Source: Suthersan, 1997
The first transient behavior after initiation of air injection into the saturated zone
The second transient behavior before reaching steady state during air sparging
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(3) Distribution of airflow pathways
Source: Suthersan, 1997 9
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(4) Groundwater mixing
may significantly reduce the diffusionlimitation for mass transfer during air spargingwithout generating any changes in the bulk groundwater flow
is important during air sparging to effectively transport dissolved oxygen for in situBioremediation
can be effective if it occurs at the porescale as well as over site-scale distances
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12.4 Enhanced air sparging technologies
(1) Horizontal trench sparging
Trench sparging developed to apply air sparging under less permeable (the hydraulic conductivities (in the horizontal direction) are less than 10–3 cm/s) geologic conditions when depth of contamination is less than 30 ft
Generally applicable where there is a shallow depth to groundwater and the formation is fine grained
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Horizontal trench sparging (Plan view)
Horizontal trench sparging (Section view)
12Source: Suthersan, 1997
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(2) In-well air sparging
to use air as the carrier of contaminants to overcome the difficulties of injecting air into “non-optimum” geologic formations
Injection of air into the inner casing induces an “air lifting effect” water column inside the inner casing lifted upward and overflow over the top contaminated water drawn into the lower screen and continuously “air lifted” in the inner tube strippable VOCs captured for treatment
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In-well air sparging
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Source: Suthersan, 1997
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(3) Biosparging
To remediate a dissolved plume of contaminant, which is a nonstrippable but extremely biodegradable compound
Injection of air at very low flow rates (0.5 cfm to less than 2 to 3 cfm per injection point) into water-saturated formation to enhance biodegradation
(4) Vapor recovery via trenchesA minor modification to conventional air sparging
that involves the recovery of stripped vapors from fine-grained formations of a shallow depth to groundwater
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Air sparging with vapor recovery through trenches
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Source: Suthersan, 1997
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(4) Pneumatic fracturing for vapor recovery
Using pneumatic fracturing to enhance vapor recovery
Applicable to sites with fine-grained formations that extend below the water table and depths to water that prohibit trenching
Increased hydraulic and vapor flow conductivity near the top of the water table and in the overlyingunsaturated zone allowing stripped contaminants to be collected without spreading out laterally
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Air sparging combined with pneumatic/hydraulic fracturing18
Source: Suthersan, 1997
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Spring 2012 Faculty of Engineering & Applied Science
Lecture 13: Pump and Treat
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ENGI 9621 – Soil Remediation Engineering
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Pumping groundwater to the surface removing the contaminants then either recharging the treated water back into the ground or discharging it to a surface water body or municipal sewage plant
Goals of pump and treat (P&T)
13.1 Introduction(1) Definition
Hydraulic containment of contaminated groundwater Prevent contamination from spreading to
uncontaminated areasTreatment of contaminated groundwater Reduce
concentrations in groundwater to below cleanup standards (MCLs)
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Source: Hardisty, 2005 Schematic of pump and treat
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Source: US EPA, 2002
Until the very recent past, almost all groundwater cleanup systems installed involved variations of the P&T technology
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(2) Applicability contaminants with high solubility, low sorption capability to aquifer materials(3) Advantages very effective at containing and removing the dissolved phase contaminants(4) Lmitations
long duration from years to even decadeshighly dependant on the chemical nature of the
contaminants and the subsurface geology(5) Costs typically $50,000 - $5 million per case
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Cross section along the x axis showing the cone of depression for a single extraction well superimposed on the regional water table
help to determine the number of extraction wells
13.2 Design of the pumping system
Capture Zone
Source: Fetter, 199924
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T = transmissivityi = hydraulic gradient
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Source: Fetter, 1999
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BU = Ti and Q = pumping rate
Source: Shanahan, 2004
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Injection and withdrawal well pair
Source: Shanahan, 2004
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(1) Oil/water separation
13.3 Treatment options
LNAPLs are floating on top of the groundwater table recovery of such contamination can be accomplished in two ways: (1) recovery of the LNAPLs separately from the contaminatedgroundwater, or (2) recovery of the LNAPLs and contaminated water as total fluids
If the LNAPL thickness is small and the site hydrogeologicconditions are less permeable total fluids recovery will be the preferred technique for recovering the floating separate phasecontamination separation of the recovered oil and water becomes necessary prior to further treatment of the contaminatedgroundwater
