alternative cleanup methods for chlorinated vocsweb.cecs.pdx.edu/~fishw/ecr-cvoc_slideshow.pdf ·...

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8/4/2008

W. Fish, PSU

Alternative Cleanup Methods for Chlorinated VOCs

Getting beyond pump and treat

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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

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Soil Vapor Extraction


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W. Fish, PSU

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

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W. Fish, PSU

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

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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


8/4/2008

W. Fish, PSU

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)

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W. Fish, PSU

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

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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


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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

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W. Fish, PSU

Air Sparging

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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


8/4/2008

W. Fish, PSU

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

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W. Fish, PSU

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

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W. Fish, PSU

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)


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W. Fish, PSU

Enhanced BiodegradationSOLUTION (?): Inject materials that microbes need to degrade contaminants

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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)

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W. Fish, PSU

Enhanced Biodegradation


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W. Fish, PSU

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

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W. Fish, PSU

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

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W. Fish, PSU

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)


8/4/2008

W. Fish, PSU

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

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W. Fish, PSU

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

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W. Fish, PSU

ORC: Slurry Injection Method


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W. Fish, PSU

ORC Injection Scheme

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W. Fish, PSU

Regenesis ORC: Results

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W. Fish, PSU

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


8/4/2008

W. Fish, PSU

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%

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8/4/2008

W. Fish, PSU

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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W. Fish, PSU

Permeable Reactive Barriers


8/4/2008

W. Fish, PSU

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)

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W. Fish, PSU

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.

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8/4/2008

W. Fish, PSU

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

alternative cleanup methods for chlorinated vocsweb.cecs.pdx.edu/~fishw/ecr-cvoc_slideshow.pdf ·...

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