Alternative Cleanup Methods for Chlorinated VOCs

Transcription

1 Slide 1 Alternative Cleanup Methods for Chlorinated VOCs Getting beyond pump and treat Slide 2 Soil Vapor Extraction Vacuum is applied through extraction wells Creates a pressure gradient that induces gas-phase volatiles to be removed from soil Also is known as: in situ soil venting in situ volatilization enhanced volatilization soil vacuum extraction Slide 3 Soil Vapor Extraction

2 Slide 4 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 ft Extraction wells can be either vertical or horizontal Slide 5 SVE: Applicability Target contaminant groups: Volatile compounds (chlorinated or not) Fuels (especially lighter fractions) Will not remove heavy oils, metals, PCBs, or dioxins Can promote in-situ biodegradation of low-volatility organic compounds Slide 6 SVE: Limitations Low permeability soil or high degree of saturation requires higher vacuums (increasing costs) Heterogeneous subsoils may require large screened intervals to get even flows of vapor Reduced removal rates when soil is highly sorptive (high organic content) Off-gases may require treatment

3 Slide 7 SVE: Possible Improvements Impermeable cap on soil surface can improve removal rates (but not always that effective) Horizontal wells may be efficiently laid in trenches; can improve removal De-watering by pump drawdown can expose more unsaturated zone (especially with floating LNAPLs) Slide 8 SVE: Performance Has worked well at many sites, but often find lower removal rates, higher costs than expected Site-specific pilot study needed to establish feasibility and fine tune the design Intermittent (pulsed) extraction can improve efficiency be allowing vapor levels to build up between pulses Slide 9 SVE: Pulsed Operation No-pump interval VOC

4 Slide 10 Air Sparging Air is injected through wells into a contaminated aquifer Air traverses horizontally and vertically through the soil column Creates an in-situ air stripper Usually used in conjunction with SVE to capture contaminant-rich vapors Slide 11 Air Sparging Slide 12 Air Sparging: Applicability As with any stripping system, limited to volatile compunds (VOCs) and light components of fuels Can double as a source of oxygen to stimulate biodegradation of hydrocarbons

5 Slide 13 Air Sparging: Limitations Physics of air-flow in saturated zone poorly understood Preferential channels can short circuit much of the air, by-passing much of the contaminated zone Contaminated air may escape the capture zone of SVE system In heterogeneous aquifer only the porous zones will get much air flow; little removal from less permeable layers Slide 14 Air Sparging: Performance Has been used successfully at many sites But still very hard to generalize from that experience Hard to say why it is working in some cases Not very effective if there is extensive DNAPL free-product below the sparging zone Slide 15 Enhanced Biodegradation Microbes can degrade most pollutants But rate can be VERY slow if they lack proper conditions Groundwater often lacks what they need: electron acceptors (like oxygen) nutrients (N, P, K, trace elements) co-metabolites (for chlorinated cmpds)

6 Slide 16 Enhanced Biodegradation SOLUTION (?): Inject materials that microbes need to degrade contaminants Slide 17 Examples: Add oxygen via sparging Add oxygen via hydrogen peroxide Add alternate electron acceptor (nitrate that substitutes for oxygen) Micro nutrients Hydrogen-releasing compounds (for reductive dehalogenation) Slide 18 Enhanced Biodegradation

7 Slide 19 E.B.: Limitations If heterogeneous, very difficult to deliver the nitrate or hydrogen peroxide evenly Safety precautions when handling hydrogen peroxide Concentrations of H2O2 > 100 to 200 ppm is inhibiting to microorganisms A groundwater circulation system must be created so contaminants don t escape from zones of active biodegradation Many states prohibit nitrate injection Slide 20 Regenesis Corp. Mfr of proprietary solid-phase products for enhancing biodegradation ORC: Oxygen Release Compound (patented Mg peroxide) Stimulates aerobic breakdown) HRC: Hydrogen Release Compound (poly-lactate gel) Stimulates reductive dechlorination of chlorinated solvents Slide 21 Regenesis ORC: Case Study Service station in Wisconsin, underground storage tank (UST) leakage Contaminants: Gasoline, BTEX and MTBE Treatment: ORC Slurry Injection Soil Type: Loose to medium to course grain sand Project Cost: $16,150 (ORC Only)

8 Slide 22 Regenesis ORC: Case Study UST 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 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 ORC: Slurry Injection Method

9 Slide 25 ORC Injection Scheme Slide 26 Regenesis ORC: Results Slide 27 Regenesis ORC: Results Both 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

10 Slide 28 ORC: Reputed Savings Compared with Air Sparging plus Vapor Containment Site AS/SVE ORC Savings % Savings Oklahoma $158,000 $46,000 $112,000 70% California 180,000 80, ,000 55% Alabama 99,000 26,000 73,000 74% All values were derived independently by the sites consultants. The costs are full systems costs with the objective of site closure. [Regenesis] Slide 29 Permeable Reactive Barriers A permeable barrier zone is placed across front of contaminant plume Contaminant can passively flow into barrier Chemical or biological reactions in barrier destroy or otherwise remove contaminants from water Slide 30 Permeable Reactive Barriers

11 Slide 31 Permeable Reactive Barriers Most common material used are zero-valent (metallic) iron (ZVI) ZVI removes chlorines from chlorinated solvents Chemistry not completely understood but it certainly works Also interest in ion-exchange barriers (for metals, etc.) and biological barriers (zones of enhanced bacteria) Slide 32 Slide 33

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15 Slide 43 PRBs: Limitations Passive 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 Natural Attenuation Not an action but a methodology for closing out a site safely with no further action We ll discuss this more in Wednesday s lecture