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Gravity separation, the primary and most common treatment is based on the specific gravity difference between water
and immiscible oil globules is used to move free oil to the surface of a water body for subsequent skimming of oil
Schematic description of an oil/water separator
Source: Suthersan, 1997
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(2) Air stripping tower
Use air stripping technique to remove volatile organic compounds (VOCs) present in pumped contaminatedgroundwater
Source: Fetter, 1999
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(3) Carbon adsorptionGranular activated carbon (GAC) is a granular media, approximately the size of medium fine sand has high interstitial surface area, 800 to 1100 m2/g the surface activation allows organic molecules to adsorb to the interstitial surface remove dissolved organic compounds from water
Source: Shanahan, 200431
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(4) Metals precipitationTransforming a soluble metallic ion into an
insoluble precipitate through the addition of chemicals the most common technique used for treatment of
metal-containing waters
Hydroxide Precipitation
Sulfide Precipitation
Carbonate Precipitation
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Spring 2012 Faculty of Engineering & Applied Science
Lecture 14: In Situ Reactive Walls
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ENGI 9621 – Soil Remediation Engineering
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an emerging technology that have been evaluated, developed, and implemented only within the last few years
is also known as “funnel and gate systems” or “treatment walls”
involves the installation of impermeable barriers downgradient of the contaminated groundwater plume and hydraulic manipulation of impacted groundwater to be directed through porous reactive gates installed within the impermeable barrier
14.1 Introduction(1) Definition
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Slurry walls a means of placing a low-permeability, subsurface reactive wall
Sheet-pile walls an impermeable or low-permeable reactive wall installed using closely spaced steel sheet piles
(2) Types of reactive walls
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Source: Houlsby, 2006
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Grout curtains thin, vertical, grout walls constructed by pressure-injecting grout directly into the subsurface at closely spaced intervals
(a) Crossover from one direction to the other (b) Transition smoothly from one direction
(a) (b) (b) Source: Houlsby, 2006
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Most common reactive wall technologyPossible materials include
14.2 Slurry walls
Soil and bentonite clay (SB) have lower K, are less expensive Typical K = 10-7 cm/sec and Reported K’s as low as 5 x 10-9 cm/sec
Cement-bentonite (CB) have greater shear strength, lower compressibility use on slopes where strength is important use in areas where appropriate soils (for SB) are not available
Additives to enhance CB and SB Fly ash to increase carbon for adsorption Liners or sheet pile installed within wall to decrease K
(1) Introduction
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Source: Grubb, D. G. and N. Sitar, 1994
Slurry wall construction
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(2) Structure of a slurry wall
Typical vertical section for slurry wallShanahan, Waste Containment and Remediation Technology, 2004 39
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Alternative vertical section for “hanging” slurry wall for LNAPLS
Shanahan, Waste Containment and Remediation Technology, 2004 40
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Alternative horizontal section for slurry wall
Shanahan, Waste Containment and Remediation Technology, 200441
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During construction, wall stability maintained byhigher head in trench than in ground water
Slurry density should be 0.25 g/cm3 lighter than emplaced backfill
Shanahan, Waste Containment and Remediation Technology, 2004 42
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Slurry walls may leakConstruction can be difficultWaste may compromise wallRequires long-term pumping in slurry wall enclosuresSlurry walls are good barriers to advection, but not to
diffusion
(3) Limitation of a slurry wall
EPA review of slurry wall success Reviewed 130 sites and 36 had adequate data8 of 36 met remedial objective4 met objective except not yet for long term13 appear to have met objective4 appear not to have met objective7 are uncertain4 of 36 leaked and required repairs (leaks most often at “key” with floor) 43
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In situ reactive wall system can cooperated with other soil remediation technologies/system lead to advanced technologies and successful implementation
14.3 Applicable reactive processes
(1) + Air sparging
Horizontal trench spargingSource: Suthersan, 1997 44
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(2) + Adsoprtion
Easily replaceable, porous reactive cassettes at gates of the reactive walls
Liquid-phase granular activated carbon (GAC) used to remove manyorganics, especially those not easily removable by air stripping or biodegradation
Ion exchange resins used to remove dissolved heavy metals
Source: Suthersan, 1997 45
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Key cost drivers Economy of scaleQuantity of material treated has a large impact Width of the plume to be treatedChoice of supplemental amendmentsAdditional monitoring required by regulators
$1000-2000 per cubic yard of the reactive wall
14.4 Economic consideration
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