Multiphase Extraction for Soil and Groundwater Remediation

1. Multiphase Extraction for Soil and Groundwater Remediation Nick Swiger Spring 2007

2. Introduction

3. Contaminant Releases • When a contaminant is released into the environment, it will partition into four phases. • Bulk liquid if insoluble (LNAPL or DNAPL) • Adsorb to soil particles • Vapor phase in soil gas • Dissolve into soil moisture • To effectively remediate the environment from a chemical spill, all phases will need to be recovered. • Types of remediation systems have greatly evolved in last 20 years as everyone is always looking for a more effective and efficient process. • Multiphase extraction is a relatively “new” enhancement to soil vapor extraction.

4. What is Multiphase Extraction? • Extractions of multiple phases of fluids from the subsurface. • Fairly recent enhancement for the increased recovery and efficiency (in right subsurface conditions) of Soil Vapor Extractions. • Two Types: • Dual Phase Extraction (Not focused on in paper/presentation) • Extraction of multiple phases of fluids utilizing separate pumps and conduits. For example, installing a submersible pump in a well to lower the elevation of the groundwater to enhance an existing soil vapor extraction system. • Multiphase Extraction • Extraction of multiple phases of fluids utilizing one pump and one conduit.

5. Typical Schematic of Multiphase Extraction

6. Multiphase Extraction vs. Soil Vapor Extraction • Both vapor and hydraulic conductivity are a function of moisture content in the vadose zone. • Vapor conductivity decreases with increasing moisture content. • With many chemical releases, they will migrate vertically until a lower permeability unit is reached or the surface of the saturated soil is reached (capillary fringe above the groundwater). • Chemicals with a lower density than water (LNAPL) with stop at the saturated soil. • Chemicals with a higher density than water (DNAPL) with stop at a lower permeability unit or force balance (i.e. capillary, hydrostatic, etc.)

7. Multiphase Extraction vs. Soil Vapor Extraction cont. • In both cases, the areas typically with the majority of the contaminants have a high moisture content. • The decreased vapor conductivity in the areas with the majority of the contaminants makes soil vapor extraction less efficient. • The increased moisture content also limits the mass transfer of contaminants from the liquid phase to vapor phase.

8. Multiphase Extraction vs. Soil Vapor Extraction cont. • The mass transfer limited systems are less efficient than flow limited systems. • Mass transfer in high moisture content dominated by Henry’s Law • KH = Cv/Cl (conc. vapor/conc. liquid) • Exponential Decrease vs. Linear Decrease

9. Multiphase Extraction vs. Soil Vapor Extraction cont. • By the removal of multiple phases of fluids it forces systems to be flow limited. Monthly Mass Removed Time

10. Typical Uses of Multiphase Extraction • As with soil vapor extraction, the volatile compounds that tend not to adhere to the soil are amenable to multiphase extraction. The following chemical properties are typically used for indicators. • Relatively low distribution coefficient (ratio of concentration in soil to concentration in liquid) • Kd = Cs/Cl • Relatively high vapor pressure (pressure exerted by vapor in equilibrium with bulk fluid) • Indicative of the compounds volatility or tendency to exist in gaseous phase. • Multiphase extraction is most efficient in porous media with moderate permeability – allows removal of both phases with the greatest remedial influence. • Highly permeable media, the dominant phase removed will be water and there will be less influence on the soil gas. Near the aquifer, mostly groundwater removed with very little soil gas. • Low permeable media, the dominant phase removed will be gas, but the water will have much less influence. Near aquifer, there will be much drawdown, with little influence on the vadose zone.

