Alternative Cleanup Methods for Chlorinated VOCsweb.cecs.pdx.edu/~fishw/ECR-CVOC_Slideshow.pdf ·...
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Transcript of Alternative Cleanup Methods for Chlorinated VOCsweb.cecs.pdx.edu/~fishw/ECR-CVOC_Slideshow.pdf ·...
Slide 1
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8/4/2008
W. Fish, PSU
Alternative Cleanup Methods for Chlorinated VOCs
Getting beyond pump and treat
Slide 2
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W. Fish, PSU
Soil Vapor ExtractionVacuum is applied through extraction wells Creates a pressure gradient that induces gas-phase volatiles to be removed from soilAlso is known as:
in situ soil ventingin situ volatilizationenhanced volatilizationsoil vacuum extraction
Slide 3
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W. Fish, PSU
Soil Vapor Extraction
Slide 4
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Soil Vapor Extraction
Works only in the vadose (unsaturated) zone Typically used with shallow extraction wells (5-10 ft)Has been used as deep as 300 ftExtraction wells can be either verticalor horizontal
Slide 5
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SVE: ApplicabilityTarget contaminant groups:
Volatile compounds (chlorinated or not)Fuels (especially lighter fractions)
Will not remove heavy oils, metals, PCBs, or dioxinsCan promote in-situ biodegradation of low-volatility organic compounds
Slide 6
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W. Fish, PSU
SVE: LimitationsLow permeability soil or high degree of saturation requires higher vacuums (increasing costs)Heterogeneous subsoils may require large screened intervals to get even flows of vaporReduced removal rates when soil is highly sorptive (high organic content)Off-gases may require treatment
Slide 7
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SVE: Possible ImprovementsImpermeable cap on soil surface can improve removal rates (but not always that effective)Horizontal wells may be efficiently laid in trenches; can improve removalDe-watering by pump drawdown can expose more unsaturated zone (especially with floating LNAPLs)
Slide 8
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SVE: PerformanceHas worked well at many sites, but often find lower removal rates, higher costs than expectedSite-specific pilot study needed to establish feasibility and fine tune the designIntermittent (pulsed) extraction can improve efficiency be allowing vapor levels to build up between pulses
Slide 9
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W. Fish, PSU
SVE: Pulsed Operation
0
20
40
60
80
100
120
1 3 5 7 9 11 13 15 17
VOC
No-pump interval
Slide 10
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W. Fish, PSU
Air SpargingAir is injected through wells into a contaminated aquiferAir traverses horizontally and vertically through the soil columnCreates an in-situ air stripperUsually used in conjunction with SVE to capture contaminant-rich vapors
Slide 11
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Air Sparging
Slide 12
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Air Sparging: Applicability
As with any stripping system, limited to volatile compunds (VOCs) and light components of fuelsCan double as a source of oxygen to stimulate biodegradation of hydrocarbons
Slide 13
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Air Sparging: LimitationsPhysics of air-flow in saturated zone poorly understoodPreferential channels can “short circuit” much of the air, by-passing much of the contaminated zoneContaminated air may escape the capture zone of SVE systemIn heterogeneous aquifer only the porous zones will get much air flow; little removal from less permeable layers
Slide 14
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Air Sparging: PerformanceHas been used successfully at many sitesBut still very hard to generalize from that experienceHard to say why it is working in some casesNot very effective if there is extensive DNAPL free-product below the sparging zone
Slide 15
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Enhanced BiodegradationMicrobes can degrade most pollutantsBut rate can be VERY slow if they lack proper conditionsGroundwater often lacks what they need:
“electron acceptors” (like oxygen)nutrients (N, P, K, trace elements)co-metabolites (for chlorinated cmpds)
Slide 16
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Enhanced BiodegradationSOLUTION (?): Inject materials that microbes need to degrade contaminants
Slide 17
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Examples:Add oxygen via spargingAdd oxygen via hydrogen peroxideAdd alternate electron acceptor (nitrate that substitutes for oxygen)Micro nutrientsHydrogen-releasing compounds (for reductive dehalogenation)
Slide 18
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Enhanced Biodegradation
Slide 19
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E.B.: LimitationsIf heterogeneous, very difficult to deliver the nitrate or hydrogen peroxide evenlySafety precautions when handling hydrogen peroxideConcentrations of H2O2 > 100 to 200 ppm is inhibiting to microorganismsA groundwater circulation system must be created so contaminants don’t escape from zones of active biodegradation Many states prohibit nitrate injection
Slide 20
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Regenesis Corp.Mfr of proprietary solid-phase products for enhancing biodegradationORC: Oxygen Release Compound (patented Mg peroxide)
Stimulates aerobic breakdown)HRC: Hydrogen Release Compound (poly-lactate gel)
Stimulates reductive dechlorination of chlorinated solvents
Slide 21
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Regenesis ORC: Case StudyService station in Wisconsin, underground storage tank (UST) leakageContaminants: Gasoline, BTEX and MTBETreatment: ORC Slurry InjectionSoil Type: Loose to medium to course grain sand Project Cost: $16,150 (ORC Only)
Slide 22
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Regenesis ORC: Case StudyUST was removed along with some of the contaminated soils Residual soil and groundwater contamination remained in source area. Continuing groundwater plume contained MTBE up to 800 ppb and BTEX concentrations ranging up to 14,000 ppb
Slide 23
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ORC slurry was applied into the source area via Geoprobe® injection A total of 1,700 pounds of ORC powder were injected in a slurry
Slide 24
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ORC: Slurry Injection Method
Slide 25
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ORC Injection Scheme
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Regenesis ORC: Results
Slide 27
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Regenesis ORC: ResultsBoth BTEX and MTBE were apparently degraded by > 99.9 % within 10 months of ORC application Post-treatment monitoring throughout a complete hydrogeologic cycle, showed no significant rebound in contaminant concentrations
Slide 28
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ORC: Reputed SavingsCompared with Air Sparging plus Vapor Containment
“All values were derived independently by the sites’ consultants. The costs are full systems costs with the objective of site closure.” [Regenesis]
Site AS/SVE ORC Savings % SavingsOklahoma $158,000 $46,000 $112,000 70%California 180,000 80,000 100,000 55%Alabama 99,000 26,000 73,000 74%
Slide 29
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Permeable Reactive BarriersA permeable “barrier” zone is placed across front of contaminant plumeContaminant can passively flow into barrierChemical or biological reactions in barrier destroy or otherwise remove contaminants from water
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Permeable Reactive Barriers
Slide 31
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Permeable Reactive BarriersMost common material used are zero-valent (metallic) iron (ZVI)ZVI removes chlorines from chlorinated solventsChemistry not completely understood but it certainly worksAlso interest in ion-exchange barriers (for metals, etc.) and biological barriers (zones of enhanced bacteria)
Slide 32
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Slide 33
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Slide 34
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Slide 35
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Slide 36
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Slide 37
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Slide 38
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Slide 39
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Slide 40
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Slide 41
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Slide 42
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Slide 43
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PRBs: LimitationsPassive treatment walls may lose their reactive capacity, requiring replacement of the reactive medium. Passive treatment wall permeability may decrease due to precipitation of metal salts Depth and width of barrier. Limited to a subsurface lithology that has a continous aquitard at a depth that is within the vertical limits of trenching equipment.
Slide 44
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Natural Attenuation Not an “action” but a methodology for closing out a site safely with no further actionWe’ll discuss this more in Wednesday’s lecture