11. Contaminant Removal with Multiphase Extraction • Phase removal can be accomplished by two mechanisms: • Overcoming hydrostatic pressure (static lift) • Entraining the liquids in the vapor • Dependant on vapor velocity

12. Contaminant Removal with Multiphase Extraction • For the static lift/hydrostatic pressure, the vacuum applied must be able to lift the most dense phase fluid (water with an LNAPL spill and contaminant with a DNAPL spill) to the surface and overcome major and minor losses. • P = ρgh (h – height of lift + head loss)

13. Contaminant Removal with Multiphase Extraction • For the liquid to be removed via vapor entrainment, the vapor velocity forces [drag (Fd) and vapor surface forces (Fv)] must overcome gravity forces (Fg): • Fg < Fd + Fv • Fg = ρw*V*g = mg • Fd = 0.5*Cd* ρa*v2*A • Fv = ρa*A*v2

14. Contaminant Removal with Multiphase Extraction • Three different schemes of multiphase flow in well (based on velocity of the vapor)

15. Multiphase Extraction Operations • Two extraction configurations are typically utilized and are based on the location of the applied vacuum. • Vacuum can be applied down in the well with the use of a “stinger” or “drop” tube. • Vacuum can be applied to the top of the well.

16. Multiphase Extraction Operations • The “stinger” tube configuration would be most applicable very near or at the water table. • The vacuum configuration would be more applicable the high moisture content soil in the vadose zone

17. Multiphase Extraction Operations • The fluid removal is provided by a fan, compressor blower, or pump depending on the subsurface conditions (i.e. flow rate needed, vacuum needed, etc.). • With all fans, blowers, and compressors the fluid stream must be routed through a tank, commonly called a knockout tank, as they are not designed for liquid movement. • Liquid ring pumps do not need to have a knockout tank – designed for multiple phase fluids.

18. Multiphase Extraction Operations • All phases of fluids will have to be treated prior to discharge (for the most part – depending on regulations); however, with non aqueous phase contaminants, there will be minimal treatment of water as the vapor acts as in situ air stripping. • Multiple ways to treat the fluids such as chemical (i.e. oxidation with ozone or peroxide), physical (i.e. carbon adsorption), and biological (i.e. bioreactors)

19. Determining Treatment Area • Each extraction point will have a set “area of influence” or “radius of influence” if radial flow is assumed. • Darcy’s Law can be used in both the saturated and unsaturated media to approximate radius of influence (capture). • Integrated, steady state Darcy’s Law for unconfined aquifers: • Q = [K2π (Ho2-Hw2)] / ln (Ri/Rw) • Where: Ho – original water elevation • Hw – water elevation in extraction well • Ri – radius of influence • Rw – radius of extraction well

20. Determining Treatment Area • For the vadose zone, the Darcy’s Flux, related to pressure changes (instead of head) is: • q = -k/µ (dP/dx) • Using the above and flow equations for radial flow to wells, Jeff Kuo in Practical Design Calculations for Soil and Groundwater Remediation presented the following: • uw = (k/2µ){Pw/[ Rw (ln (Rw/Ri)]}[1-(Pri/Pw)2] • where: uw - vapor flux at the extraction well • Pw - pressure at the extraction well • Pri - pressure at the radius of influence • Rw - radius of the extraction well • Ri - radius of influence • ur = (k/2µ){{Pw/[ r (ln (Rw/Ri)]}[1-(Pri/Pw)2]/ {1 + [1-(Pri/Pw)2] [(ln (r/Rw)/ln (Rw/Rri)]}0.5} • where: ur - vapor velocity at r • r – radial distance r

21. Determining Treatment Area • The two equations can be utilized to find the capture area in the vadose zone and vapor velocity at certain radial points with in the vadose zone. • May soil and groundwater remediation engineers utilize only the “radius of influence” as the soil gas capture. • This provides the extent of capture, but not necessarily the extent of remediation!!!!!

22. Summary • Multiphase extraction is a enhancement that can be made to soil vapor extraction system to increase efficiency of remediation of volatile, moderately soluble, low soil adsorption, chemicals in the vadose zones with higher moisture contents. • The most efficient use of multiphase extraction is with moderately permeable porous media, so both phases of fluids can be extracted. • Can apply vacuum to the top of well (mostly in high moisture content vadose zone) or with a “stinger” pipe (mostly near the surface of the aquifer/capillary fringe). • The extraction can be accomplished with many types of fluid pumps, but must have a knockout tank with all except a liquid ring pump. The fluids will have to treated prior to discharge. • The extent of remediation can be determined by Darcy’s Law. In the vadose zone, caution must be used as the zone of vapor capture is not necessarily the zone of remediation.

23. Questions or Comments? swig2380@uidaho.edu or swigern@michigan.gov